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LASER ABLATION STUDIES OF METAL ALLOYS
USING LIBS AND TIME OF FLIGHT MASS
SPECTROMETRY
By
Nasar Ahmed
Reg. No. 2004-GRTB-5680
A Thesis
Submitted in Partial Fulfillment of the Requirements for the Degree of
Doctorate of Philosophy
in
Physics
Session 2011 -2014
Department of Physics
Faculty of Sciences
University of Azad Jammu and Kashmir, Muzaffarabad, Pakistan.
ii
DEPARTMENT OF PHYSICS
UNIVERSITY OF AZAD JAMMU & KASHMIR
MUZAFFARABAD
SUBMISSION CERTIFICATE
The thesis entitled: "Laser Ablation Studies of Metal Alloys using LIBS and Time of
Flight Mass Spectrometry” by Nasar Ahmed, is satisfactory for evaluation and open
public defense.
SUPERVISORY COMMITTEE
Supervisor: Prof. Dr. Muhammad Aslam Baig (H.I, S.I, T.I) ________
Co-supervisor: Prof. Dr. Muhammad Rafique ________
Member: Prof. Dr. Abdul Rauf Khan ________
Nasar Ahmed ________
PhD scholar
Submission date ________
Chairman
Department of Physics
Dean Director
Faculty of Sciences Advance Studies and Research
iii
DEPARTMENT OF PHYSICS
UNIVERSITY OF AZAD JAMMU & KASHMIR
MUZAFFARABAD
CERTIFICATE OF APPROVAL
This is to certify that the research work presented in this thesis, entitled: "Laser
Ablation Studies of Metal Alloys using LIBS and Time of Flight Mass Spectrometry”
was conducted by Mr. Nasar Ahmed under the supervision of Distinguished
National Professor Dr. M. Aslam Baig (H.I, S.I, T.I). No part of this thesis has been
submitted anywhere else for any other degree. The thesis is submitted to the
Department of Physics in partial fulfilment of the requirement for the degree of Doctor
of Philosophy in field of Physics, University of Azad Jammu & Kashmir.
Scholar Name: Nasar Ahmed Signature: ____________
EXAMINATION COMMITTEE:
a) External Examiner 1:
Prof. Dr. Raheel Ali Signature: ____________
Quaid-i-Azam University, Islamabad,
Pakistan
b) Internal Examiner:
Prof. Dr. M. Aslam Baig (H. I, T. I. S. I) Signature:____________
Director, Atomic and Laser Physics Department,
National Centre for Physics, Islamabad,
Pakistan
Supervisor: Prof. Dr. M. Aslam Baig (H. I, T. I. S. I) Signature: ____________
Co-Supervisor: Prof. Dr. Muhammad Rafique Signature:____________
Chairman: Prof. Dr. Muhammad Qayyum Khan Rauf K Signature:____________
Dean Faculty of Sciences:
Prof. Dr. Muhammad Qayyum Khan Rauf Signature:____________
Director Advanced Studies and Research:
Prof. Dr. Azhar Saleem Signature: ____________
iv
Author’s Declaration
I Mr. Nasar Ahmed hereby states that my Ph.D thesis entitled: Laser Ablation
Studies of Metal Alloys using LIBS and Time of Flight Mass Spectrometry “is my
own work and has not been submitted by previously by me for taking any degree from
this University; University of Azad Jammu & Kashmir, Muzaffarabad or anywhere
else in the country/world.
At any time, if my statement is found to be incorrect even after my graduation, the
university has the right to withdraw my PhD degree.
Nasar Ahmed
Ph.D Scholar
Date: _____________
v
Plagiarism Undertaking
I solemnly declare that research work presented in this thesis titled: “Laser Ablation
Studies of Metal Alloys using LIBS and Time of Flight Mass Spectrometry" is
solely my research work with no significant contribution from any other person. Small
contribution/help wherever taken, has been acknowledged and that complete thesis has been
written by me.
I understand the zero tolerance policy of HEC and University of Azad Jammu & Kashmir
towards plagiarism. Therefore, I declare that no portion of my thesis has been plagiarized and
any material used as references is properly referred/cited.
I undertake that if I am found guilty of any formal plagiarism in the above titled thesis, even
after award of Ph.D degree, the university has the right to withdraw/revoke my Ph.D degree
and that HEC and university has the right to publish my name on HEC/ university website,
with the name of students who submitted plagiarized thesis.
Scholar/Author Signature: _______________
Name: Nasar Ahmed
vi
DEDICATED
TO
My Father (Late)
(May Allah Rest his soul in eternal peace)
My Mother, Family members, Caring Spouse and Lovely daughter
vii
List of Contents
LIST OF FIGURES .................................................................................................................. x
LIST OF TABLES ................................................................................................................. xiv
LIST OF PUBLICATIONS .................................................................................................... xv
ABREVIATIONS .................................................................................................................. xvii
ACKNOWLEDGEMENTS ................................................................................................. xviii
ABSTRACT ............................................................................................................................. xx
CHAPTER 1 ............................................................................................................................... 1
INTRODUCTION ..................................................................................................................... 1
1.1 LASER ....................................................................................................................... 1
1.2 LASER ABLATION ................................................................................................. 4
1.2.1 Laser Induced Plasma Formation ................................................................... 5
1.3 CONDITIONS FOR LASER PLASMA DIAGNOSTICS .................................... 9
1.4 PLASMA DIAGNOSTICS .................................................................................... 13
1.5 PLASMA TEMPERATURE ................................................................................. 14
1.5.1 Intensity Ratio Method ................................................................................... 14
1.5.2 Boltzmann Plot Method .................................................................................. 15
1.5.3 Saha Boltzmann Plot Method ........................................................................ 16
1.6 ELECTRON NUMBER DENSITY (ne) ............................................................... 18
1.6.1 Electron Number Density Using Stark Broadening Method....................... 18
1.6.2 Electron Number Density using Saha-Boltzmann Relation ........................ 21
1.7 APPLICATIONS OF LASER PRODUCED PLASMA ...................................... 22
1.8 MASS SPECTROSCOPY ...................................................................................... 24
1.8.1 Principle ........................................................................................................... 24
1.9 LINEAR TIME OF FLIGHT MASS SPECTROMETER .................................. 25
1.10 CALIBRATING OF THE MASS SPECTRUM .................................................. 27
1.10.1 Mass Resolution ............................................................................................... 28
1.11 AIM OF THE PRESENT WORK ......................................................................... 28
CHAPTER 2 ............................................................................................................................. 30
REVIEW OF LITERATURE ................................................................................................ 30
2.1 DIFFERENT TECHNIQUES USED FOR COMPOSITIONAL ANALYSIS .. 31
viii
CHAPTER 3 ............................................................................................................................. 38
MATERIALS AND METHODS ........................................................................................... 38
3.1 LIBS EXPERIMENTAL SETUP .......................................................................... 38
3.1.1 Q-switched Nd-YAG Laser ............................................................................ 39
3.1.2 Focusing Lens .................................................................................................. 40
3.1.3 Fiber Optics ..................................................................................................... 41
3.1.4 Avantes spectrometer...................................................................................... 41
3.2 FABRICATION OF LASER ABLATION TIME OF FLIGHT MASS
SPECTROMETER (LA-TOF-MS) ....................................................................... 41
3.2.1 Design Parameters .......................................................................................... 43
3.2.1 Space Focusing Parameters ............................................................................ 44
3.3 METHODS FOR COMPOSITIONAL ANALYSIS............................................ 47
3.3.1 One Line Calibration Free LIBS (OL-CF-LIBS) ......................................... 48
3.3.2 Self-Calibration free LIBS (SCF-LIBS) ........................................................ 49
3.3.3 Internal Reference Line Self Absorption Correction LIBS (IRSAC-LIBS)
52
3.3.4 Algorithm Based calibration free (AB-CF-LIBS) ........................................ 55
3.3.5 Compositional Analysis using LA-TOF-MS ................................................. 59
CHAPTER 4 ............................................................................................................................. 60
LASER ABLATION TIME OF FLIGHT MASS SPECTROMETER FOR ISOTOPE
MASS DETECTION AND ELEMENTAL ANALYSIS OF MATERIALS ...................... 60
4.1 CALIBRATION OF LINEAR LA-TOF-MS ....................................................... 61
4.2 Spatial and Temporal Kinetic Energies Distributions ........................................ 62
CHAPTER 5 ............................................................................................................................. 68
LASER ABLATION STUDIES OF DIFFERENT KARATS OF GOLD USING LIBS
AND TIME OF FLIGHT MASS SPECTROMETER ......................................................... 68
5.1 EMISSION STUDIES ............................................................................................ 70
5.2 DETERMINATION OF PLASMA TEMPERATURE ....................................... 74
5.3 DETERMINATION OF ELECTRON NUMBER DENSITY ............................ 78
5.4 SPATIAL BEHAVIOR OF PLASMA PARAMETERS ..................................... 81
5.5 EFFECTS OF LASER IRRADIANCE ON PLASMA PARAMETERS ........... 83
5.6 COMPOSITIONAL ANALYSES USING SCF-LIBS ......................................... 84
5.7. LIMITS OF DETECTION .................................................................................... 85
ix
5.8 COMPOSITIONAL ANALYSIS USING LASER ABLATION TIME OF
FLIGHT MASS SPECTROMETER (LA-TOFMS)............................................ 88
CHAPTER 6 ............................................................................................................................. 91
LASER ABLATION STUDIES OF BRASS ALLOY USING LIBS AND LA-TOF-MS . 91
6.1 OPTICAL EMISSION STUDIES ......................................................................... 92
6.2 COMPOSITIONAL ANALYSIS USING SAC-LIBS AND IRSAC-LIBS ........ 97
6.3 QUANTITATIVE ANALYSIS USING LASER-ABLATION TIME OF
FLIGHT MASS SPECTROMETER (LA-TOF-MS) .......................................... 99
CHAPTER 7 ........................................................................................................................... 102
A COMPARATIVE STUDY OF COPPER NICKLE ALLOY USING LIBS, LA-TOF-
MS, EDX AND XRF ............................................................................................................. 102
7.1 EMISSION STUDIES .......................................................................................... 103
7.2 DETERMINATION OF PLASMA TEMPERATURE ..................................... 105
7.3 DETERMINATION OF ELECTRON NUMBER DENSITY .......................... 107
7.4 NUMBER DENSITY USING SAHA-BOLTZMANN RELATION ................ 109
7.5 QUANTITATIVE ANALYSIS USING OL-CF-LIBS, SCF-LBS AND AB-CF-
LIBS TECHNIQUES ............................................................................................ 109
7.6 QUANTITATIVE ANALYSIS BY LA-TOF-MS, EDX AND XRF ................. 113
CHAPTER 8 ........................................................................................................................... 116
ON THE ELEMENTAL ANALYSIS OF DIFFERENT CIGARETTE BRANDS USING
LIBS LA-TOF-MS ................................................................................................................ 116
8.1 OPTICAL EMISSION STUDIES ....................................................................... 117
8.2 DETERMINATION OF PLASMA TEMPERATURE ..................................... 121
8.3 DETERMINATION OF ELECTRON NUMBER DENSITY: ......................... 123
8.4 COMPOSITIONAL ANALYSIS USING OL-CF-LIBS ................................... 125
8.5 ELEMENTAL ANALYSIS BY LASER ABLATION TIME OF FLIGHT
MASS SPECTROMETER ................................................................................... 126
CHAPTER 9 ........................................................................................................................... 129
SUMMARY ........................................................................................................................... 129
9.1 CONCLUSIONS ................................................................................................... 129
9.2 FUTURE RECOMMENDATIONS .................................................................... 133
LITERATURE CITED......................................................................................................... 134
x
LIST OF FIGURES
Figure 1.1: Energy level diagram of three level laser system 2
Figure 1.2: Energy level diagram of four level laser system 3
Figure 1.3: Schematic representation of laser produced plasma plume 6
Figure 1.4: Graphical representation of mechanisms of laser induced ablation 8
Figure 1.5: Schematic diagram of single stage Linear Time of Mass spectrometer 25
Figure 1.6: Tailing effect in time of flight mass spectrum (TOF-MS). 27
Figure 3.1: Schematic diagram of LIBS setup 39
Figure 3.2: A schematic diagram of the experimental setup of the Laser
ablation/ionization TOF-MS system. 42
Figure 3.3: Schematic diagram of LA-TOF-M showing lagging in the mass spectrum
due to different initial kinetic energies. 45
Figure 3.4: A schematic diagram of the force experienced by the charged particle in
the magnetic field. 46
Figure 3.5: Lorentzian Fit of lead (208) for calculation of resolution. 47
Figure 4.1: Calibration curve for the locally fabricated linear time of flight mass
spectrometer 61
Figure 4.2: Comparison of the TOF mass spectra of lead at low Vac without
magnetic lens (a), at high Vac without magnetic lens (b) and at high Vac
with magnetic lens.(c). 63
Figure 4.3: Laser ablation time of flight mass spectrum (TOF-MS) of Lithium. Two
isotopes of lithium; Li6 and Li
7 are evident at -1600 V operating
voltage. 64
Figure 4.4: Laser ablation/ionization time of flight mass spectrum (TOF-MS) of
pure cadmium. 65
Figure 5.1 Typical optical emission spectra of the Laser produced plasmas at the
gold alloys, 24K, 22K, 20K, 19K and 18K, covering the spectral region
250- 870nm using laser energy 100mJ and 2µs time delay. 71
xi
Figure 5.2: Emission spectra of the Laser produced plasmas of different Karat of the
gold covering the spectral region 508 - 547nm showing variations in the
line intensities of copper, silver and gold lines. 72
Figure 5.3: Variation of emission line intensity of Cu I at 510, Ag I at 328 Au I at
312nm with the variable laser energy (5-130) mJ laser energy of 18K
gold alloy. 73
Figure 5.4: Boltzmann plots of the 22K gold alloy using emission lines of Cu I, Ag
II and Au I using Laser pulse energy 100 mJ and at 2µs time delay. 77
Figure 5.5: Stark broadened line profile of Ag I line at 328.07 nm (a), Calculation of
full width at half area of hydrogen Hα line at 656.28 nm at 100mJ laser
energy (b) Calculation procedure for FWHA using numerical
integration (c). 79
Figure 5.6: Variation of electron number density along the direction of the laser
produce plasma plume. 82
Figure 5.7: Variation of excitation temperature as a function of distance along the
direction of the laser produces plasma plume. 82
Figure 5.8: Variation of electron number density with the laser pulse energy. 83
Figure 5.9: Variation of excitation temperature with the laser pulse energy. 84
Figure 5.10: Calibration curves of copper and silver obtained by drawing the
normalized line intensities against concentrations. 87
Figure 5.11: Laser Ablation Time of Flight Mass spectra of 24K, 22K, 20K, 19K and
18K gold alloys at 5mJ Laser pulse energy 88
Figure 5.12: Enlarge spectra with Lorentz fit to the experimental data points of the
laser ablation time of flight mass spectra of gold alloy samples 89
Figure 5.13: Bar graph showing the compositional analysis of all Karats of gold by
CF-LIBS and LA-TOF-MS 90
Figure 6.1: Optical emission spectrum of the laser produced brass plasma, covering
the spectral region 463 – 527 nm. 93
Figure 6. 2: Typical Boltzmann Plots to estimate the plasma temperatures from the
Cu I and Zn I spectral lines 94
xii
Figure 6.3: Stark broadened line profile of copper line at 465.01 nm along with the
Voigt fit. 95
Figure 6.4: Typical Boltzmann Plots of copper and zinc after self-absorption 98
Figure 6.5: The mass spectrum of brass alloy measured by LA-TOF mass
spectrometer. 100
Figure 6.6: A histogram of the results of the composition of the copper–zinc based
brass alloy acquired using different analytical techniques. 101
Figure 7.1: Optical emission spectrum of the Laser produced Cu-Ni alloy plasma
covering the spectral region 295- 307 nm. The spectral lines of Cu-I and
Ni I are assigned in the blue and red colour respectively. 103
Figure 7.2: Optical emission spectrum of the Laser produced Cu-Ni alloy plasma
covering the spectral region 350 – 475 nm. 104
Figure 7.3: Optical emission spectrum of the Laser produced Cu-Ni alloy plasma
covering the spectral region 506 – 579 nm. 104
Figure 7.4: Typical Boltzmann-Plots for estimating the plasma Temperature,
emission lines from singly ionized Cu and Ni are used for obtaining
temperature. 107
Figure 7.5: Stark broadened profile of copper line at 510.55 nm along with the Voigt
fit FWHM 0.09 nm. 108
Figure 7.6: Time of Flight Mass Spectrum of the Cu-Ni alloy. 113
Figure 7.7: Energy Dispersive X-ray spectrum of the Cu-Ni alloy. 114
Figure 7.8: Histogram across different techniques vs composition of Cu-Ni alloy 115
Figure 8.1: Optical emission spectrum of the Laser produced Kisan Cigarette tobacco
plasma covering the spectral region 250- 870nm. 118
Figure 8.2: Optical emission spectrum of the Laser produced tobacco plasma
covering the spectral region from (a) (280nm-324nm), (b) (400nm-
4700nm), (c) (490nm-590nm) and (d) (650nm-780nm). 120
Figure 8.3: Variation of Intensity of emission line of Ca I at 527.03nm at different
delay times between laser pulse and acquisition time of Kisan brand.
121
xiii
Figure 8.4: (a) Boltzmann plots of all the tobacco brands using Ca II spectral lines.
(b) Shows the Saha Boltzmann plot for Ca along with an inset showing
the Boltzmann plots of Kisan cigarette brand. 123
Figure 8.5: Bar graph showing the variation of number densities in the emission
spectra of different cigarette brands. 124
Figure 8.6: Laser Ablation Time of Flight Mass spectrum of Kisan Tobacco. 126
Figure 8.7: Bar graph showing the compositions of metals in different cigarette
brands. 128
xiv
LIST OF TABLES
Table 4.1: Measured isotope ratios for Li, Cd and Pb samples compared with
natural abundance (NIST database, 2016) 67
Table 5.1: Spectroscopic parameters of the Cu, Ag and Au emission lines used to
construct the Boltzmann Plots. 75
Table 6.1: Quantitative results for the copper–zinc based brass alloy 100
Table 7.1: Spectroscopic parameters of copper and nickel lines taken from NIST
database. 106
Table 7.2: Quantitative calculation by self-calibration free (SCF-LIBS) method 111
Table 7.3: The density ratio (ncu I
/nNi-II
) for the calibration free quantitative
analysis 111
Table 7.4: Comparison of the experimentally and theoretically values derived at
0.82 eV plasma temperature. 112
Table 7.5: Compositional analysis using different techniques. 115
Table-8.1: Spectroscopic parameters of the emission lines of Ca I and Ca II (NIST
data base, 2016) to construct the Boltzmann plot. 122
Table 8.2: Average elemental composition of Pakistani Cigarette Brands 127
xv
LIST OF PUBLICATIONS
1. Nasar Ahmed, Rizwan Ahmed, Zeshan A Umar, Usman Liaqat, Umair
Manzoor and M.A. Baig, Qualitative and Quantitative Analyses of Copper
Ores collected from Baluchistan, Pakistan using LIBS and LA-TOF-MS,
Applied Physics B, 124, 160(2018)
2. Nasar Ahmed, M. Abdullah, Rizwan Ahmed, N.K. Piracha and M. Aslam Baig,
Quantitative analysis of brass alloy by CF-LIBS and Laser Ablation Time of
Flight Mass Spectrometer, Laser Phys. 28 (2018) 016002 (7pp)
3. Nasar Ahmed, Rizwan Ahmed, M. Aslam Baig, Analytical Analysis of
Different Karats of Gold Using Laser Induced Breakdown Spectroscopy (LIBS)
and Laser Ablation Time of Flight Mass spectrometer (LA-TOF-MS), Plasma
Chem Plasma Process 38 (2018) 207-222
4. Nasar Ahmed, Zeshan A. Umar, Rizwan Ahmed, M. Aslam Baig, On the
elemental analysis of different cigarette brands using laser induced breakdown
spectroscopy and laser-ablation time of flight mass spectrometry, Spectrochimica
Acta Part B 136 (2017) 39–44
5. Nasar Ahmed, Rizwan Ahmed, Z. A. Umar, M. Aslam Baig, Laser Ionization
Time of Flight Mass Spectrometer for Isotope Mass Detection and Elemental
Analysis of Materials, Laser Phys. 27 (2017) 086001 (6pp)
6. Nasar Ahmed, Rizwan Ahmed, M. Rafiqe, and M. Aslam Baig, A comparative
study of Cu–Ni Alloy using LIBS, LA-TOF, EDX, and XRF, Laser and Particle
Beams, 35 (2016), 1-9.
7. Zeshan A. Umar, Nasar Ahmed, Rizwan Ahmed, Usman Liqat, M. A. Baig,
Elemental composition analysis of granite rocks using LIBS and LA-TOF-MS,
Applied Optics, 57(2018), 4985-4991.
8. Mahmood Akhtar, Abdul Jabbar, Shaukat Mehmood, Nasar Ahmed, Rizwan
Ahmed, M. A. Baig, Magnetic Field Enhanced Detection of Heavy Metals in Soil
using Laser Induced Breakdown Spectroscopy, Spectrochimica Acta Part B 148
(2018) 143–151
xvi
9. Zeshan A. Umar, Nasar Ahmed, Rizwan Ahmed, M. Arshad, M. Anwar-Ul-
Haq, T. Hussain, M. Aslam Baig, Substrate temperature effects on the Structural,
Compositional and Electrical Properties of VO2 thin films deposited by pulsed
laser deposition, Surface and Interface Analysis, 50(2018) 297– 303
10. Nasar Ahmed, Abdul Majid, M. A. Khan, M. Rashid, Z. A. Umar, M. A. Baig,
Synthesis and Characterization of Zn/ZnO microspheres on indented sites of
silicon substrate by hydrothermal route, Material Science Poland, 36(2018),
DOI: 10.2478/msp-2018-0058
11. Qaswer Abbass, Nasar Ahmed Rizwan Ahmed, M. Aslam Baig, A Comparative
Study of Calibration Free Methods for the Elemental Analysis by Laser Induced
Breakdown Spectroscopy, Plasma Chem Plasma Process, 36(2016), 1287–1299.
12. Rizwan Ahmed, Nasar Ahmed, J. Iqbal, and M. Aslam Baig, An inexpensive
technique for the time resolved laser induced plasma spectroscopy, Plasma
chemistry plasma process, 23(2016), 083101
xvii
ABREVIATIONS
AB-CF-LIBS : Algorithm Based Calibration Free LIBS
AES : Atomic Emission Spectrometry
CCD : Charged Coupled Device
COG : Curve of growth
CW : Continuous Wave
EDX : Energy Dispersive X-ray Spectroscopy
FWHM : Full width Half Maximum
ICCD : Intensified Charged Coupled Device
IRSAC : Internal Reference Line Self Absorption Correction
LIBS : Laser Induced Breakdown Spectroscopy
LA-TOF-MS : Laser Ablation Time of Flight Mass Spectrometer
LTE : Local Thermodynamical Equilibrium
ML : Magnetic Lens
ONCF-LIBS : One Line Calibration Free LIBS
SCF-LIBS : Self-Calibration Free LIBS
XPS : X-ray Photo electron Spectroscopy
XRF : X-ray Fluorescence
xviii
ACKNOWLEDGEMENTS
All commendations to Almighty Allah the most Merciful and Ubiquitous, who
enabled me to complete this research work successfully and all respects for the Holy
Prophet MUHAMMAD (P.B.U.H), the foundation of the knowledge and guidance for
all.
I acknowledge my deepest gratitude to my respectable and kind Supervisor
Distinguished National Professor Dr. Muhammad Aslam Baig (H.I, S.I, T.I) for
the guidance and encouragement provided to me throughout my research work. I
consider myself to be very fortunate to get the chance of working under his
supervision. He always trusted in me and given me freedom of doing independent
research work, which made me self-confident to gain the deep understanding of the
scientific research work.
I express my sense of indebtedness to my Co-Supervisor Prof. Dr.
Muhammad Rafique, Director, QEC, UAJ&K, for his invaluable guidance, moral
support and remarkable efforts for my study leave for my PhD studies. In fact, without
his efforts it was difficult for me to fulfil the task. I have also been fortunate in
precious suggestions at every stage of my studies.
My Sincere gratitude and wishes for Dr. Rizwan Ahmed for his guidance,
motivations and help throughout my research work. I am indebted to pay thanks to Dr.
Zeshan Adeel Umar, for his valuable encouragement and help during my research
work.
xix
I am thankful to Dr. Abdul Rauf Khan Chairman Department of Physics for his
moral support. I am thankful to D.G. NCP, Dr. Hafeez Hoorani, Dr. Riffat Mahmood
Quershi and the other officials for their help and support. I am grateful to the
Administration of my parent University; The University of Azad Jammu & Kashmir for
sanctioning the study leave. I would like to thank all my lab/PhD fellows Qaswer Abbass,
M. Abdullah, Muhammd sajid, Mehmood Akhter, Abdul Jabber, Dr. Javed Iqbal, Shahab
Abbasi, Shaujat Bukhari, Tariq Iqbal, Sana Jamil, Abida Zafar and Amir Fayyaz.
I would like to thank Higher Education Commission of Pakistan (HEC) for
providing the Indigenous Scholarship. I found no words to thanks my family and all of my
teachers for their support and patience during my Study.
I am deeply indebted to my mother whose prayers are real asset of my life. I am
extremely grateful to my father (Late) who gave the moral and financial support
throughout my education. I am obliged to my elder brother Muhammad Razzaq for his
moral and pecuniary support and my brothers, sisters, relatives and friends for their moral
support and encouragement during the hectic time of study.
In the last but not the least, a special appreciation to my spouse Maryam Qasim for
the continuous support and encouragement. Without her sacrifice and patience, it was next
to impossible to complete my Ph.D. I feel pleasure to appreciate my lovely daughter;
Horain Fatima who managed to survive without her due care from my side.
Nasar Ahmed
xx
ABSTRACT
Laser ablation is a versatile technique used for the investigations of
technological advanced and industrially important materials. In this technique, the
interaction of a short and intense laser pulse forms a plasma plume. The laser produced
plasma plume consists of radiation which arises due to transitions of electrons from the
excited states of atoms and ions. The aim of this thesis is the fabrication of the laser
ablation time of flight mass spectrometer (LA-TOF-MS) with an improved resolution
and to compare the compositional results of mass spectra from LA-TOF-MS with the
emission spectra obtained from laser induced breakdown spectroscopy (LIBS). The
compositional analysis using calibration free (CF-LIBS) techniques is based on the
observed emission spectra of the laser produced plasma plume whereas, the elemental
composition analysis using laser ablation time of flight mass spectrometer (LA-
TOFMS) is based on the mass spectra of the ions produced by laser ablation.
We have successfully designed and locally fabricated an improved version of
the laser ablation time-of-flight mass spectrometer (LA-TOF-MS) for isotope mass
analysis and elemental analysis of materials. This system is coupled with a Q-switched
Nd: YAG laser, which is capable of delivering the energy of about 850 mJ at 1064 nm
and 500 mJ at 532 nm. The resolution of system has been improved by adjusting
spatial and space focusing, and optimizing other parameters. The capability of the
system has been exploited by the isotopic analysis and compositional analysis of
different alloy samples, having certified composition. The laser ablation time of flight
mass spectrometer complementary with Laser induced breakdown spectroscopy has
xxi
been used for the quantitative determination of constituents of certified samples;
different Karats of gold (18K, 19K, 20K, 22K, 24K), Brass alloy (Cu 62%, Zn 38%)
and Cu-Ni Alloy (75% Cu, 25% Ni). Moreover, the samples with the unknown
compositions such as different brands of the cigarette available in Pakistan have also
been investigated using LIBS and LA-TOF-MS techniques. Initially five Karats of
gold alloys, 18K, 19K, 20K, 22K and 24K having certified composition of gold as
75%, 79%, 85%, 93% and 99.99% were selected and their precise elemental
compositions were determined by LIBS and LA-TOF-MS. Here internal reference line
self-absorption correction laser induce breakdown spectroscopy (IRSAC-LIBS)
technique have been utilized for the quantitative determination of constituents present
in different Karats. Elemental compositions of these gold alloys were also determined
using a Laser Ablation time of flight mass spectrometer (LA-TOF-MS). The
quantitative analysis of brass alloy has been studied using Laser Induced Breakdown
Spectroscopy (LIBS), Energy Dispersive X-ray Spectroscopy (EDX) and Laser
Ablation Time of Flight Mass Spectrometry (LA-TOF-MS). The emission lines of
copper (Cu I) and zinc (Zn I) are used to calculate the plasma parameters. Here we
have compared the elemental composition obtained by SCF-LIBS and IRSAC-LIBS
with LA-TOF-MS and EDX. After utilizing SCF-LIBS and IRSAC-LIBS for
quantitative analysis, we have compared the composition for Cu-Ni alloy using three
calibration free LIBS techniques other than IRSAC-LIBS, and also compared the
results with laser ablation LA-TOF-MS. For the quantitative determination of
constituents of Cu-Ni Alloy (Pakistani five rupee coin of year 2004) of known
composition (Cu 75%, Ni 25%), we have utilized one line calibrations-free (OL-CF-
xxii
LIBS), self-calibration free (SCF-LIBS), and algorithm based calibration free (AB-CF-
LIBS) techniques. Results obtained by these LIBS based techniques have also been
compared with LA-TOF-MS. The samples of fourteen different brands of cigarettes
available in Pakistan have also been analyzed using the above mentioned techniques.
We have also performed compositional analysis of the trace elements in different
brands of tobacco available in Pakistan using one line calibration free (OLCF-LIBS)
and Laser ablation Time of Flight Mass Spectrometer (LA-TOFMS). The results
obtained by (CF-LIBS) are comparable with (LA-TOF-MS). The compositional results
obtained by OL-CF-LIBS, SCF-LIBS, IRSAC-LIBS and algorithm based AB-CF-
LIBS are in agreement with that of LA-TOF-MS and other standard techniques. The
analysis of different industrial important alloys and different brands of cigarettes
demonstrates that LIBS complemented with LA-TOF-MS are powerful techniques for
the elemental analysis of the major and trace elements in any solid samples.
1
CHAPTER 1
INTRODUCTION
This chapter consists of introduction, properties and application of
sophisticated laser systems, Introduction and application of Laser Induced Breakdown
Spectroscopy (LIBS) and Laser Ablation Time of Flight Mass Spectrometer (LA-TOF-
MS). This chapter also includes introduction to the plasma parameters, optically thin
and LTE condition of plasma, mass spectroscopy, time of flight mass spectroscopy,
mass calibration and mass resolution.
1.1 LASER
The term LASER is the abbreviation of the light (L) amplification (A) by
stimulated (S) emission (E) of radiation (R). LASER is a device which can produce
monochromatic, coherent and intense light. Due to the unique properties of laser, it can
be applied in different fields as; industry, space science, medical science, agriculture
and other fields (Griem, 1997; Noll, 2012; Cremers 2006). Charles and Arthur, (1958)
provided the basic idea about the laser operation. The helium neon laser was the first
continuous wave (CW) laser. The semiconductor diode laser and air-cooled ion lasers
have also been introduced (Cremers 2006; Noll, 2012). To operate a laser, an active
medium is required which generates population inversion and optical resonator with a
positive feedback produces a highly collimated and monochromatic beam (Hegazy et
al., 2014).
Active medium may be in the form of solid, liquid or gas containing energy
levels for the absorption and emission of optical radiations. This medium is placed
2
between two highly reflecting mirrors forming an optical resonator. Population in the
upper energy level (E2) is enhanced by an excitation process known as pumping, in
which atoms are raised from a lower state to an upper state. The electrical and optical
pumping are the common ways for pumping, which can be achieved by stimulated
absorption, energy levels are pumped by an intense irradiation. However, multiple
systems are necessary to achieve population inversion and laser action. For the laser
action we can use three or four level systems.
Figure 1.1: Energy level diagram of three level laser system
The population inversion cannot be achieved in a two level systems, which is a
necessary condition for the laser action. Therefore, either three or four level system is
required to obtain the population inversion. The active medium having three energy
levels; the ground state, metastable state and the excited state is shown in the Fig.1.1.
A large number of atoms are excited from the ground state to the excited state. A non-
Pumping
Non-Radiative decay
Lasing Transition
Ground state
Metastable state
Excited state
N2
N1
3
radiative decay of the atoms from the excited stats allows the atom to decay in the
metastable state (Hegazy et al., 2014; Griem, 1997). The population inversion cannot
be achieved until the population in the metastable state is greater than the population in
the ground state i.e N2 > N1. The lasing action occurs between the metastable state and
the ground state (Hegazy et al., 2014; Griem, 1997).
Figure 1.2: Energy level diagram of four level laser system
A four level laser system is shown in the Fig. 1.2. The pumping process of a
four level laser systems is similar to the three level laser system. Four level laser
systems have an extra energy level above the ground state having very short lifetime.
The population of the lower laser level E2 decay rapidly to the ground state, so
practically it remains empty. Here population inversion is achieved by rapid population
of the upper laser level E3, through the higher energy level E4. Four level laser systems
requires lower pumping and no need to pump more than 50% of the atoms to the upper
Population number (N)
N1 E1
E2
E3
E4
N2
N3
N4
Pumping
Fast transition decay
Fast decay
Laser Radiation
4
laser level to create the population inversion. Thus, a continuous operation of the four
level lasers is possible even if 99% of electrons remain in the ground state (Griem,
1997).
1.2 LASER ABLATION
When a high power laser beam is focused on any solid sample it generates
plasma, which is characterized by the emitted light and an acoustical shock wave
generates high-velocity expansion of matter (Noll, 2012). For the generation of
plasma, numbers of phenomenon are involved including melting of the target material,
evaporation, excitation and ionization (Brill, 1997). If a metal is heated at enough high
temperature, the electrons gain sufficient energy to overcome the natural barrier, so
thermionic emission will occur. For metals, the work function 𝜑 is the energy required
to remove an electron from the Fermi level to infinity. The ionization potential I is the
energy required for removing an outer electron of the atom to infinity. The energy
necessary for the transition is ∑, so the ionization potential is the sum of transition
energy and the work function (Davydov, 2002; Davis, 1998):
𝐼 = ∑ + 𝜑 1.1
The valence bands are filled up to the Fermi energy (EF). The energy difference
between Fermi energy and continuum level corresponds to the work function (ϕ)
(Davis, 1998). To generate plasma on the target surface, sample should be evaporated.
Evaporation occurs when the energy absorbed by the target exceeds the Latent heat of
vaporization Lv of material. For the plasma generation, laser energy must be greater
than the target's threshold fluence, below which no evaporation occur (Corti, 2001;
5
Davis, 1998). The threshold fluence 𝜑𝑡ℎ(𝐽𝑐𝑚−2) also depends on the density of the
target material, latent heat of vaporization, thermal conductivity, specific heat, laser
wavelength and ionization energy (Marucco, 2004; Marucco, 1998; Honkimaki, 1996).
Exposing an atom to an intense laser beam may cause multiphoton ionization/excitation.
An atom possesses discrete and continuum energy states. When a high power laser
beam interacts with an atom, the outermost electron absorbs the photon energy ℎ𝜗 and
jumps to the next energy state E2 provided that the photon energy resonate with the
energy difference between the states (𝐸2 − 𝐸1) involved.
ℎ𝜗 = 𝐸2 − 𝐸1 1.2
Here, ϑ is the frequency of the absorbed photon. This process can be completed by a
single photon or by the several photons. If the ionization occurs with the help of
several photons it is called Multiphoton Ionization. High laser intensities can deform
the atomic potential. As a result, those electrons for which the photon energy is not
enough to overcome the potential barrier may also come out. Such ionization is known
as Tunneling Ionization (Argaon et al., 2014).
1.2.1 Laser Induced Plasma Formation
Formation of plasma by mean of laser is rapid process and is under exploration for
the many years. When a high power laser light of very short duration delivers its energy
to the target surface it excites, ionizes and vaporizes the material called as ‘plasma
plume’. It has three main regions as shown in Fig. 1.3. First region is hottest and the
densest part of the plasma, called core of the plasma. This region is near the target
surface, plasma temperature is very high and mostly ionized from of material exist.
6
Second layer is Knudsen layer, which have thickness equal to a few mean free paths
and it exist closest to the target surface as shown in figure below. This is the layer
where particles achieve an equilibrium velocity distribution from non-equilibrium
distribution (Argaon et al., 2008). In the central region of the laser produced plasma,
both neutrals and ions exist due to the continuous ionization/recombination processes.
The last layer (outer) of the laser produced plasma is relatively cold. In the outer layer
of the plasma plume the population of neutrals dominates. The shock waves are
produced beyond the outer layer of the plasma plume due to the explosive expansion.
The shock waves travel in front of the plasma plume (Aguilera et al., 2004) as shown
in Fig. 1.3.
Figure 1.3: Schematic representation of laser produced plasma plume
Quantum mechanics best describes the phenomenon of laser matter interaction.
When a material is exposed in front of high power laser the sample undergoes some
changes resulting ablation. Any energy applied to an atom can be fully absorbed by the
7
atoms or molecules or the electron may jump to any higher energy level or it may
leave the bound state and gets ionized (Harilal et al., 2005). If the applied energy is
high enough, it can detach more electrons by overcoming second or third ionization
potentials. These detached electrons emit radiation when they recombine with the ions.
Usually the ionization takes place immediately after focusing the beam on a target and
is completed before the pulse ends. An electric field is set up due to this charge
separation which, consequently, knocks the ions out of the target by transferring
momentum. These free electrons and ions are collectively termed as plasma, giving off
a glowing spark. Spark formation is followed by the absorption of light and production
of charged particles (Harilal et al., 2005). Charged particles, specifically the free
electrons, come from the atoms in the sample when applied energy of the laser beam is
greater than the ionization energy of the sample. These detached electrons recombine
with ions resulting in the emission of light. In addition to the visible and UV
radiations, high plasma temperature can also lead to the emission of radiation which
may fall into the X-ray region (Harilal et al., 2005). Fig. 1.4 explains the steps of laser
induced plasma formation (Aguilera et al., 2004). In the thermal ablation the absorbed
laser energy is completely converted into heat. Due to the high temperature on the
surface of the sample ablation occurs.
8
Figure 1.4: Graphical representation of mechanisms of laser induced ablation
If the incoming photons have appropriate energy, the absorption of such
photons can introduce the defects in the target material. Such high energy beam can
break the bonds of atoms, ions and molecules. The ablation occurred due to the defects
and bond breaking is termed as photo-chemical ablation. However, thermal and non-
thermal mechanisms can cause photo-physical ablation. In Fig. 1.4 thermal, non-
thermal and photo-physical ablation processes are graphically represented. When high
power laser beam strikes any sample, material is ablated by forming plasma plume,
which expands perpendicular to the surface of target. The expansion of the plasma
depends on the initial ablated mass and energy in the plume. Initially, plume expansion
is adiabatic; afterward irradiation and collisional process become responsible for the
energy loss. Finally condensation takes place during the decay process of the laser
produced plasma.
9
1.3 CONDITIONS FOR LASER PLASMA DIAGNOSTICS
To use laser plasma as an analytical technique for qualitative and quantitative
analysis, it is necessary that the plasma should be optically thin and in local
thermodynamical equilibrium (LTE). If plasma doses not fulfill these conditions then
there may be saturation in the optical spectrum or self-absorption is contributing.
When radiation are emitted from the plasma without being significantly absorbed or
scattered, the plasma is said to be optically thin and the spectral line intensity of such
line is given by (Cremers 2006; Noll, 2012).
𝐼𝑢𝑣 =1
4𝜋𝑁𝑢𝐴𝑢𝑣ℎ𝑣𝑢𝑣𝐺 1.3
Where, 𝐴𝑢𝑣 is the spontaneous emission coefficient, 𝑁𝑢 is the population of the
state, ℎ is the plank constant and 𝐺 is the instrumental factor of the system. It is
essential to confirm that the plasma is optically thin for the lines used for estimation
plasma temperature. The self-absorption effect depends on degeneracy, oscillator
strength of the energy levels, as well as the plasma parameters such as plasma
temperature, electron number density and densities of different species present in the
plasma (Harilal et al., 2005; Aguilera et al., 2004). It is also discussed by Argaon et
al., (2008) and summarize as:
a) For the multiple lines of an element in which lower or upper terms have a
single level, the line intensity ratios of such lines should be in accordance
with the statistical weight ratio (Chen et al., 2012; Adamson et al., 2007;
Sabsabi and Cielo, 1995; Simeonsson and miziolek, 1993; Radziemski et al.,
1983)
10
b) The integrated line intensity ratio of two atomic spectral lines having the same
upper energy level should be equivalent to the branching ratio (Hegazy et al.,
2010; Hegazy et al., 2010)
𝐼1
𝐼2=
𝐴1𝑔1𝜆1
𝐴2𝑔2𝜆2 1.4
c) The optical depth of the plasma should be much lower than 1 such as:
𝑘(𝜆0)𝐷(𝑐𝑚) ≪ 1. Here 𝑘(𝜆0) is the absorption coefficient of the material and
𝐷(𝑐𝑚) is the thickness of the plasma (Adamson et al., 2007; Colon et al.,
1993).
d) The value of self-absorption coefficient 𝑆𝐴 = (∆𝜆
2𝜔𝑠𝑛𝑒)
1𝛼⁄ should be very close
to 1, here ∆𝜆 is the experimental Stark width of the line profile, 𝜔𝑠 is the Stark
broadening parameter of the line, 𝑛𝑒 is the number density and 𝛼 is a constant
equal to -0.54 (Cristoforetti et al., 2010; Sherbini et al., 2005)
e) The COG of the line should be a straight line (Aragon et al., 2010; Gornushkin
et al., 1999)
f) The line selection should be made according to the following criteria to avoid
the effects of self-absorption (Sabsabi and Cielo, 1995; Simeonsson and
miziolek, 1993).
g) Avoid the resonance lines and lines having the lower level below 6000 cm-1
.
h) Spectral lines having low transition probabilities should also be excluded.
i) Avoid the high intensity lines because those lines have high transition
probability to overestimate the population.
11
An important method to examine the optically thin condition is the linearity of
the Boltzmann plot (Cristoforetti et al., 2010). A second way to check the condition is
to compare the intensities ratios of any two lines of same charge state with the
theoretical one (Cristoforetti et al., 2010) as.
𝐼1
𝐼2=
𝜆𝑛𝑚
𝜆𝑘𝑖
𝐴𝑘𝑖
𝐴𝑛𝑚
𝑔𝑘
𝑔𝑛exp [
𝐸𝑛−𝐸𝑘
𝑘𝐵 𝑇] 1.5
where I1 is the intensity of a line at wavelength λki due to a transition from an upper
level k to a lower level i, Aki is the corresponding transition probability, gk and Ek are
the statistical weight and the energy of the upper level respectively and I2 is the
intensity of the line at wavelength λnm due to a transition from an upper level n to a
lower level m, Anm is the corresponding transition probability, gn and En are the
statistical weight and the energy of the upper level respectively, kB is the Boltzmann
constant and T is the plasma temperature. If a laser produced plasma follows
thermodynamical equilibrium then the velocity of all types of particles should hold
Maxwellian distribution, the population of all energy levels follow Boltzmann law
distribution, ionization equilibrium follow Saha equation, intensity of emitted
radiations is described by the Planck’s equation and all the processes should possess a
unique temperature (Cremers 2006).
When the atoms de-excited by some process of collisions instead of any radiative
process (i.e. when collisions dominate the radiative process), we speak of Local
Thermodynamic Equilibrium. Low-lying energy levels appear to have high values of
Einstein coefficients for spontaneous emission. This means that these energy levels are
quickly depopulated as compared to the high energy levels. Hence these levels are
12
more likely to suffer from radiative disequilibrium. By excluding the transitions among
such levels, one can assure the existence of LTE.
Complete thermodynamic equilibrium exists when all kinds of distributions are
defined at the same temperature T. If complete LTE established, the Principle of
detailed balance which requires each process to be balanced by its inverse must hold.
One of the criteria reported by McWhirter is considered as a proof of the existence of
LTE. It estimates a critical density to ensure LTE (Noll, 2012; Cristoforetti et al., 2010;
Cramers, 2006; Griem, 1997) as.
𝑛𝑒 ≥ 1.6 × 1012 𝑇1
2 (∆𝐸)3 1.6
Where, T (K) is the plasma temperature and ∆E (eV) is the maximum energy
difference between the upper and lower energy level. In addition, to the above criteria,
the condition of the validity of LTE in inhomogeneous plasma should also be
performed. The above condition is necessary but not sufficient to declare that the
plasma follows LTE. Therefore, the condition of LTE in inhomogeneous plasma
should also be validated (Cristoforetti et al., 2013; Cristoforetti et al., 2010). For this
criterion, the characteristic variation length in the plasma “d” must be much larger
than 10Dλ i-e 10𝐷𝜆 ≪ 𝑑. The diffusion length 𝐷𝜆 can be calculated using the
following equation (Cristoforetti et al., 2013; Cristoforetti et al., 2010; Cramers, 2006).
𝐷𝜆 ≈ 1.4 × 1012 × ((𝑘𝐵𝑇)
34⁄
𝑛𝑒) × (
∆𝐸
𝑀𝐴𝑓12(��))
12⁄ × 𝑒
∆𝐸2𝑘𝐵𝑇⁄
1.7
Where, 𝑘𝐵 is Boltzmann constant, 𝑇 is plasma temperature, 𝑛𝑒 is the number density,
𝑀𝐴 is the atomic mass of the species, g is the gaunt factor. This criterion should
necessarily be fulfilled in order to ensure LTE.
13
1.4 PLASMA DIAGNOSTICS
Plasma is a rich source of electromagnetic radiations ranging from IR to X-
rays; these radiations are emitted by the excited atoms of the plasma. Various
techniques have been exploited for the diagnostics of the radiation emitting from the
plasma. Diagnostic techniques include optical, electrical diagnostic and diagnostics
using solid-state detectors. Optical diagnostics gives information about the
characteristics of plasma i.e plasma temperature and electron number density.
However, electrical diagnostic provide information about the electron and ion
emissions from the plasma. For taking information about the energy distribution of
particles emitting from plasma, solid-state detectors are used. Due to the fast process
of plasma formation and its short lifetime; all the optical diagnostic techniques are not
fruitful for analysis. Most commonly, the laser-induced emission spectroscopy is used
for the optical diagnostics. Radiation emitted from plasma is registered by a
spectrometer of the range of electromagnetic spectrum by a charge-coupled device
(CCD) detector or with intensified CCD for even better time resolved results. The
obtained spectrum is a combination of characteristic discrete emissions as well as of
continuum emissions. These spectra carry information about the plasma environment.
Different characteristics of the obtained spectrum can be utilized to obtain specific
information about the plasma. For example, Stark broadening of the emission line is
related to the number density of the plasma, the line height or integrated line intensity
of the emission line is proportional to the quantity of the emitter, Doppler broadening
of the line tells about the velocity of the emitting particle in the plasma while the ratio
14
between the intensity of the emission lines and the continuum can provide information
about the temperature of the plasma (Cramers, 2006).
1.5 PLASMA TEMPERATURE
From the optical emission spectrum of plasma, the plasma temperature can be
determined by several spectroscopic methods including the intensity ratio method,
Boltzmann plot method and Saha Boltzmann plot method etc. One method may be
more suitable than others under specific conditions. After a few microseconds of
plasma formation, the line intensities dominate in the spectrum; in such a situation, the
intensity ratio method , Boltzmann plot method or Saha Boltzmann Plot method have
been employed for the estimation of plasma temperature (Cramers, 2006). These
methods are described in the following section.
1.5.1 Intensity Ratio Method
Assuming that the plasma is in local thermodynamic equilibrium (LTE), the
plasma temperature can be calculated through the intensity ratio of a pair of spectral
lines of atom or ion of same ionization stage. If the population in the excited state the
Boltzmann distribution law, then the integrated line intensity of a transition ( j → i )
can be represented as (Cramers, 2006)
𝐼𝑖𝑗 = 𝑛𝑖𝑠𝐴𝑖𝑗 1.8
Where, nis represents the population density of “s” element in level ‘i’ given as
𝑛𝑖𝑠 =
𝑔𝑖
𝑈𝑠(𝑇)𝑛𝑠𝑒
𝐸𝑖𝑘𝐵𝑇 1.9
Therefore, intensity Iij can be written as
15
𝐼𝑖𝑗 =𝐴𝑖𝑗𝑔𝑖
𝜆𝑖𝑗𝑈𝑠(𝑇)𝑛𝑠𝑒
𝐸𝑖𝑘𝐵𝑇 1.10
Where, 𝑔𝑖 is the statistical weight of the level ‘𝑖 , 𝐴𝑖𝑗 is the transition probability of
transition 𝑖 − 𝑗, 𝑛𝑠 is the total number density of an element, 𝐸𝑖 is energies of the upper
level, 𝑘𝐵 is the Boltzmann constant, T is the plasma temperature and 𝑈(𝑇) is the
partition function of the species “s”.
Now, consider another emission line of the same element with different transition from
“m” to “n” i-e having different upper and lower energy levels. The plasma temperature
can be calculated by taking the intensity ratio of these two spectral lines and
simplifying as follows (Noll, 2012; Cremers 2006; Griem, 2006; Chaudhary et al.,
2016)
𝑇 = 𝐸𝑖 − 𝐸𝑚 [𝑘𝐵𝑙𝑛 (𝐼𝑛𝑚𝐴𝑖𝑗𝑔𝑖𝜆𝑛𝑚
𝐼𝑖𝑗𝐴𝑛𝑚𝑔𝑚𝜆𝑖𝑗)]
−1
1.11
As the response of a detector remain approximately the same when we consider
wavelengths as close as possible. Therefore, it is better to choose spectral lines having
different upper-level energies but close in wavelengths. If we chose different
wavelength regions it will limit the device response and can cause variation in the
measurement of intensities of the lines.
1.5.2 Boltzmann Plot Method
Boltzmann plot is the most reliable method for the calculation of plasma
temperature. The emission intensity of a spectral line is can be written as:
𝐼𝑖𝑗 =ℎ𝑐
4𝜋
𝐴𝑖𝑗𝑔𝑖
𝜆𝑖𝑗𝑈(𝑇)𝑛𝑒
−𝐸𝑖
𝑘𝐵𝑇⁄ 1.12
16
Where, ℎ, is the Plank’s constant, 𝑐 is the speed of light, 𝑘𝐵 is the Boltzmann constant,
𝑇 is plasma temperature, 𝑈(𝑇) is the partition function, 𝐴𝑖𝑗 is the transition
probability, 𝑔𝑖 is the degeneracy of the upper level, 𝐸𝑖 is the upper-level energy, 𝜆𝑖𝑗 is
the emission wavelength and 𝑛 is the total population density of the emitting species,
respectively. Taking logarithm and re-arranging the Eq. 1.12 we obtain
𝐿𝑁 (𝐼𝑖𝑗𝜆
𝐴𝑖𝑗𝑔𝑖) = −
𝐸𝑖
𝑘𝐵𝑇+ 𝐿𝑁(
ℎ𝑐𝑛
4𝜋𝑈(𝑇)) 1.13
This is a straight line equation and the slope of this line is equal to −1
𝑘𝐵𝑇 . From the
slope, plasma temperature ‘T’ can easily be estimated (Noll, 2012; Cremers 2006;
Griem, 2006). The Boltzmann plot method is more reliable and more precise because it
uses several lines, which averages out uncertainties involved in the measurements. The
intensity ratio method makes use of only pair of emission lines (Griem, 2006). The
value of plasma temperature depends upon laser–matter interaction, characteristics of
the ambient environment and laser energy. However, it shows exponential decay with
time (Griem, 2006).
1.5.3 Saha Boltzmann Plot Method
As populations of different excited levels obey the Boltzmann distribution law
therefore, the emissivity of a particular transition of the species at a given position of
plasma can be expressed as (Cristoforetti et al., 2010; Griem, 2006):
𝜀𝑛𝑚 =ℎ𝑐
𝜆𝑛𝑚𝐴𝑛𝑚𝑔𝑛 𝑁
exp (−𝐸𝑛
𝑘𝐵𝑇)⁄
𝑈(𝑇) 1.14
17
Experimentally the emissivity 𝜀𝑛𝑚 is replaced by the line intensity (Inm). The
population distribution of two successive ionization stages of the same element can be
explained by Saha–Eggert distribution law (Cristoforetti et al., 2010) as
𝑛𝑒𝑛𝑧+1
𝑛𝑧 =2𝑈𝑧+1(𝑇)
𝑈𝑧(𝑇)(
𝑚𝑒𝑘𝑇
2𝜋ℏ2 )3
2⁄ exp (−𝐸∞ − ∆𝐸
𝑘𝐵𝑇) 1.15
Where 𝑛𝑒 represents the electron number density, 𝑛𝑧 is the number density of neutral
atoms and 𝑛𝑧+1 is the density of the ionized atoms, 𝑚𝑒 is the mass of electron, 𝐸∞ is
the first ionization energy of an isolated system, ∆𝐸 is the correction of 𝐸∞ for
interactions in the plasma and is equal to ∆𝐸 = 3𝑧𝑒2
4𝜋𝜖(
4𝜋𝑛𝑒
3)
13⁄ (Harilal et al., 1997).
Combing above equations and considering neutral and ionization stages of the atoms
we can get Saha-Boltzmann two line equation as (Samek et al., 2000; Yalcin el al.,
1999):
𝐼𝑍+1
𝐼𝑧= 2
(2𝑚𝑒𝐾𝑇)3
2⁄
𝑛𝑒ℎ3 (𝐴𝑔
𝜆)
𝑧+1(
𝜆
𝐴𝑔)
𝑧exp (−
𝐸𝑖𝑜𝑛+𝐸𝑧+1+𝐸𝑧
(𝑘𝐵𝑇)𝑧+1) 1.16
Where 𝐸𝑖𝑜𝑛 the ionization energy of the atom, 𝐸𝑧+1is the excitation energy of the ionic
line and 𝐸𝑧 is the excitation energy of the neutral line and 𝑇 is the ionization
temperature. Detalle et al., (2001) used a method to calculate the plasma temperature
by varying the value of ionization temperature until the calculated value of the number
density 𝑛𝑒 becomes equal to the experimentally measured number density 𝑛𝑒 with
about 1% uncertainty. The Saha–Boltzmann equation can be obtained by combining
Eqs. 1.14 and 1.15 which yields (Cremers 2006; Aguilera et al., 2004; Griem, 1997):
𝐿𝑁 (𝐼𝑛𝑚𝜆
𝐴𝑛𝑚𝑔𝑛)
∗
= −1
𝐾𝑇𝐸𝑛
𝑧∗ + 𝐿𝑁(ℎ𝑐𝑁0
4𝜋𝑈0(𝑇)) 1.17
18
Where superscript 0 stands for the neutral atoms and the terms having superscript '*' can
be expressed as:
𝐿𝑁 (𝐼𝒏𝒎𝜆
𝐴𝒏𝒎𝑔𝒏)
∗
= 𝐿𝑁 (𝐼𝒏𝒎𝜆
𝐴𝒏𝒎𝑔𝒏) − 𝑧𝐿𝑁[2 (
𝑚𝑒𝑘𝑇
2𝜋ℏ2)
𝟑
𝟐
𝑻𝟑𝟐
𝒏𝒆] 1.18
𝐸𝑛𝑧∗ = 𝐸𝑛
𝑧 + ∑ (𝐸∞𝑘 −𝑧−1
𝑘=0 ∆𝐸∞𝑘 ) 1.19
From the above equation it is clear that the ionization energy is added to the excitation
energy, thus the term 𝐸𝑛𝑧 has an even broader range as compared to the Boltzmann
plot. The newly added term 𝑧𝐿𝑁[2 (𝑚𝑒𝑘𝑇
2𝜋ℏ2 )
3
2
𝑇32
𝑛𝑒] depends on the temperature deduced
from the plot. An iterative procedure is applied (Aguilera et al., 2004) to get a more
accurate temperature as compared to the plasma temperature deduced by the
Boltzmann plot method. Initially, the data is plotted irrespective of the newly added
term and a starting temperature value is obtained. After deducting the initial value of
the plasma temperature this value is introduced into the term and a new plot provides a
new temperature. This procedure is repeated until the convergence value of the plasma
temperature is obtained.
1.6 ELECTRON NUMBER DENSITY (ne)
Electron number density in the plasma can be calculated using the Stark broadening of
a spectral line or through the intensity ratio of two different emission lines of the same
element by using Saha Boltzmann Equation.
1.6.1 Electron Number Density Using Stark Broadening Method
Utilizing the Stark-broadening parameter for the measurement of the electron
number density is considered as a more reliable method. The broadening of a spectral
19
line due to the Stark effect is a direct significance of the presence of charged particles
around the emitter. For estimation of electron number density using this method, we
make use of the Stark broadening of an emission line. The emission lines are normally
broadened by a combination of major three broadening mechanisms including natural
broadening, the Doppler broadening and the Stark broadening. The contribution of
Doppler broadening is due to the thermal motion of the emitter and the Stark
broadening is due to the splitting of energy level because of the electric field strength
of charged particles near the emitter. The Doppler broadening becomes more
prominent at high plasma temperatures, whereas the Stark broadening dominates at
high densities of charged particles in the plasma, which is also called as collision or
pressure broadening. Natural broadening is related to the uncertainty in the energy of
an excited state ‘ΔE’ for a limited excitation time ‘Δt’ of an electron through
Heisenberg’s uncertainty principle as: ~∆𝐸. ∆𝑇 ≅ ℏ.
Doppler line broadening appears, as a result of thermal motion of the emitter along the
direction of observation. The variation in the wavelength is explained on the basis of
Doppler Effect. If movement of the emitter is towards the detector, a slightly shorter
wavelength is recorded, and if the movement is away from the detector, a slightly
longer wavelength is observed by the detector. Consequently, a broader emission line
with a Gaussian profile is observed. For an emitter of atomic mass m, Doppler
broadening ΔλD of an emission line at wavelength λ for a particular electron
temperature T can be calculated as
𝛥𝜆𝐷 = 2𝜆√2𝑘𝑇𝑙𝑛2
𝑚𝑐2 1.20
20
In addition to the above-described broadenings, the emission line is superimposed by
another broadening contributed by the spectrometer itself that is referred as
instrumental broadening. It can be determined by using a narrow line laser beam.
Typically, Doppler and Stark are the main competing broadening mechanisms. Stark
broadening of the well isolated line can be used for the estimation of electron number
density. An estimation of the full width at half maximum ∆𝜆1/2(𝑠𝑡𝑎𝑟𝑘) is given by
(Cremers 2006; 2004; Griem, 1997) as.
∆𝜆1/2(𝑠𝑡𝑎𝑟𝑘) = 2𝜔 (𝑛𝑒
1016) + 3.5𝐴 (𝑛𝑒
1016)1/4
[1 −3
4𝑛𝐷
−1/3]𝜔 (𝑛𝑒
1016) 1.21
Where, 𝜔(𝑛𝑚) is the electron impact width parameter, A (nm) is the ion broadening
parameter, 𝑛𝑒 (𝑐𝑚−3) is the electron number density and 𝑛𝐷 is the number of the
particles in the Debye sphere. The first term represents the broadening due to electron
contribution whereas; the second term is the ion broadening. The observed line profile
can be fitted with the Voigt function, which takes into account instrumental width,
Doppler width and Stark broadening. The FWHM is deduced using the relation
(Cremers 2006; 2004; Griem, 1997;)
∆𝜆𝐹𝑊𝐻𝑀 =𝑊𝐿
2+ √( 𝑊𝐺)2 +
𝑊𝐿
2 1.22
Where, WG and WL are the Gaussian and Lorentzian contributions. Stark broadening is
directly linked with the electron density through electron impact parameter as 𝜔𝐹𝑊𝐻𝑀
by the following relation:
𝑛𝑒(𝑐𝑚−3) =∆𝜆𝐹𝑊𝐻𝑀
2𝜔𝑠(𝜆,𝑇𝑒)× 𝑁𝑟 1.23
21
Here, ∆𝜆𝐹𝑊𝐻𝑀 is the Stark contribution to the total line profile, 𝜔𝑠(𝜆, 𝑇𝑒) is the Stark
broadening parameter which is slightly wavelength and temperature dependent and its
values are available in the literature, Nr is the reference electron density which is equal
to 1016
(cm-3
) for the neutral atomic line and 1017
(cm-3
) for the ionized one (Griem,
1997). The Stark line widths ∆𝜆𝐹𝑊𝐻𝑀 have been determined by deconvoluting the
observed line profiles as a Voigt profile. The electron number density in plasma
depends on number of experimental parameters such as laser energy, background gas,
ambient pressure and characteristics of the target. However, it represents a temporal
profile that follows an exponential decay as a function of plasma lifetime (Hegazy et
al., 2014; Griem, 1997).
Electron number density can also be calculated using the line profile of the Hα
line of hydrogen. For this purpose, we calculated the full width at half area FWHA
using numerical integration; it is the distance between the points that give areas
between 1/4 and 3/4 of the total area (Praher et al., 2010; Gigososa et al., 2003). The
electron density is calculated using the relation (Gigososa et al., 2003; Cremers 2006).
𝐹𝑊𝐻𝐴 = 0.549 𝑛𝑚 × (𝑛𝑒
1023𝑚−3)0.67965 1.24
1.6.2 Electron Number Density using Saha-Boltzmann Relation
The Saha-Boltzmann equation relates the number density of a particular element in the
two consecutive charged states Z and Z+1 (Unnikrishnan el. al., 2012; Tognoni et al.,
2007).
𝑛𝑒𝑛𝛼,𝑧+1
𝑛𝛼,𝑧= 6.04 ∗ 1021(𝑇𝑒𝑉)
3
2𝑈𝛼,𝑧+1
𝑈𝛼,𝑧exp [−
𝜒𝛼,𝑧
𝑘𝐵𝑇] 1.25
22
where, ne (cm-3
) is the electron density, nα,z+1
is the density of atoms in the upper
charged state z+1 of the element α, nα,z
is the density of atoms in the lower charged
state z of the same element α, χα,z (eV) is the ionization energy of the element α in the
charged state z, Uα,z+1 and Uα,z are the partition functions of the upper charged state
z+1 and of the lower charged state z respectively whereas T(eV ) is the plasma
temperature in electron volt. The Eq. 1.25 can also be written in terms of intensities of
the atomic and ionic lines as (Unnikrishnan el. al., 2012)
𝑛𝑒 = 6.04 × 1021 ∗Ὶ𝑧
Ὶ𝑧+1(𝑇𝑒𝑉)
3
2 exp [−𝐸𝑘,𝛼,𝑧+1+𝐸𝑘,𝛼,𝑧−𝜒𝛼,𝑧
𝑘𝐵 𝑇] 1.26
where, Ek,α,z is the upper level energy of the element α in the charged state z, Ek,α,z+1 is
the upper level energy of the element α in the charged state z+1and Ὶ𝑧 =𝜆𝑘𝑖𝐼𝑘𝑖
𝐴𝑘𝑖𝑔𝑘.
1.7 APPLICATIONS OF LASER PRODUCED PLASMA
LIBS is an analytical detection technique and it has attracted much attention in
various industries for the compositional analysis because of its fast-response,
noncontact, and multidimensional features (Cremers, 2006; Davis, 1999). With the
development of laser and fast detection systems, this technique has been successfully
applied in various fields, including metallurgy, food, human, Mars and combustion.
Many applications have been successfully demonstrated such as monitoring of plant
control factors. Laser induced breakdown spectroscopy has been used in different
industries such as iron and steel, thermal power and waste disposal industry.
Environmental monitoring and safety applications have also been studied using this
23
technique. The merits and demerits of Laser Induced Breakdown Spectroscopy for
elemental determination compared to traditional techniques are presented as:
1. The ability of laser to evaporate and excite any type of a solid in a single step
without any sample preparation.
2. This technique can be applied on any type of material solid, liquid or gas
irrespective of their conduction.
3. It is a non-destructive technique; nominal amount of the material is evaporated.
4. We can get multi elemental analysis of any material including super hard
materials such as ceramics, glasses and superconductors.
5. This technique has no waste, no pollution, no explosion and no fire.
6. There is no need for extraction or any chemical treatment
7. This technique has very good resolving power and micro regions can also be
easily analyzed.
8. Using this technique, multi elemental samples can be analyzed easily.
9. LIBS analysis is quick and simple.
10. Using fiber optics, remote sensing can be achieved.
11. Samples can be analyzed in ambient environment.
12. Under water analysis is also possible.
13. There are some demerits of this technique such as the system used in LIBS
analysis is costly, required safety measurements, detection limit and precision
is lower then the conventional techniques.
24
1.8 MASS SPECTROSCOPY
Mass spectrometry (MS) is an analytical technique that ionizes atoms and
separates those ions on the basis of their mass-to-charge ratio. Mass spectrometry is
used in many fields and is applied to pure samples as well as on complex mixtures. A
typical mass spectrum is a plot of the ions signal as a function of the m/z ratio. These
spectra are used to determine the elemental or isotopic signature of a sample, the
masses of particles and of molecules, and to elucidate the chemical structures of
molecules, such as peptides and other chemical compounds.
Mass spectrometry has progressed extremely rapidly during the last two
decade, especially between 1995 and 2005. In a typical mass spectrometer, three
components are essential to perform mass analysis: (i) ion source; (ii) mass analyzer;
and (iii) ion detector. The performance of all the components reflects the quality of the
mass spectrum. It must be emphasized that generally these three components are
spatially separated to get the ionization and mass analysis separated in time
(Brinckerhoff et al., 2000; Stuke et al., 1996).
1.8.1 Principle
The first step in the mass analysis of any type of the sample is to produce the
ions of the sample by ionization. The Ions are separated in the mass spectrometer
according to their mass-to-charge ratios and their relative abundances. A typical Mass
spectrum is a plot of ion abundance versus mass-to-charge ratio. In the spectrum of a
pure compound, the molecular ion, if present, appears at the highest value of m/z and
gives the molecular mass of the compound.
25
1.9 LINEAR TIME OF FLIGHT MASS SPECTROMETER
Linear time of flight mass spectrometers consists of extraction/ionization
region, drift region and the detector. In the source region electric fields (E=V/s) are
usually defined by the applied voltages, these fields are used to accelerate the ions to a
constant energy. Drift region is field free and bounded by the extraction/ionization
grid. A schematic diagram of a linear time of flight mass spectrometer is shown in Fig
1.5.
Figure 1.5: Schematic diagram of single stage Linear Time of Mass spectrometer
Ions are formed in the source region and then accelerated through the
extraction region to the final kinetic energy. Ions cross the drift region with their
velocities are inversely proportional to the square root of their masses, thus lighter ions
have higher velocity and reach at the detector sooner as compared to the heavier once
(Demtroder, 2010; Stuke et al., 1996) as:
1
2𝑚𝑣2 = 𝑒𝑉 1.27
In the drift region, the velocities of the ions and their flight time can be obtained by
modifying the Eq. 1.27 as:
+
+
s D d
+
+
Es E = 0 Ed
eV+U0
eV
26
𝑣 = √2𝑒𝑉
𝑚 1.28
𝑇𝑂𝐹 = (𝑚
2𝑒𝑉)
12⁄ × 𝐷 1.29
From Eq. 1.29 it is clear that the flight time of the ions is proportional to the square
root of their masses. If the ions formed at some distance s between the extraction grid
and backing plate, those ions will spend short time in the source region. Following
problems affect the resolution of the linear time of flight mass spectrometers
(Demtroder, 2010):
1. The ions formed in extraction region gain different kinetic energies e. g some ions
born in the extraction region with some value of initial kinetic energies as shown in
Fig. 1.5. The actual flight time of these ions in drift region is represented
𝑇𝑂𝐹 = (𝑚
2𝐾𝐸)
12⁄ 𝐷 1.30
Where, K.E = eV + U0. Ions with initial K.E arrive sooner than the ions with no
initial K.E, resulting in tailing of mass spectra peak towards low mass side as
shown in Fig. 1.6.
2. Initial velocity of some ions directed away from the source exit. At the initial stage
acceleration of these ions turned around and exit the source with the same energy
(Uo+eV) as those, which initially moving in forward direction. These ions have
higher velocities and shorter flight times in drift region, but exit the source later
time known as turned around time. It contributes tailing in the mass spectra
towards higher mass as shown in Fig. 1.6.
27
Figure 1.6: Tailing effect in time of flight mass spectrum (TOF-MS).
These tailing effects in the mass spectra can be removed using higher acceleration
voltages, einzel lenses and long flight tubes.
1.10 CALIBRATING OF THE MASS SPECTRUM
It is clear that the mass scale follows a square root law regardless of the relative
size of the extraction and acceleration regions or other accelerating regions
(Demtroder, 2010) as.
𝑇𝑂𝐹 = 𝑎𝑚1
2⁄ + 𝑏 1.31
Thus, the mass spectra can be calibrated by measuring the flight time of two known
masses to determine the values of constants a and b. These constants take in to account
any time offset due to the laser interaction time, triggering of the recording system, etc.
Initial K.E effect
Due to different
direction of motion
28
1.10.1 Mass Resolution
In the mass spectra the mass resolution is defined as; 𝑚
∆𝑚 (Demtroder, 2010). In
a time of flight mass spectrometer the ions are accelerated to constant energy so the
mass resolution is calculated as; 𝑡
2∆𝑡 . Where ∆𝑡 is commonly measured as FWHM
(Full Width Half Maximum) of the peak (Demtroder, 2010). As mass resolution
depends on the time resolution and therefore, it also depends on the laser pulse widths,
detector response, recorder band widths and initial kinetic energies and velocities of
the ions. The basic resolution equation can be derive by rearranging the Eq. 1.29
𝑚 = (2𝑒𝑉
𝐷2 )𝑡2 1.32
As ions formed in the extraction/ionization region are accelerated to constant energy,
therefore by taking the derivative of the above equation one can get relation for mass
resolution for TOF-MS as.
𝑑𝑚 = (2𝑒𝑉
𝐷2 ) 2𝑡𝑑𝑡 1.33
𝑀𝑎𝑠𝑠 𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 = 𝑚
∆𝑚=
𝑡
2∆𝑡 1.34
1.11 AIM OF THE PRESENT WORK
The aim of this study is to design and fabricate a laser ablation time of flight
mass spectrometer (LA-TOF-MS) for the elemental analyses of the solid samples and
to compare the compositional results with different calibration free laser induced
breakdown spectroscopy (CF-LIBS) techniques. We have successfully fabricated an
improved version of a linear time of flight mass spectrometer which yields improved
mass resolution about 700(m/∆m). The problems related to the spatial distribution and
different directions of motion of the ions along the axis of the flight tube are
29
minimized by introducing multistage accelerating voltages and by inserting a magnetic
lens of about 1Tesla field strength after the extraction region. After improving the
resolution, the isotopes of lithium, cadmium and lead have been well resolved in
accordance to the natural abundances, reflecting the performance of our locally
developed system.
LA-TOF-MS has been used for the quantitative determination of constituents of
certified samples; different Karats of gold (18K, 19K, 20K, 22K, 24K), Brass alloy
(Cu 62%, Zn 38%) and Cu-Ni Alloy (75% Cu, 25% Ni) and some unknown
composition samples such as different brands of cigarettes available in Pakistan. Four
calibration-free CF-LIBS techniques including OL-CF-LIBS, SCF-LIBS, IRSAC-
LIBS and algorithm based AB-CF-LIBS techniques have also employed for
quantitative determination of constituents of these samples. The compositional result
obtained from different calibration free (CF-LIBS) techniques are in excellent
agreement with the results obtained from LA-TOF-MS. The analysis of different
industrially important alloys and different brands of cigarettes demonstrates that LIBS
complemented with LA-TOF-MS is a powerful technique for the elemental analysis of
the trace elements in any solid sample.
30
CHAPTER 2
REVIEW OF LITERATURE
Laser Induced Breakdown Spectroscopy (LIBS) is a multipurpose technique
that is being used in industries (Cremers, 2006), environmental diagnostics (Bassiotis
et al., 2001; Bulajic et al., 2001), and in biomedical research (Mowery et al., 2002;
Sing et al., 2001). Any material from the Periodic Table with any shape can be easily
analyzed using LIBS technique. As LIBS is a portable and non-distractive technique so
we can use this technique for compositional analysis (Noll, 2014; Cremers, 2006). Due
to these features, LIBS has advantage, over the other standard techniques. In this
technique a high-power pulsed laser is focused on the surface of a solid (Cramer, 2006;
Baig et al., 2012), liquid (Fichet et al.,2003; Charft et al., 2002; Noll, 2001), or on the
gaseous target (Hohreiter, 2005), to generate plasma. Elemental composition of any
material can be obtained from the emission spectrum of the laser produced plasma
(Hahn and Omenetto, 2012; Cremers, 2006). For the last couple of decades, LIBS has
been used for the qualitative and quantitative analysis (Winefordner et al., 2004; Cucci
et al., 1999) by the calibration curves (Galbacs et al., 2001) and also by the calibration
free methods (Unnikrishnan et al., 2012; Tognoni et al., 2010; Galbacs et al., 2001). In
the calibration curve method, reference samples are needed for drawing calibration
curves between the emission lines intensities versus known compositions. The
composition of the unknown samples is then estimated by comparing the emission line
intensity from the calibration curves (Unnikrishnan et al., 2012; Gupta et al., 2011;
Tognoni et al., 2010; Galbacs et al., 2001). Calibration curve LIBS (CC-LIBS)
31
technique is frequently used for accurate analysis of the samples with similar
compositions but this technique have some drawbacks such as matrix effect, self-
absorption, spectral overlap and other experimental uncertainties. The matrix effect
arises due to different electro negativity and ionization potentials between the
constituents of the sample, which results in the non-linearity of the spectral line
intensities as a function of elemental composition. However, in the calibration free
LIBS (CF-IBS) method, not any reference sample is needed (Aguilera et al., 2009;
Burakov et al., 2007). For the quantitative analysis of a sample, the plasma needs to be
optically thin (Unnikrishnan et al., 2010; Cremers 2006) and fulfills the local
thermodynamic equilibrium (LTE) condition (Cristoforetti et al., 2010).
2.1 DIFFERENT TECHNIQUES USED FOR COMPOSITIONAL
ANALYSIS
Different CF-LIB techniques have been employed for the quantitative analysis
including One Point Calibration Free LIBS, Self-Calibration Free LIBS, Self-
Absorption Correction CF-LIBS, and algorithm based CF-LIBS. Cucci et al.,(1999)
initially proposed the calibration-free approach for the quantitative analysis of
materials. Based on the Local Thermal Equilibrium (LTE), the CF-LIBS allowed
quantitative analysis of any material without any standards. Corsi et al., (2001)
demonstrated a new procedure for the accurate determination of precious alloys
compositions. Bulajic et al., (2002) delegated a new procedure for correcting the self-
absorption in calibration-free laser induced breakdown spectroscopy. Corsi et al.,
(2002) devolved a different approach for the coal and combustion products. Corsi et
32
al., (2003) devolved a calibration-free algorithm for the biological samples, Crosi et
al., (2004) performed three dimensional analysis of laser produced plasma to improve
the results of calibration-free LIBS technique. Lazic et al., (2005) used CF-LIBS
technique for quantitative determination of the constituents in the archaeological
material. Salle et al., (2006) compared different methodologies for quantitative
analysis of the rocks and the geological samples. Aguilera et al., (2009) used
calibration-free LIBS technique for the quantitative analysis of copper based alloys.
Bellagio et al., (2104) used CF-LIBS technique for quantitate determination of outer
space objects. Gaudiuso et al., (2010) presented a comprehensive review on the
quantitative analysis using LIBS for the environmental, cultural heritage and space
applications.
In recent years, several researchers are working on the use of CF-LIBS to
improve the trueness of LIBS analysis. Different approaches for Calibration-Free LIBS
analysis have been proposed, that are progressively abandoned the idea of a complete
analysis without calibration. Analysis of different copper base alloys was made to
determine the best approach for CF-LIBS and to show the advantages and limits of
different algorithms. Different groups have proposed variations in the CF-LIBS
algorithms to overcome the problems associated to poor knowledge of the spectral
parameters and partially compensate self-absorption effects (Cavalcanti et al., 2013).
Andrae et al., (2015) demonstrated the basic assumption of the One Point Calibration
Free (OPCF-LIBS) method provided that the laser-induced plasma is close to LTE. A
secondary assumption was the stoichiometric condition; so that the plasma chemical
composition is identical to the composition of the sample or the condition of optically
33
thin plasma (Cremers, 2006). The starting point of the one point calibration free
algorithm is the Boltzmann equation, which shows the dependence of the emission line
intensities on the elemental compositions of the corresponding species. Using one line
calibration free laser induced breakdown spectroscopy; only a single spectral line is
used to measure the elemental composition. Gaudiuso et al., (2012) proposed another
calibration-free inverse method for quantitative determination of copper base alloys.
The idea of this method was similar to that of OPCF-LIBS, but in this method the
information recovered from the analysis of the known sample is limited. In the
Boltzmann Plot method the Boltzmann plots are drawn for all the species present in the
sample. The intercepts of the Boltzmann plots are related to the species concentrations
and the proportionality constant is determined by normalizing the concentrations of all
species to unity. This method has been successfully applied for compositional analysis
of precious alloys (Gaudiuso et al., 2012; Corsi et al., 2001), gases and archeological
samples (Burakov et al., 2007) and for caratage analysis (Corsi et al., 2001). There are
some conditions in these methods; the emission lines should be free from self-
absorption and plasma should be optically thin and in LTE. If these conditions hold
than these methods can be used for compositional analysis (Tognono et al., 2007). To
minimize the error in the compositional results Sun et al., (2009) demonstrated a
slightly different technique, known as internal reference self-absorption correction
(IRSAC) for the quantitative analysis of the materials. In this technique, initially the
spectral lines are corrected for self-absorption using an internal standard line. Many
authors reported the self-absorption correction to improve the CF-LIBS results (Dong
et al., 2015; Sun et al., 2009). Another self-calibration method for the quantitative
34
analysis by LIBS was initially developed by Ciucci et al., (1999) in this method the
concentration is determined from the Boltzmann plots, the slope yields the plasma
temperature and the intercept is proportional to the compositions. The Boltzmann plot
method requires at least four to five optically thin spectral lines for the determination
of composition. As for the trace elements, it is not always possible to find four to five
optically thin spectral lines in the emission spectrum. Therefore, there are limitations
to use the Boltzmann plot method for the quantitative analysis of the trace elements in
the sample. To overcome this difficulty Goma et al.,(2001) developed a new CF-LIBS
technique in which concentration of the elements can be estimated by comparing the
theoretically obtained electron density and the ratio of the number densities of neutral
and singly ionized species of the same elements as well as of different elements with
the experimentally measured electron densities. However, accurate values of electron
density ne and temperature Te are important in CF-LIBS. The electron temperature is
mostly calculated using the Boltzmann plot method (Sherbini et al., 2012; Goma et
al.,2001; Borgia et al., 2000; Joseph et al., 1994) and electron number density ne can
be deduced from the Stark broadening of the spectral lines (Cremers 2006; Borgia et
al., 2000) or by the Saha-Boltzmann equation (Cremers 2006; Borgia et al., 2000;
Andrzej et al., 1946).
To validate the results obtained from different calibration-free LIBS (CF-LIBS)
techniques it is necessary to compare the LIBS results with some standard techniques
such as EDS, XRF, PIXE and LA-TOF-MS. EDS, XRF and PIXE have some
limitation; these technique do not give more accurate results for the elements having
low atomic numbers such as H, Li etc. The LA-TOF-MS technique is one of the best
35
alternate that can be used for the validation of the LIBS results. As laser ablation is an
efficient source of ion production and the consumed amount of the ablated target
material is also very nominal therefore, LIBS can easily be coupled with a
conventional time of flight mass spectrometer (TOF-MS) for the ionic mass analysis.
In this setup a single laser pulse is used to ablate the material (LIBS) and the
corresponding mass spectra are obtained with the Laser Ionization.
For the commercial analysis Stephens, (1946) proposed the utilization and
construction of ‘‘a pulsed mass spectrometer, using time dispersion for isotopic mass
and compositional analysis in a meeting of the American Physical Society at the
Massachusetts Institutes of Technology. Two years later, Cameron and Eggers, (1948)
working at the Oak Ridge Y-12 plant, reported the first TOF-MS that fulfilled the
proposal by Stephens. In this instrument the ions were accelerated to 300eV and
traveled down a 3 meter flight tube to the detector. Here the flight path distance is
taken about 317cm. The singly charged masses in their spectra were calculated. The
mass resolution was much poor, but the principle was demonstrated. This system was
capable to resolve only singly and multiply charged ions of mercury, but not their
isotopes. Wolff and Stephens, (1953) proposed that pulsed voltage used to accelerate
the ions should turn off before ions reached the full acceleration voltage. Under these
conditions, all ions acquire the same momentum instead of same energy as in the
conventional TO-FMS arrangement. In this instrument a ten-stage copper-beryllium
dynode electron multiplier was used. Here 100cm field free flight length was 300V
accelerating voltage was used. Subsequently, Smith, (1951) described another version
of the ‘‘magnetic period mass spectrometer in which three electrostatic lenses were
36
used to form and deflect pulses of ions. All lenses were grounded and a square wave
pulsed voltage was applied to the element. The development of a modern commercial
time of flight mass spectrometers began with the Wiley and McLaren, (1955) designed
TOF. New design did not improve the resolution of the magnetic TOF-MS but it
represented advancement in the non-magnetic TOF-MS. Here the electron beam of
finite spatial width was directed between the plates of the TOFMS. The ions created at
different positions within the spatial width of the electron beam fall through different
voltages, resulting in a spread in the time of arrival distribution. Wiley and McLaren,
(1955) attempted to partially correct this spatial dispersion and to improve the
resolution. A number of authors have also proposed higher order corrections to the
Wiley McLaren dual source space focusing conditions (Seccombe et al., 2001; Evan et
al., 2000; Eland et al., 1993). All of these authors discussed different methods for
improving Wiley and McLaren design. The introduction of additional ion grid in the
source region achieved higher order space focusing. The ultimate goal is to achieve the
focusing condition so that the ions of the same mass generated anywhere in the ion-
source region arrive simultaneously at the detector. In this design a third grid was
added to produce a second-order space focusing. Tabrizchi et. al., (2016) designed,
constructed and calibrated the modified version of linear time of flight mass
spectrometer. In this system the ions were generated by the laser ablation. The
ionization chamber consists of an accelerator and an ion lens to focus the ions into a
one meter linear flight tube mass analyzer. A Quartz window was used to enter the
laser beam in the ionization chamber and plasma was generated between the extraction
and ionization region. Mass spectra were investigated for the gas samples as well as
37
for the solid samples. The mass calibration was achieved by measuring the flight time
of the known alkali ions; Li+, Na
+, K
+, Cs
+ and Rb
+ ions. An average mass accuracy of
0.016% was reported and a mass resolution of 540 (𝑚∆𝑚⁄ ) was reported (Tabrizchi et.
al., 2016).
After improvement in the resolution of the commercial time of flight mass
spectrometers, this technique was used for isotopic mass analysis and elemental
compositional analysis of solid samples by many authors (Kohn et al., 2008;
Brinckerhoff et al., 2000; Sneddon et al., 1997; Beekman et al., 1996). As a time of
flight mass spectrometer is a simplest analyzing technique used for the separation of
ions therefore, this technique have also been used for optical diagnostics, bio-imaging
and compositional and elemental analysis of metallic targets (Jurowski et al., 2013;
Saleem et al., 2006; Labazan et al., 2005; Amoruso et al., 1996; Stuke et al., 1996;
Koumenis et al., 1995; Macler et al., 1994; Wang et al., 1991). Several analytical
techniques have also been used for the isotope analysis such as glow discharge mass
spectrometry (GD-MS), laser-ablation inductively coupled plasma mass spectrometry
(LA-ICP-MS), secondary ion mass spectrometry (SIMS) and laser ablation/ionization
time-of-flight mass spectrometry (LA-TOF-MS) (Demtroder, 2010; Stuke et al., 1996;
Koumenis et al., 1995).
38
CHAPTER 3
MATERIALS AND METHODS
Major part of this chapter has published in the journal, “Laser Physics”. This
article is also selected for the “2017 Highlights Collections” of the journal. In this
contribution author have successfully designed and fabricated a modified version of linear
(LA-TOF-MS.
3.1 LIBS EXPERIMENTAL SETUP
A Q-switched Nd:YAG (Quantel Brilliant) pulsed laser was used, to ablate the
targets, having 5ns pulse duration and 10 Hz repetition rate, capable of delivering
pulse energy about 850 mJ at 1064 nm and 500 mJ at 532 nm. The energy of the laser
pulse was varied by adjusting the flash lamp Q-switch delay. A quartz lenses (convex)
of 20 cm focal length was used to focus the laser beam on the target sample placed in
air at atmospheric pressure. The measured diameter of the focused laser beam was
about (0.10 ± 0.01) cm; the focal spot area was 7.85x10-3
cm2. The laser energy was
measured by an energy meter (Nova-Quantel, France). To prevent the formation of
deep craters, the sample was placed on a rotating stage for providing fresh surface of
the target to every laser shot. In order to prevent the air breakdown in front of the
sample, it was necessary to keep the distance between the lens and the sample less than
the focal length. An optical fiber (high – OH, core diameter about 600µm) was used to
collect the plasma radiation with a collimating lens (0-450 field of view) which was
placed normal to the laser beam. The emitted radiation was captured by a set of four
spectrometers (Avantes, Holand) each having 10 µm slit width and covering the
wavelength range of 250 - 870 nm. To correct the emission signal, the dark signal was
39
subtracted from the observed signal using the LIBS software. Schematic diagram of
LIBS setup used in the work is shown in Fig. 3.1.
Figure 3.1: Schematic diagram of LIBS setup
Different components of this system used in our experiment are mentioned below:
1. Q-switched Nd-YAG Laser (Brilliant, Quantel, France )
2. Focusing lens
3. Fiber optics cable
4. Avantes spectrometer
5. Computer system
3.1.1 Q-switched Nd-YAG Laser
Neodymium Yttrium Aluminum Garnet (Nd: YAG) laser is a four level solid
state laser and was first demonstrated by J.E. Geusic et. al., (1964). Flash lamp or
semiconductor laser was used for pumping the Nd: YAG laser. Basically it operates at
40
wavelength 1064 nm and shift from infrared to visible or ultraviolet due to the 2nd
or
3rd
harmonics. In the Nd: YAG laser Neodymium and Yttrium Aluminum Garnet
(Y3Al5O12) act as active and host medium respectively. The energy levels higher than
4F3/2 having the wavelength 730 nm and 800 nm are populated using optical pumping;
by the absorption from the ground level 4I9/2. The fast non-radiative decay occur from
the upper levels to the metastable state 4F3/2 which has a much longer life time (0.23
ms) and therefore has high population. Due to the stimulated emission, transitions
occur from the 4F3/2 to
4I1/2 called the strongest transition and light having wavelength
1064 nm is emitted. To maintain the population inversion between the 4F3/2 and
4I1/2
levels, the non-radiative decay occurs from the unstable 4F3/2 level to the ground level
4I9/2 (Cremers, 2006).
The efficiency of a Nd: YAG laser is ~ 1% and the beam profile is Gaussian.
Human eye can’t see the near infrared light having the wavelength 1064 nm therefore,
its second harmonic is commonly used which is in the green region (532nm). Safety
goggle which block the near infrared neodymium lines and transmit the visible light
must be used in the laboratory.
3.1.2 Focusing Lens
Focusing lenses of about 10, 15 and 20cm focal lengths are used to focus the
laser light on the target to produce plasma.
41
3.1.3 Fiber Optics
Optical fiber is used to transmit the required data which consists of a bundle of
threads made up of plastic and quartz. Its working principle is the total internal
reflection. It has the capability to transmit light of the plasma to the detecting system.
3.1.4 Avantes spectrometer
To record the optical emission spectrum of the samples, the optical fiber is
connected with the Avantes detection system which is a set of four spectrometers, each
containing 10 micrometer slit width. The optical resolution of the spectrometer is
about 0.06 nm and covering the wavelength region from 250 to 870 nm. The Avantes
detection system activates laser beam and a DG535 four channel digital delay/pulse
generator to harmonize the Q-switch of the Nd: YAG laser and Avantes detection
system. Using the DH-2000-CAL standard light source, all the installed spectrometers
in the Avantes system are manufacturer calibrated in efficiency.
3.2 FABRICATION OF LASER ABLATION TIME OF FLIGHT MASS
SPECTROMETER (LA-TOF-MS)
In Fig. 3.2 we present a schematic diagram of the experimental setup for the
laser ablation/ionization TOF-MS system developed in our laboratory which is based
on the Wiley and McLaren, (1955) type instrument. Three metallic electrodes are used
in this equipment having rectangular shape (3 cm x 3cm), two of the electrodes hold 1
cm openings in the center which are covered with the fine tungsten mesh. The mass
analysis system consists of a 30 cm diameter stainless steel vacuum chamber which
hosts the ionization region, extraction electrodes and 8 cm x 100 cm drift tube. The
42
entire system is coupled with a turbo molecular pump backed by a mechanical pump to
maintain vacuum at about 2×10-6
mbar during the experimentation.
Figure 3.2: A schematic diagram of the experimental setup of the Laser
ablation/ionization TOF-MS system.
In the ionization region, the deflection plates are installed and appropriate
voltages are applied to correct the flight path of the ions up to the detector, one meter
away from the ion extraction region. For the ablation of the target sample we have
used a Q-switched Nd:YAG Laser (Brilliant, Quantal, France), which is capable of
delivering pulse energy about 850 mJ at 1064 nm and 500 mJ at 532 nm, 5ns pulse
duration and 10 Hz repetition rate. The laser beam is focused by a quartz lens of 30 cm
focal length which is placed in front of the entrance window of the vacuum chamber
and the focused beam radius at the target surface was about 0.5 mm. The electrodes are
43
located perpendicularly to the target surface, and the ions are extracted from the laser
produced plasma perpendicular to the plume expansion axis. The generated ions are
detected by a channeltron electron multiplier (Galileo, USA) operating at about 2KV.
3.2.1 Design Parameters
A linear two stage time of flight mass spectrometer consists of a vacuum
chamber containing an ionization/extraction region ‘s’, an acceleration region ‘d’, a
field free drift region ‘D’ and an ion detector at the end of the field free region. The
ions are generated by laser ablation, which are accelerated twice and then enter in the
field free region where they are separated according to their mass to charge (m/q) ratio.
Finally accelerated ions are detected by a channeltron. Our designed TOF-MS operates
in pulsed mode, the ion production source is pulsed and the accelerating fields are
constant. The lighter ions arrive at the detector earlier than the heavier ones. In our
design 𝑠 = 2𝑐𝑚, 𝑠0 = 1𝑐𝑚, 𝑑 = 2𝑐𝑚 and the two extraction grids are fixed at
𝐸𝑠 ≅ 210𝑉/𝑐𝑚, 𝐸𝑑 ≅ 1500𝑉/𝑐𝑚. Space focusing is achieved for the ions which
have identical initial kinetic energies but formed at different locations in the source
region. Using a particular set of parameters, the space focusing is found around 100cm
away from the second extraction region and the tube length was set accordingly at that
distance. The space focus plan for dual stage (LA-TOF-MS) can be achieved using
following Eq. (Demtroder, 2010; Wiley and Mclaren, 1955)
𝐷 = 2𝜎3
2⁄ [1
𝑠01
2⁄−
2𝑑
𝑠01
2⁄ (𝑠01
2⁄ +𝜎1
2⁄ )2] 3.1
Where, = 𝑠0 +𝐸𝑑
𝐸𝑠𝑑 . The value of the 𝜎 for our design can be obtained as
44
𝜎 = 𝑠0 +𝐸𝑑
𝐸𝑠𝑑 = 1 +
1500
210× 2 ≈ 15.3 𝑐𝑚 3.2
Using the above value of 𝜎 = 15.4𝑐𝑚 the space focus plan is calculated using the
equation 3.1.
𝐷 = 2(15.3)3
2⁄ [1
(1)1
2⁄−
2(2)
(1)1
2⁄ ((1)1
2⁄ +(15.3)1
2⁄ )2] ≈ 100 𝑐𝑚 3.3
The initial kinetic energy focusing is achieved using the known time lag focusing. For
problem related to turn around time and different directions of the motions of the ions
we have inserted a Magnetic Lens (ML).
3.2.1 Space Focusing Parameters
When a high intensity laser beam is focused on any target material, a small fraction
of the material is ablated and a plasma plume is generated which mainly consists of
neutrals, ions and electrons. The ions formed in the ionization region can be
accelerated by applying an accelerating potential as (Demtroder, 2010):
1
2𝑚𝑣2 = 𝑍𝑒𝑈 3.4
A positive potential is applied across the extraction region which drifts the ions that are
then accelerated in a field free region as shown in Fig. 3.3. All the produced ions
possess kinetic energies in accordance with their charged stages. The velocities of
these ions are related to their masses accordingly:
𝑣 = (2𝑍𝑒𝑉
𝑚)
12⁄ 3.5
Where z is the charge state, m is the mass of the ion, v is the velocity of the ion, e is the
electron charge and V is the acceleration potential. Thus, the Time of flight of the ions
reaching the detector, one meter, is calculated using the relation:
45
T = (𝑚
2𝑍𝑒𝑉)
12⁄ × 𝐷 3.6
Here D is the drift length and T is the time of flight of the ion which is normally in
microseconds (Demtroder, 2010). The mass to charge ratio is then calculated using the
relation:
𝑚
𝑧= (
𝑇
𝐷)2 × 2𝑒𝑉 3.7
In Fig.3.3 we present a schematic diagram showing the production of ions in the
ionization region and cause the time lagging in the mass spectrum due to different
kinetic energies of the ions. Some of the ions are formed with initial K.E but their
actual flight time in the drift region is reduced. The total kinetic energy of an ion is
written as:
K.ETotle = eU = eV + U0, 3.8
Where, Uo correspond to the initial kinetic energy. The ions with energies eV + U0
arrive earlier than the ions having zero initial kinetic energy.
Figure 3.3: Schematic diagram of LA-TOF-M showing lagging in the mass spectrum
due to different initial kinetic energies.
Due to the difference in the kinetic energies and different directions of motion of the
ions they arrive at the detector at different times as shown in Fig. 3.3. Similarly the
46
ions having same m/z with different directions of motion also take different times to
reach at the detector.
To tackle these problems, we applied an appropriate accelerating voltage Vac such
that eV is now much larger then Uo and secondly we inserted a magnetic lens after the
extraction region. The magnetic lens is a permanent magnet, 5cm long, having about
1Tesla field strength. When a positively charged ion enters the uniform magnetic field
B, it experiences a force given by equation:
F=q ( v × B ) = 𝑞𝑣𝐵𝑠𝑖𝑛𝜃 3.9
The force experienced by the moving positively charged particles in a magnetic
field is perpendicular to v and B. The charged particles moving parallel (θ = 00) or
anti-parallel (θ = 1800) to the magnetic field will experience zero force and will
continue to move along the same direction with the same velocity as shown in Fig. 3.4.
Figure 3.4: A schematic diagram of the force experienced by the charged particle in
the magnetic field.
Any charged particle which enters the magnetic field at any angle (θ) will
experience a combined effect of the linear motion along the field and the circular
motion in a plane perpendicular to the field, a helical motion. Thus, the charged
particles which enter at any angle will scatter away as shown in Fig. 3.4. In this way
B
vcosθ
vsinθ v
+ θ
+
F = qvBsin0 =0
B
F = qvBsin0 =0 + v
v B θ F = qvBsinθ
+
F = qvBsinθ
47
we have got good space focusing conditions and better mass resolution. In Fig.3.5, we
show the mass spectrum of Pb 208 isotope. The FWHM is determined by the
Lorentzian fit as 0.30 which corresponds to the mass resolution about 700.
Figure 3.5: Lorentzian Fit of lead (208) for calculation of resolution.
3.3 METHODS FOR COMPOSITIONAL ANALYSIS
LIBS technique requires only the optical approach for the compositional analysis.
The spectra provide information about the elements evaporated from the sample.
Qualitative analysis can be performed by two traditional approaches.
(1) Calibration Curves method (CC-LIBS)
(2) Calibration-free methods (CF-LIBS)
The calibration curve method is based on drawing of the calibration curves. This
method requires a set of standards of the same kind and of similar composition to that
48
of under investigation. By using this technique, the compositions of major components
cannot be measured easily because of the different electro-negativities and ionization
potentials of the constituents which results in a non-linearity in the calibration curves.
The disadvantage of this method is that the samples with composition similar to the
unknown samples are required which is not possible in most of the cases. Due to these
reasons we have utilized different calibration free LIBS (CF-IBS) methods. The CF-
IBS approaches do not require any reference materials, these methods are based on the
measurement of accurate plasma parameters such as plasma temperature and electron
number density. For the accuracy of CF-LIBS, the selection of optically thin lines is
important. We have utilized the five calibration free CF-LIBS techniques for
compositional analysis and results are compared with the certified composition as well
as compositions obtained by LA-TOF-MS.
3.3.1 One Line Calibration Free LIBS (OL-CF-LIBS)
In this method the Boltzmann equation is utilized to get the composition of the
neutral species. This equation links the intensities of the emission lines emitted by the
same species as (Andrea et al., 2015; Tognoni et al., 2007; Griem, 1997).
𝐹𝐶𝑧 = 𝐼𝑘𝑈𝑧(𝑇)
𝐴𝑘𝑔𝑘𝑒
(𝐸𝑘
𝑘𝐵𝑇) 3.10
Where, F factor is related to the ablated mass (constant for constant efficiency of
spectral system), 𝐼𝑘 is the line intensity, 𝐶𝑧 is the concentration of neutral atom. The
factor F can be determined by normalizing the species concentration. An average value
of plasma temperature and electron number density is used in this method. At average
plasma temperature the partition functions are taken from the NIST database and the
49
concentration of neutral atoms Cz is calculated from above equation. If the ionic lines
are not present for all the elements then the concentration of the ionized atoms Cz+1 is
calculated using the Saha–Boltzmann equation, relating the concentrations in the two
consecutive charge states Z and Z + 1 of a particular element (Unnikrishna et al., 2012;
Giacomo et al., 2007; Ciucci et al., (1999):
𝑛𝑒𝐶 ,𝑧+1
𝐶𝑧 =(2𝑚𝑒𝑘𝐵𝑇)
32
ℎ3
2𝑈𝑧+1
𝑈𝑧exp [−
𝐸𝑖𝑜𝑛
𝑘𝐵𝑇] 𝑐𝑚−3 3.11
Eq. 3.11 gives the ratio of the concentration of two charge states Z and Z+1 of the
same element (𝐶𝑧+1
𝐶𝑧 ) (Unnikrishna et al., 2012): from where we can easily calculate the
value of Cz+1
by substituting the value of Cz obtained from equation 3.10.
Total concentration of 𝐶𝑎 and 𝐶𝑏 is presented as: 𝐶𝑡𝑎 = 𝐶𝑧
𝑎 + 𝐶𝑧+1𝑎 , 𝐶𝑡
𝑏 = 𝐶𝑧𝑏 + 𝐶𝑧+1
𝑏 .
To calculate the percentage compositions, we used the following relations:
𝐶𝑎% = 𝑛𝑡𝑜𝑡
𝑎 ∗(𝑎𝑡𝑜𝑚𝑖𝑐 𝑤𝑒𝑖𝑔ℎ𝑡)
𝑛𝑡𝑜𝑡𝑎 ∗(𝑎𝑡𝑜𝑚𝑖𝑐 𝑤𝑒𝑖𝑔ℎ𝑡)+ 𝑛𝑡𝑜𝑡
𝑏 ∗(𝑎𝑡𝑜𝑚𝑖𝑐 𝑤𝑒𝑖𝑔ℎ𝑡)∗ 100 3.12
𝐶𝑏% = 𝑛𝑡𝑜𝑡
𝑏 ∗(𝑎𝑡𝑜𝑚𝑖𝑐 𝑤𝑒𝑖𝑔ℎ𝑡)
𝑛𝑡𝑜𝑡𝑎 ∗(𝑎𝑡𝑜𝑚𝑖𝑐 𝑤𝑒𝑖𝑔ℎ𝑡)+ 𝑛𝑡𝑜𝑡
𝑏 ∗(𝑎𝑡𝑜𝑚𝑖𝑐 𝑤𝑒𝑖𝑔ℎ𝑡)∗ 100 3.13
3.3.2 Self-Calibration free LIBS (SCF-LIBS)
To calculate the composition using the self-calibration LIBS technique, initially the
plasma is checked for the optically thin and LTE. The Boltzmann distribution gives an
estimation of the population of the excited state as (Cremers 2006; Griem, 1997):
𝑛𝑘𝑠 = 𝑛𝑠 𝑔𝑘
𝑈(𝑇)exp [−
𝐸𝑘
𝑘𝐵 𝑇] 3.14
where 𝑛𝑘𝑠 is the population density of the excited level k of the species S, 𝑛𝑠 is the
total number density of the species S in the plasma, gk is the statistical weight of the
50
upper level of the transition, U(T) is the partition function of the species S at the
temperature T is defined as (Cremers 2006; Griem, 1997):
𝑈(𝑇) = ∑ 𝑔𝑘 exp [−𝐸𝑘
𝑘𝐵 𝑇]𝑘 3.15
The intensity of the emission line is proportional to the population of the excited level
as:
Iki = Akinks hc
λki 3.16
The above equation can be simplified as:
𝐼𝑘𝑖 = 𝐴𝑘𝑖𝑛𝑠 ℎ𝑐
𝜆𝑘𝑖
𝑔𝑘
𝑈(𝑇)exp [−
𝐸𝑘
𝑘𝐵𝑇] 3.17
The measured intensity is also affected by the efficiency of the collecting system,
therefore, the above equation can also be written as:
𝐼𝑘𝑖 = 𝐹𝐶𝑠𝐴𝑘𝑖ℎ𝑐
𝜆𝑘𝑖
𝑔𝑘
𝑈(𝑇)exp [−
𝐸𝑘
𝑘𝐵 𝑇] 3.18
Where, 𝐼𝑘𝑖 is the measured integrated line intensity, Cs is the concentration of the
emitting atomic species and F is an experimental factor which takes into account the
efficiency of the collection system. By taking the logarithm of the above equation:
ln [𝜆𝑘𝑖𝐼𝑘𝑖
ℎ𝑐𝐴𝑘𝑖𝑔𝑘] = −
𝐸𝑘
𝐾𝐵 𝑇+ ln [
𝐹𝐶𝑠
𝑈(𝑇)] 3.19
Comparing with a straight line equation; y = mx + qs
𝑦 = ln [𝜆𝑘𝑖𝐼𝑘𝑖
ℎ𝑐𝐴𝑘𝑖𝑔𝑘] ; 𝑥 = 𝐸𝑘 ; 𝑚 = −
1
𝐾𝐵 𝑇 ; 𝑞𝑠 = ln [
𝐹𝐶𝑠
𝑈(𝑇)] 3.20
The slope of the Boltzmann plot gives the plasma temperature and intercept gives the
composition of the species as:
𝐹𝐶𝑠 = 𝑈(𝑇)𝑒𝑞𝑠 3.21
The Boltzmann plots are drawn for each element separately. If the laser produced
plasma is close to LTE, then the slopes of the Boltzmann plots for each element should
be approximately same and the intercepts may differ according to the concentration of
that element in the sample. If the neutral and singly ionized spectral lines and their
51
spectroscopic parameters of each element are present, then the Boltzmann plot can be
drawn for each element estimating FCs for each element.
Finally the concentration for each element may be deduced as:
Cs =U(T)eqs
F 3.22
If it is not possible to draw Boltzmann plot for the neutral or ionized species of an
element, then the Saha-Boltzmann equation is utilized to calculate the missing FCs
values as in the case of OL-CF-LIBS technique. In short, to calculate the concentration
of the ionized species, the Saha-Boltzmann equation is used that relates the number
densities of the neutral as well as the singly ionized species (Gomba et al., 2001):
𝑛𝑧+1 α
𝑛𝑧α . 𝑛𝑒 = 6.04×10
21 𝑇𝑒𝑣3/2
𝑈𝑧+1𝛼
𝑈𝑧𝛼
exp (−
ᵡ𝑧𝛼
𝑇𝑒𝑣) cm
-3 3.23
Here, 𝑛𝑒 is the electron number density in the plasma, 𝑛𝑧+1α 𝑎𝑛𝑑 𝑛𝑧
α are the number
densities of the neutral and the singly ionized species respectively, T(eV) is the plasma
temperature, 𝑈𝑧+1𝛼 and 𝑈𝑧
𝛼 are the partition functions and ᵡ𝑧𝛼
is the ionization energy of
the species. From this equation, we determined the density of the ionized species
whereas the density of the neutral species is calculated from Eq.3.22 (see above), thus
the total concentration is represented as sum of the neutral and ionized concentrations
of each element (Cremers 2006): The instrumental factor ‘F’ can be deduced by
normalizing the sum of the concentration of all the species to unity. The total
concentration of an element is the sum of the concentration of both neutral and the
ionized species of that element as
Ctotals = CI
s + CIIs 3.24
Finally, percentage composition is calculated same as calculated in the above method
using Eq. 3.12 and 3.13.
52
3.3.3 Internal Reference Line Self Absorption Correction LIBS (IRSAC-LIBS)
The compositional analysis from the observed emission spectra is based on
Boltzmann Plots. However, the accuracy in the calculations of plasma temperatures
and intercepts is largely influenced by the self-absorption effect (Sun et al., 2009;
Sherbini et al., 2005). After incorporating the self-absorption corrections, the data
points in the Boltzmann Plot follow a more realistic linear trend and yields more
accurate plasma temperature and compositions (Sun et al., 2009). In this technique
initially the observed line intensities are corrected for the self-absorption. Sun et al.,
(2009) proposed a relation to calculate the self-absorption coefficient using the
following relation
Iba
= 𝑓𝜆𝑏 FCS Aba
𝑔𝑏
𝑈(𝑇) 𝑒
− 𝐸𝑏
𝑘𝐵𝑇 3.25
Where, 𝑓𝜆𝑏 is the self-absorption coefficient, F is experimental factor, Aba is the
transition probability of the transition from b to a. The value of 𝑓𝜆𝑏 varies from zero to
one. If a spectral line is completely reabsorbed than value of self-absorption
coefficient 𝑓𝜆𝑏 = 0, and if 𝑓𝜆
𝑏 = 1 that means the spectral line is unaffected by self-
absorption. The emission lines having low excitation energies and higher transitions
probability are strongly affected by the self-absorption. On the other hand, self-
absorption is low for those emission lines which have high excitation energy of the
upper level and low transitions probability. The lines which have lower value of the
self-absorption are selected as internal reference lines. The self-absorption coefficients
53
(𝑓𝜆) for the other spectral lines are calculated using the following relation (Sun et al.,
2009).
𝑓𝜆𝑏
𝑓𝜆𝑅 𝑏 =
𝐼𝜆𝑏𝑎𝐴𝑚𝑛𝑔𝑚
𝐼𝜆𝑅𝑚𝑛𝐴𝑏𝑎𝑔𝑏
𝑒−
(𝐸𝑚 −𝐸𝑏)
𝑘𝐵𝑇 3.26
where, 𝐴𝑚𝑛 ,is the transition probability, 𝑔𝑚is the statistical weight of the upper level,
𝐼𝜆𝑅𝑚𝑛 𝑎𝑛𝑑 𝐸𝑚 are the line intensity and the energy of the selected internal reference line,
𝐴𝑏𝑎 , 𝑔𝑏,𝐼𝜆𝑏𝑎 𝑎𝑛𝑑 𝐸𝑏 is the transition probability, statistical weight and intensity of the
other lines in the spectrum. The internal reference line is selected for which 𝑓𝜆𝑏 ≈ 1.
The self-absorption coefficient depends on the plasma temperature which is deduced
by the Boltzmann plot method. As the internal reference line plays an important role to
extract the self-absorption coefficient therefore, the selection of internal reference line
is a key point of this method. If the value of the self-absorption coefficient is greater
than one, than there is possibility of selection of a wrong internal reference line or
presence of self-absorption (Dong et al., 2015; Sun et al., 2009). The self-absorption
coefficients for all the other lines can be calculated with the help of an internal
reference line using the relation:
𝑓𝜆𝑏 =
𝐼𝜆𝑏𝑎𝐴𝑚𝑛𝑔𝑚
𝐼𝜆𝑅𝑚𝑛𝐴𝑏𝑎𝑔𝑏
𝑒−
(𝐸𝑚−𝐸𝑏)
𝑘𝐵𝑇 3.27
To get the corrected line intensities, the self-absorption co-efficient was divided by the
observed line intensities using relation:
𝐼𝜆𝑏𝑎 =
𝐼𝜆𝑏𝑎
𝑓𝜆𝑏 =
𝐼𝜆𝑅𝑚𝑛𝐴𝑏𝑎𝑔𝑏
𝐴𝑚𝑛𝑔𝑚𝑒
−(𝐸𝑚−𝐸𝑏)
𝑘𝐵𝑇 3.28
54
The IRSAC method is based on an iterative procedure, following Eqs. 3.27 and 3.28,
which needs to be repeated until correct line intensities are achieved. Following steps
are involved in the IRSAC procedure:
I. Select an internal reference line for each element.
II. Calculate appropriate values for reference lines which depend on the
Boltzmann plots
III. Calculate the plasma temperature using Boltzmann plot.
IV. Calculate the self-absorption coefficient ( 𝑓𝜆𝑏) .
V. Estimate the corrected line intensities.
VI. Check whether the temperature difference between all species is lower than
10%
VII. If the difference between the plasma temperatures is lower than 10% and
points on the Boltzmann plots show more linear and realistic trend then
calculate the elemental compositions using the CF-LIBS procedure.
VIII. If the difference between the plasma temperatures is greater than 10% then
set the highest measured plasma temperature as a mean value of the plasma
temperature.
IX. Again calculate the self-absorption coefficient and repeat steps until the
temperature difference between all the elements is less than10% and plots
follow more linear and realistic trends.
After the intensity corrections by the IRSAC method, all the Boltzmann Plots
should have approximately the same slope with different values of intercepts. Using
55
the values of the intercepts for all the elements, the compositions are calculated using
the self-calibration free (SCF-LIBS) technique as discussed above.
3.3.4 Algorithm Based calibration free (AB-CF-LIBS)
In this technique initially exact values of plasma temperature and electron
number density are calculated. The density ratios of the species of the same element
are then calculated using the following relation.
𝑛𝛼,𝑧+1
𝑛𝛼,𝑧 = 6.04 × 1021(𝑇𝑒𝑉)3
2𝑈𝛼,𝑧+1
𝑈𝛼,𝑧exp [−
𝜒𝛼,𝑧
𝑘𝐵𝑇] ×
1
𝑛𝑒 3.29
The value of ne is obtained from the Saha-Boltzmann equation:
𝑛𝑒 = 6.04 × 1021 ×Ὶ𝑧
Ὶ𝑧+1(𝑇𝑒𝑉)
3
2 exp [−𝐸𝑘,𝛼,𝑧+1+𝐸𝑘,𝛼,𝑧−𝜒𝛼,𝑧
𝑘𝐵 𝑇] 3.30
Where Ek,α,z is the upper level energy of the element α in the charged state z , Ek,α,z+1 is
the upper level energy of the element α in the charged state z+1and the constant
Ὶ𝑧 =𝜆𝑘𝑖𝐼𝑘𝑖
𝐴𝑘𝑖𝑔𝑘. For different elements α and β in different charge states Z and Z+1
respectively, the density ratios are calculated as described by (Unnikrishna et al., 2012;
Gomba et al., 2001).
𝑛𝛼,𝑧
𝑛𝛽,𝑧+1 =Ὶ𝑧,𝛼
Ὶ𝑧+1,𝛽×
𝑈𝛼,𝑧
𝑈𝛽,𝑧+1exp [
−𝐸𝑘,𝛽,𝑧+1+𝐸𝑘,𝛼,𝑧
𝑘𝐵𝑇 ] 3.31
where Ek,α,z is the upper level energy of the element α in the charged state z , Ek,β,z+1 is
the upper level energy of the element β in the charged state z+1and Ὶ𝑧 is the measured
intensity as discussed earlier.
56
To calculate the theoretical values of ne and the ratio of number densities of the same
elements as well as the ratio of the number densities of different elements, we use an
algorithm (Unnikrishna et al., 2012; Gomba et al., 2001) built in MATLAB program.
Formulation:
The formulas for calculation of the theoretical values of ne and the ratio of the
number densities of the same elements as well as the ratio of the number densities of
different elements is constructed. If ne is the total electron number density of the
plasma and ne,α is the electron density contribution from the element α to the total
electron density ne ,then ne,α is equal to the sum of all the electrons from all the ionic
states of the element α as (Unnikrishna et al., 2012; Gomba et al., 2001):
𝑛𝑒,α = 𝑛𝛼,2 + 2𝑛𝛼,3 + 3𝑛𝛼,4 + ⋯ = ∑ 𝑧𝑛𝛼,𝑧+1𝑁𝑧=1 3.32
If there are N elements, then the total electron density ne is the sum of the electrons
from all the elements:
𝑛𝑒 = ∑ 𝑛𝑒,α𝑁α=1 3.33
The total number density ntot,α is the sum of number densities of the neutral and
ionized atoms of the element α:
𝑛𝑡𝑜𝑡,α = 𝑛𝛼,1 + 𝑛𝛼,2 + ⋯ + 𝑛𝛼,𝑧 = 𝑛𝛼,1 [1 +𝑛𝛼,2
𝑛𝛼,1 + + ⋯ ] = 𝑛𝛼,1 [1 + ∑𝑛𝛼,𝑧+1
𝑛𝛼,1𝑁𝑧=1 ] 3.34
Where, nα,z
represents the number density of the element α in charge state z.
Above equation represents the contribution of one electron to the total electron number
density; for doubly ionized atom the contribution two electrons and so on. If we deal
only with the neutral and singly ionized atoms than we consider z = 1 and 2. A new
57
function 𝑅𝛼,𝑧+1 =𝑛𝛼,𝑧+1
𝑛𝛼,1 is introduced in Eq. 3.34 that relates the ionized and neutral
states of the atom as:
𝑛𝛼,1 = 𝑛𝑡𝑜𝑡,α
1+∑ 𝑅𝛼,𝑧+1𝑁𝑧=1
3.35
Now using the value of 𝑛𝛼,𝑧+1 = 𝑛𝛼,1 × 𝑅𝛼,𝑧+1 = 𝑛𝑡𝑜𝑡,α
1+∑ 𝑅𝛼,𝑧+1𝑁𝑧=1
× 𝑅𝛼,𝑧+1 in Eq. 3.32
𝑛𝑒,α = 𝑛𝑡𝑜𝑡,α×∑ 𝑧 𝑅𝛼,𝑧+1𝑁
𝑧=1
1+∑ 𝑅𝛼,𝑧+1𝑁𝑧=1
3.36
Again we define another parameter 𝑆𝛼,𝑧+1 = 𝑛𝑒 𝑛𝛼,𝑧+1
𝑛𝛼,𝑧 to obtain the value of Rz+1
Where 𝑆𝛼,𝑧+1 = 6.04 × 1021(𝑇𝑒𝑉)3
2𝑃𝛼,𝑧+1
𝑃𝛼,𝑧exp [−
𝜒𝛼,𝑧
𝐾𝐵 𝑇] 3.37
Now 𝑅𝛼,𝑧+1 can be written as:
𝑅𝛼,𝑧+1 =𝑛𝛼,𝑧+1
𝑛𝛼,1 = ∏𝑆𝛼,𝑧+1
(𝑛𝑒)𝑧𝑧𝑖=1 3.38
If there are only two elements α and β then we can repeat the same calculations for the
other element β.
Algorithm
The algorithm involves following steps to calculate the theoretical values:
I. Use an estimated value of plasma temperature T (eV). Initially hypothetical
values of densities ne, ntot,α and ntot,β in the range 1014
to 1017
(cm-3
) are selected.
II. Taking the above hypothetical values using the equations we determined a new
value of ne.
58
III. If ne(new) and ne(hypothetical) are approximately not the same values then by
varying the value of ne(hypothetical) repeat the Step II until ne(new) and ne
(hypothetical) give approximately the same results.
IV. Now using the above converged value of ne and initially supposed values of
ntot,α , ntot,β we can find out the number densities of neutral and singly ionized
atoms of both the elements using the Eq. 3.33.
V. Using the values obtained in the step IV we can easily calculate the density
ratio of the same as well as different elements. These steps are repeated until
hypothetical values of ne, and density ratios match with the experimentally
found values.
VI. When these hypothetical values match with the experimental values then the
convergent values of ntot,α, ntot,β are used for the calculation of the new value of
ne.
VII. Now step II is repeated until ne(new) and ne(hypothetical) values give
approximately the same results (or converged).
VIII. After that, using the above converged value of ne and the initially hypothetical
values of ntot,α, ntot,β we can find out the number densities of neutral and singly
ionized atoms of both the elements using Eq. 3.34, 3.35, 3.36.
IX Using the values obtained in the step IV we can easily calculate the density
ratios. If these theoretically found ratios do not match with the experimentally
found ratios then we use the above converged value of ne and vary the values of
59
ntot,α, ntot,β until the theoretical value of ne and also the density ratios match
with the experimentally found values.
X Save the convergent values of ntot,α, ntot,β for which the theoretical results match
with the experimental results.
Finally, we can estimate the relative compositions of the elements α, β, ϒ … using the
saved values given in Eq. 3.12 and 3.13:
3.3.5 Compositional Analysis using LA-TOF-MS
Since laser ablation is an efficient source for the production of ions, LA-TOF-
MS is an efficient tool for the compositional analysis. For the compositional analysis
of the mass spectra, integrated line intensities of the ionic signals are used. Following
formula is used for the calculation of the composition of an element.
𝐶𝑎% =𝐼𝑎
𝐼𝑎+𝐼𝑏+𝐼𝑐+𝐼𝑑+𝐼𝑒+⋯∗ 100 3.38
𝐶𝑏% =𝐼𝑏
𝐼𝑎+𝐼𝑏+𝐼𝑐+𝐼𝑑+𝐼𝑒+⋯∗ 100 3.39
𝐶𝑐% =𝐼𝑐
𝐼𝑎+𝐼𝑏+𝐼𝑐+𝐼𝑑+𝐼𝑒+⋯∗ 100 3.40
.
.
.
.
Where, 𝐼𝑎 is the integrated ion signal intensity of element “a”, 𝐼𝑏 is the integrated ion
signal intensity of element “b” and 𝐼𝑐 is the integrated ion signal intensity of element
“c”.
60
CHAPTER 4
LASER ABLATION TIME OF FLIGHT MASS SPECTROMETER
FOR ISOTOPE MASS DETECTION AND ELEMENTAL
ANALYSIS OF MATERIALS
Major part of this chapter has been published in the journal, “Laser Physics”. This
article is also selected for the “2017 Highlights Collections” of the journal. In this
contribution the author has successfully resolved the isotopes of pure elements and
determined the composition of brass alloy having certified composition of Cu and Zn.
In this chapter we present the results and discussion of modified linear time of
flight mass spectrometer with improved mass resolution. This system consists of a
laser ablation/ionization unit based on a Q-switched Nd:YAG laser (532 nm, 500 mJ,
5ns pulse duration) integrated with a one meter linear time of flight mass spectrometer
coupled with an electric sector and a magnetic lens and outfitted with a channeltron
electron multiplier for ions detection. The entire setup is discussed in detail in Chapter
3. The resolution of system is improved by optimizing the accelerating potential and
inserting a magnetic lens after the extraction region. The isotopes of lithium, lead and
cadmium samples have been well resolved and detected in accordance with their
natural abundance. The capability of the system has been further exploited to
determine the elemental composition of a brass alloy, having certified composition of
zinc and copper. Our results are in excellent agreement with its certified composition.
This setup is found to be extremely efficient and convenient for fast analyses of any
solid sample.
61
4.1 CALIBRATION OF LINEAR LA-TOF-MS
Since laser ablation is an efficient source for the production of ions and it
consumes nominal amount of the target material therefore, LA-TOF-MS is an efficient
tool for the ionic mass analysis. In this setup a single laser pulse is used to ablate the
material (LIBS) and the corresponding mass spectra are obtained with the LA-TOF-
MS arrangement which makes it a powerful tool for the isotope abundance studies and
the compositional analysis of any solid samples. Calibration of the system is
mandatory before starting the analysis. The mass scale follows a square root law
regardless of the extraction and acceleration voltages or other parameters as:
𝑇𝑂𝐹 = 𝑎𝑚1
2⁄ + 𝑏 (4.1)
Figure 4.1: Calibration curve for the locally fabricated linear time of flight mass
spectrometer
To calibrate the mass spectra we have taken standard samples with known
elements (Li, Cu, Sn and Pb). The mass spectra is calibrated by measuring the flight
62
time of these elements to determine the values of constants a and b as shown in Fig.
4.1. These constants take in to account any time offset due to laser interaction time,
triggering of the recording system, etc. The values of the constants for our system are
estimated as a =0.39 and b = 3.04. The calibration curve shows good linearity, R2
i.e
correlation factor for the linear fit of copper and silver is 0.99 within the experimental
uncertainty.
4.2 Spatial and Temporal Kinetic Energies Distributions
In the linear LA-TOF-MS system; the effect of initial kinetic energies on mass
resolution is resolved by applying appropriate accelerating voltages whereas; the effect
of different directions of motion of charged particles is reduced by introducing the
magnetic lens after the extraction region as discussed in detail in the chapter 3.
The production of ions in the ionization region with different initial kinetic energies
causes the time lagging in the mass spectrum. Some of the ions are formed with initial
K.E but their actual flight time in the drift region is reduced. The total kinetic energy
of an ion is the sum of the kinetic energy gained by the ion and the initial kinetic
energy of the ion. Due to the difference in the kinetic energies and due to dissimilar
directions of motion of the ions they arrive at the detector at different times and results
lagging in the mass spectra. Lagging in the mass spectra due to initial kinetic energies
is removed by applying an appropriate accelerating voltages and lagging in the mass
spectra due to different directions of the motions of the ions is removed by applying a
magnetic lens.
63
Figure 4.2: Comparison of the TOF mass spectra of lead at low Vac without magnetic
lens (a), at high Vac without magnetic lens (b) and at high Vac with magnetic lens.(c).
In Fig. 4.2 (a,b) we show the spectra of lead isotopes taken at 300 V and 1500
V accelerating (Vac) voltages respectively, but without the magnetic lens. The tailing in
the mass spectra on the lower mass side is evident when the accelerating voltage is 300
V. However, the tailing in the spectrum disappears when a higher accelerating voltage
is applied because now eV is larger than Uo. The lowest lowermost spectrum Fig. 4.2
(c) is taken after inserting the magnetic lens while the accelerating voltage was 1500
Vac. Evidently, the peaks shapes are now much improved on both sides of the mass
a
b
c
64
spectrum. The four isotopes of lead at (m/z = 204, 206, 207, 208) are well resolved.
The reported relative abundance of the isotopes of Pb204
, Pb206
, Pb207
and Pb208
is 1.4%,
24.1%, 22.1% and 52.4% NIST database, (2016). In the present work, we have
determined their relative abundance as 3.7% 25.0%, 21.7% and 50% respectively.
Figure 4.3: Laser ablation time of flight mass spectrum (TOF-MS) of Lithium. Two
isotopes of lithium; Li6 and Li
7 are evident at -1600 V operating voltage.
In Fig. 4.3 we show the laser ablation mass spectrum of lithium metal. The
spectrum shows two well resolved peaks around m/z = 6 and 7. Since lithium has two
isotopes Li6 and Li
7, indeed two well resolved peaks are evident in the figure. The
reported relative abundance of the lithium isotopes; Li6 and Li
7 is 7.6 %, 92.4 %
respectively, NIST database, (2016). The integrated line intensities yield the
abundance of Li6 and Li
7as 9 % and 91 %, which are very close to the reported values.
65
Figure 4.4: Laser ablation/ionization time of flight mass spectrum (TOF-MS) of pure
cadmium.
Fig. 4.4 we show the laser ablation/ionization mass spectrum of cadmium.
Eight peaks corresponding to eight isotopes of Cd (m/z = 106, 108, 110, 111, 112, 113,
114, 116) are evident. Using the LAI-TOF-MS system, we have determined the
isotopic abundance for the eight isotopes at m/z = 106, 108, 110, 111, 112, 113, 114,
116 as 1.4%, 1.1%, 15.0%, 15.5%, 20.0%, 11.02%, 29.2% and 7.28% respectively.
These values are in good agreement with that reported in the literature NIST database,
(2016) which reflects the performance of this improved equipment. We have also
exploited the capability of this system for the quantitative analysis of brass sample
having certified composition (62% Cu, 38% Zn). At lower laser energy about 2mJ,
only two peaks appear which correspond to the major isotopes of copper (Cu63
) and
zinc (Zn64
) as shown in Fig. 4.5 as inset. At higher laser energy 5mJ, all the isotope
66
masses of copper (m/z = 63, 65) and zinc (m/z = 65, 66, 67 and 68) are evident. The
isotope of zinc at m/z = 70 is not clear because of its very low abundance as shown in
Fig. 4.5(b). The elemental compositional is performed using the major isotopes of Cu
at m/z = 63 and Zn at m/z = 64. To deduce the elemental composition from the
observed two peaks, we calculated the integrated line intensities (area under the peak)
which yield the elemental composition of this alloy as: Cu (62 ± 1%) and Zn (38 ±
1%). The results are in excellent agreement with its certified composition.
Figure 4.5: Time of Flight Mass Spectrum of brass alloy taken at 2mJ laser energy as
an inset (a) and the Mass Spectrum of brass alloy taken at 5mJ laser energy (b).
The measured isotope ratios for these samples are summarized in Table 1. The
measured experimental data agree reasonably well with the natural abundance, but
some differences are still present. The error in the more abundant isotopes is very
small and it may be attributed to the saturation of the mass signal, insufficient mass
resolution, interfaces or due to noise. Much larger errors appear in the low abundant
(a) (b)
67
isotopes as the lighter isotopes deplete faster as microscopic mass is ablated with laser.
The difference between the natural and the measured abundances may be due to
collisions inside the plasma and due to the space charge effect as explained by
Koumenis et al., (1995) and Song et. al., (1999). From the observed data, we can
conclude that the isotopes as well as the trace elements can be analyzed easily using
this simple modified laser ablation/ionization time of flight mass spectrometer.
Table 4.1: Measured isotope ratios for Li, Cd and Pb samples compared with
natural abundance (NIST database, 2016)
Element
m/z
Natural
Abundance (NIST database, 2016)
Measured Abundance by
Laser Ablation TOF-MS
Li 6
7
7.6%
92.4%
9%
91%
Cd 106
108
110
111
112
113
114
116
1.3%
0.9%
12.5%
12.8%
24.1%
12.2%
28.7%
7.5%
1.4%
1.1%
15.0%
15.5%
20.0%
11.0%
29.2%
7.3%
Pb
204
206
207
208
1.4%
24.1%
22.1%
52.4%
3.7%
25.0%
21.7%
50.0%
68
CHAPTER 5
LASER ABLATION STUDIES OF DIFFERENT KARATS OF
GOLD USING LIBS AND TIME OF FLIGHT MASS
SPECTROMETER
Major part of this chapter has been published in the journal, “Plasma Chemistry
Plasma Process”. In this contribution the author has successfully determined the
compositions of precious alloy samples using LA-TOF-MS and LIBS.
In this chapter we discriminate the precious gold alloys caratage using laser
induced breakdown spectroscopy (LIBS) complemented with the laser ablation time of
flight mass spectrometer (LA-TOFMS). Five Karats of gold alloys 18K, 19K, 20K,
22K and 24K having certified composition of gold as 75%, 79%, 85%, 93% and
99.99% are tested and their precise elemental compositions are determined using the
laser produced plasma technique. The plasma is generated by focusing the beam of a
Nd: YAG laser on the target in air and its time integrated emission spectra are
registered in the range 250-870 nm. The calibration free LIBS technique (CF-LIBS) is
used for the quantitative determination of the constituent elements present in different
Karats of gold. Elemental compositions of these gold alloys are also determined using
a Laser Ablation time of flight mass spectrometer (LA-TOF-MS). The LIBS limit of
detection (LOD) is calculated from the calibration curves for copper, silver and gold.
Results of CF-LIBS and LA-TOF-MS are in excellent agreement with the certified
values. It is demonstrated that LIBS coupled with LA-TOFMS is an efficient technique
that can be used to analyze any precious alloys in a fraction of a second.
69
Gold is a soft, reddish yellow, shiny and precious metal, being extensively used
in Ornaments, Jewelry, Medals, Electronics and Finances. China, Russia, the United
States are the world dominant gold producing countries with massive reserves. Precise
determination of elemental composition in different Karats of gold is an attractive
subject in gold mining industry as its composition must meet the international
standards. Therefore, certification of the caratage of gold alloys is always a
challenging and demanding process. The standard methods used for the identification
of different Karats of gold are accurate but there are a number of drawbacks associated
with the traditional methods. These methods are unwieldy, destructive and require
preliminarily information about the gold percentage in the sample. The information
about the Karat is only possible for the newly made jewelry but for the unknown
Karats of jewelry, a repetitive analysis is necessary to get exact information about the
gold percentage (Derby et al., 1917). To resolve the above mentioned problems,
several techniques have been employed (Corti, 2001 and Brill, 1997). To overcome
these problems for the gold alloy composition determination, we combined the laser
induced breakdown spectroscopy (LIBS) (Abbas et al., 2016; Gomba et al., 2001) and
laser ablation time of flight mass spectrometry (LA-TOFMS) (Ahmed et al., 2016)
techniques. Laser ablation is a good alternate to the above mentioned techniques due to
its superior limit of detection and better depth profiling capability. Laser produce
plasma induces crater with diameter up to ⩯100 micrometers and depth of more than
10μm depending upon the laser beam profile, focusing optics and laser energy
(Cremers 2006). Any material holding any shape can be easily analyzed using this
technique. As LIBS is a non-destructive technique and its portable versions are also
70
available, therefore this technique can be used for quality control in industries
(Cremers 2006; Griem, 1997). Due to these features, LIBS technique has advantages
over the other standard techniques. Calibration curve LIBS technique has been hardly
used for accurate analysis of gold Karats due to the matrix effect, self-absorption and
other experimental uncertainties. In the calibration free LIBS approach, all the
elements of the sample can be detected and it requires no external standards for
calibration. However, CF-LIBS is useful only in the case where the plasma is optically
thin and is in the local thermodynamical equilibrium (LTE). This method has been
successfully applied for the compositional analysis of precious alloys (Rafai et al.,
2017), gases and archeological samples (Burakov et al., 2007) and caratage analysis
(Corsi et al., 2001). Provided the optically thin and LTE conditions are satisfied for all
the lines, this method can be used to get accurate compositional analysis (Ahmed et
al., 2016). If self-absorption is present, then errors are bound to appear. To improve
the CF-LIBS results, an internal reference self-absorption correction (IRSAC) method
is proposed to correct the emission line intensities with reference to an internal
standard line. In the present work, we have utilized the calibration free CF-LIBS
technique to analyze the LIBS spectra for precious alloy compositions without any
external calibrations. All the five different Karats (18K, 19K, 20K, 22K and 24K)
having certified composition of gold as 75%, 79%, 85%, 91% and 99.9% are also
analyzed using a Laser Ablation Time of Flight Mass Spectrometer (LA-TOF-MS).
5.1 EMISSION STUDIES
The laser produced gold alloy plasma is generated using a high-power Q-
switched Nd:YAG Laser (Brilliant-B Quantel, France).
71
Figure 5.1 (a, b): Typical optical emission spectra of the Laser produced plasmas at
the gold alloys, 24K, 22K, 20K, 19K and 18K, covering the spectral region 250-
870nm using laser energy 100mJ and 2µs time delay.
A quartz lens (convex) of 20 cm focal length is used to focus the laser beam on the
target sample placed in air at atmospheric pressure. The laser pulse energy was varied
from 80 to 120 mJ and the measured diameter of the focused laser beam was about
(0.10 ± 0.01) cm (laser fluence about 10-15 J/cm-2
). The emitted radiation was
captured by a set of four spectrometers (Avantes, Hollands). To correct the emission
signal, the dark signal is subtracted from the observed signal using the LIBS software.
72
Same gold alloys are also analyzed using laser ablation time of flight mass
spectrometer. The LIBS spectrum consists of spectral lines of the constituent elements
which give information about the major and trace elements present in the sample. The
intensities of the observed spectral lines are proportional to their concentrations. The
plasma at the surfaces of different gold alloys is generated by focusing the Nd:YAG
laser beam with pulse energy of about 400 mJ at 532 nm. As soon as the plasma is
generated, the plasma plume expands perpendicular to the target surface and after a
few micro seconds, it cools down and emits the characteristics spectra of the
constituent elements.
Figure 5.2: Emission spectra of the Laser produced plasmas of different Karat of the
gold covering the spectral region 508 - 547nm showing variations in the line intensities
of copper, silver and gold lines.
Typical spectra of five different Karats (18K, 19K, 20K, 22K and 24K) of gold
produced plasma generated by focusing a 100 mJ and 5ns laser pulse at 532nm (Laser
73
fluence about 12 J/cm2) are shown in Fig.5.1 (a, b) covering the wavelength region
from 250 to 870 nm. The characteristic lines of gold, silver and copper are evident
labeled in the figure corresponding to their wavelengths. The resonance lines of Au,
Ag and Cu are observed which facilitates the identifications of the observed spectra.
As the plasma is generated in air at an atmospheric pressure, the lines of hydrogen,
oxygen and nitrogen are also present in the spectra. In Fig. 5.2 we show the emission
spectra of gold alloys in the range 508nm to 548nm, showing variations in the
emission line intensities of Cu I lines at 510.29nm, 515.32nm and 521.82nm, Ag I
lines at 520.90nm and 546.55nm, Au I at 523.026nm for different Karat of the gold.
The observed line intensities of copper, silver and gold are in accordance to the Karats
of the gold samples, the larger the concentration the higher the line intensity.
Figure 5.3: Variation of emission line intensity of Cu I at 510, Ag I at 328 Au I at
312nm with the variable laser energy (5-130) mJ laser energy of 18K gold alloy.
The spectrum of 24 Karat of gold shows only the gold lines and not a single
line of silver and copper is present which guarantees the quality and purity of the gold
74
sample. The effect of laser energy on the emission line intensities is observed, as laser
energy is increased the intensities and widths of spectral lines increases. The variation
in the line intensities of neutral lines of Cu, Ag, and Au at 510.29nm, 328.07nm and at
312.27nm are observed with varied laser energy from 5mJ to 130mJ as shown in Fig.
5.3. At lower laser energy, plasma is produced at the leading edge of laser pulse.
However, with an increase in the laser energy the rate of evaporation as well as the
laser absorption increases, which causes an increase in intensity and width of the
spectral lines.
5.2 DETERMINATION OF PLASMA TEMPERATURE
The plasma temperature is calculated from the relative line intensities of Cu,
Ag and Au using the Boltzmann plot method. The spectral lines used to construct the
Boltzmann Plot along with the other spectroscopic parameters taken from the
Literature (Beideck et al., 1993; Hannaford et al., 1981; Migdelek et al., 1978;
Migdelek et al., 1976; Bielski et al., 1975) are listed in the Table. 5.1. Errors are bound
to be present in the determination of the plasma temperature by the Boltzmann plot
method due to uncertainties in the transition probabilities and the measured line
intensities; therefore, the electron temperature is determined with about 10% error.
Absorption of photons within the laser-induced plasma may cause self-absorption and
it will be more evident in the emission lines where the lower level of transition is equal
or close to the ground state. The effect of self-absorption apparently reduces the peak
intensity and line broadening.
75
Table 5.1: Spectroscopic parameters of the Cu, Ag and Au emission lines used to
construct the Boltzmann Plots.
Wavelength
𝛌(nm)
Transition Ak
(107 s
-1)
Ek (eV) gk
Upper level Lower level
Cu I
261.84 3d10
5p 2P3/2 3d
94s
2 2D5/2 3.07 6.12 4
282.43 3d94s4p
2D5/2 3d
94s
2 2D5/2 0.78 5.78 6
293.30 3d94s4d
4G7/2 3d
94s4p
4P5/2 0.12 9.30 8
296.12 3d94s4p
2F7/2 3d
94s
2 2D5/2 0.37 5.57 8
299.73 3d94s4p
2D5/2 3d
94s
2 2D3/2 0.12 5.78 6
324.75 3d10
4p 2P3/2 3d
104s
2S1/2 13.9 3.82 4
327.39 3d10
4p 2P1/2 3d
104s
2S1/2 13.7 3.79 2
330.79 3d94s4d
4G11/2 3d
94s4p
4F9/2 22.2 8.82 12
465.11 3d94s5s
4D7/2 3d
94s4p
4F9/2 3.80 7.74 8
510.55 3d10
4p 2P3/2 3d
94s
2 2D5/2 0.20 3.82 4
515.32 3d10
4d 2D3/2 3d
104p
2P1/2 6.0 6.19 4
521.82 3d10
4d 2D5/2 3d
104p
2P3/2 7.5 6.19 6
529.25 3d94s5s
4D7/2 3d
94s4p
4D7/2 1.09 7.74 8
570.02 3d10
4p 2P3/2 3d
94s
2 2D3/2 0.02 3.82 4
578.21 3d10
4p 2P1/2 3d
94s
2 2D3/2 0.16 3.79 2
809.26 3d10
5s 2S1/2 3d
104p
2P3/2 4.59 5.35 2
Ag II
250.71 4d95d
2[1/2]1 4d
95p
2[3/2]2 9.00 16.21 3
266.04 4d95p
2[3/2]1 4d
95s
2[3/2]2 1.51 10.37 3
276.75 4d95p
2[7/2]3 4d
95s
2[3/2]2 1.01 10.19 7
283.76 4d96s
2[3/2]1 4d
95p
2[3/2]1 0.40 15.51 3
289.63 4d96s
2[3/2]2 4d
95p
2[3/2]2 8.40 15.55 5
822.48 4d85s5p
5G4 4d
96s
2[5/2]3 0.17 16.45 9
825.48 4d96p
2[5/2]3 4d
96s
2[5/2]2 1.40 16.49 7
843.15 4d97s
2[3/2]1 4d
96p
2[3/2]1 2.40 18.48 3
Au I
264.15 5d96s6p
2P3/2 5d
96s
2 2D5/2 3.30 5.83 4
267.59 5d10
6p 2P1/2 5d
10 6s
2S1/2 16.5 4.63 2
268.87 5d9 6s6p J=5/2 5d
9 6s
2 2D3/2 1.34 7.27 6
270.09 5d9 6s6p J=5/2 5d
9 6s
2 2D5/2 0.566 5.72 6
274.83 5d9 6s6p
4F7/2 5d
96s
2 2D5/2 4.82 5.65 8
288.34 5d9 6s6p
4D3/2 5d
9 6s
2 2D3/2 0.94 6.96 4
302.92 5d9 6s6p
4P5/2 5d
9 6s
2 2D5/2 0.80 5.23 6
Continued
76
If the self-absorption contribution is large, a self-reversal dip appears at the top of the
line which is a sign of strongly inhomogeneous plasma. Even slightly self-absorbed
lines used in CF-LIBS may introduce error in determining the compositions. However,
in the present experimentation there is not a single line which shows such a distorted
line profile. On the safe side and to minimize such minor effects of self-absorption, we
have corrected the observed line intensities of Cu, Ag and Au for self-absorption
(Sherbini et al., 2005) but this correction doesn’t exceed 10%.
Table Page 2
Wavelength
𝛌(nm)
Transition Ak
(107 s
-1)
Ek
(eV)
gk
Upper level Lower level
Au I
312.28 5d10
6p 2P3/2 5d
96s
2 2D5/2 1.92 5.11 4
322.60 5d10
10d 2D3/2 5d
10 6p
2P3/2 0.106 8.95 4
406.51 5d10
6d 2D3/2 5d
10 6p
2P1/2 8.35 7.68 4
479.26 5d10
6d 2D5/2 5d
10 6p
2P3/2 8.90 7.69 6
481.16 5d10
6d 2D3/2 5d
10 6p
2P3/2 1.51 7.68 4
583.74 5d10
7s 2S1/2 5d
10 6p
2P1/2 2.64 6.76 2
627.82 5d10
6p 2P1/2 5d
9 6s
2 2D3/2 0.337 4.63 2
751.07 5d10
7s 2S1/2 5d
10 6p
2P3/2 3.92 6.76 2
77
Figure 5.4: Boltzmann plots of the 22K gold alloy using emission lines of Cu I, Ag II
and Au I using Laser pulse energy 100 mJ and at 2µs time delay.
Fig 5.4 shows typical Boltzmann plots of the 22K gold alloy using the intensity
corrected neutral Cu lines, singly ionized Ag lines and neutral Au lines. The linearities
in the Boltzmann Plots reveals the selection of the appropriate lines to deduce the
plasma temperatures. The plasma temperature estimated from the Cu I lines is (9500 ±
500) K, Ag II lines (10500 ± 500) K and Au I lines (10000 ± 500) K. The average
values of the plasma temperatures estimated for the 18K, 19K, 20K and 22K samples
are (9800 ± 500) K, (10300 ± 500) K, (11000 ± 500) K and (10000 ± 500) K
78
respectively. These average plasma temperatures are used to estimate the elemental
compositions of different Karat of gold alloys.
5.3 DETERMINATION OF ELECTRON NUMBER DENSITY
In order to estimate the electron number density, we selected the Stark
broadened and a well isolated Ag I line at 328.07nm and the Stark broadened line
profile of the hydrogen Hα at 656.28 nm. The width ΔλFWHM of these lines are
determined by de-convoluting the observed line profiles as a Voigt profile, which takes
into account the Instrumental width, Doppler width and Stark broadening. The
instrumental width of our spectrometer is about 0.06 ± 0.01nm whereas; the Doppler
width is about 0.005 nm. The Stark width as discussed in detail in chapter 1. In Fig:
5.5(a) we show the Stark broadened line profile of the Ag I line at 328.07 nm. The
spectrum is recorded using a Nd: YAG laser at 532 nm with pulse energy 100 mJ and
at about 2 µs time delay between the laser pulse and the data acquisition system. The
dots are the experimental data points and the full line is the Voigt fit. The FWHM of
the observed experimental line profile is extracted as (0.11 ± 0.01) nm. The Stark
broadening parameter for this line is reported in the literature as 4.65×10-2
nm
(Dimitrijevic et al., 2003). The electron number density is calculated using Eq. 1.23 as
(2.0 ± 0.5) ×1017
cm-3
. Calculations of the full width at half area of hydrogen Hα line at
656.28 nm is presented in Fig. 5.5 (b) showing the experimentally observed line
profile bounded by two vertical lines representing FWHA as (0.86 ± 0.08) nm. In Fig.
5.5 (c) we present the calculation procedure for full width at half area FWHA using
numerical integration, it is the distance between the points that give areas between 1/4
and 3/4 of the total area.
79
Figure 5.5: Stark broadened line profile of Ag I line at 328.07 nm (a), Calculation of
full width at half area of hydrogen Hα line at 656.28 nm at 100mJ laser energy (b)
Calculation procedure for FWHA using numerical integration (c).
The electron density is calculated using the relation discussed in chapter 1, Eq.
1.24 as (1.9 ± 0.5) × 1017
cm−3
. A good agreement between the number densities
derived from the Stark Broadening parameter of Ag I line at 328.07 and hydrogen Hα
line is observed. Average values of the number densities are used to determine the
compositions of the samples using CF-LIBS. The electron number density for the gold
c
80
alloys is estimated in the range (1-3 ± 0.5) ×1017
cm-3
which is subsequently used to
calculate the compositions of the elements.
In order to use the LIBS spectra for the quantitative analysis, it is necessary
that the laser produced plasma is optically thin and also holds the Local
Thermodynamic Equilibrium (LTE) condition. To validate the condition for the
optically thin plasma (Cremers 2006; Griem, 1997), the corrected line intensities of
various emission lines of Cu, Ag and Au are compared with the ratio of their transition
probabilities (Cremers 2006; Griem, 1997) which are in agreement within 10%. To
check the local thermodynamic equilibrium (LTE), a criteria proposed by McWhirter
has been validated for the Au I line at 267.59 nm, Ag I at 328.06 nm and Cu I at
324.75 nm lines. The lower limit for the electron density is calculated using the Eq. 1.6
(McWhirter, 1965) discussed in chapter 1. The lower limit for electron densities,
calculated from the emission lines of Au I at 267.59nm, Ag I at 328.06nm and Cu I at
324.75nm are in the range of (0.5-1.5± 0.5) × 1014
cm-3
respectively. Evidently, the
number densities obtained from the McWhirter criteria are much lower than that
determined from the Stark broadening parameters, which are in the range (1-3 ± 0.5)
×1017
cm-3
. Thus the plasma may be considered close to LTE.
In addition to the McWhirter criterion for stationary and homogeneous plasma, the
condition of the validity of LTE in inhomogeneous plasma is also validated using the
spectral lines of gold, copper and silver. The diffusion length 𝐷𝜆 is calculated using the
Eq. 1.7 (Cristoforetti et al., 2013; Cristoforetti et al., 2010) discussed in chapter 1. The
diffusion length 𝐷𝜆 is calculated as 0.0001 cm, much larger than the characteristic
81
variation length “d” equal to 0.2 cm, which is in accordance to the criteria i.e. 10𝐷𝜆<d.
In the light of the above conditions it can be safely assumed that the plasma is very
close to LTE.
5.4 SPATIAL BEHAVIOR OF PLASMA PARAMETERS
In this section we have done new set of experiments to analyze the spatial
behavior of the electron number density and electron temperature in the plume. Fig.
5.6 we have shown the special behavior of electron number density at fix laser energy
about 100 mJ. The electron number densities ne near to the target surface is about 5.5×
1017
cm-3
. The value of number density decrease to 9.8× 1016
cm-3
at distance of 6.0 mm
from the target surface as shown in Fig: 5.6. It is observed that electron number
density is maximum near to the target surface and decreases exponentially as the
distance from the target is increased. The Special behavior of electron temperature has
also been calculated at fix laser energy of about 80mJ. The variation of plasma
temperature as a function of special distance along the direction of plasma expansion
as shown in Fig: 5.7. The electron temperature at 0.5 mm from the target is about
9000K as the distance increased the plasma temperature have also showed decreasing
trend The decrease of the excitation temperature is due to the rapidly conversion of
thermal energy into kinetic energy as thermal energy causes expansion of the plasma.
The reflection of the light from the metal surface also affects the plasma temperature.
Since the region near the surface of the target material constantly absorbs radiation
during the laser irradiation therefore, it causes higher temperature near the target
surface.
82
Figure 5.6: Variation of electron number density along the direction of the laser
produce plasma plume.
The laser energy is absorbed by the electrons via inverse bremsstrahlung absorption
process which causes a higher value of the electron temperature.
Figure 5.7: Variation of excitation temperature as a function of distance along the
direction of the laser produces plasma plume.
As the laser produced plasma expands thermally therefore, it transfers the energy to the
surroundings. It causes the electron temperature and the electron number density to
decrease along the direction of expansion of the plasma plume. It is observed that the
electron temperature and electron number densities are both maximum near to the
target.
83
5.5 EFFECTS OF LASER IRRADIANCE ON PLASMA PARAMETERS
In this section we present the experiments to study the variation of electron
temperature and electron number density as a function of the laser pulse energy using a
Nd: YAG laser at 532 nm. It is observed that the intensities and widths of the spectral
lines increase with the increase in the laser energy. The electron number density is
calculated by varying the laser pulse energy from 5 to 70mJ as shown in Fig. 5.8. The
electron temperature has also been determined by varying the laser energy from 5mJ to
70mJ shown in Fig. 5.9. The identical trends of electron number density and plasma
temperature are observed. The observed increase in the electron temperature and
electron number density by the increase of laser pulse energy is attributed to the
absorption of the laser energy by the plasma.
Figure 5.8: Variation of electron number density with the laser pulse energy.
84
Figure 5.9: Variation of excitation temperature with the laser pulse energy.
5.6 COMPOSITIONAL ANALYSES USING SCF-LIBS
For the compositional analysis the Internal Reference Line Self-Absorption
Correction (IRSAC-LIBS) method is utilized as discussed in chapter 3 section 3.3.3. In
short the line intensities are initially corrected for the self-absorption then the
Boltzmann plots are drawn as shown in Fig.5.4. For the quantitative determination of
the constituents of Cu, Ag and Au in gold alloys, the Boltzmann relation is used. The
values of CCu, CAg , and CAu are estimated as follows:
Cs =P(T)× eqs
F (5.1)
Due to the insufficient lines of ionized gold and copper and neutral lines of
silver, it is not probable to draw the Boltzmann plots. Thus, the Saha-Boltzmann
85
equation (Gomba et al., 2001 ) is used to determine the compositions of the ionized
species of gold and copper and neutral species for silver. The total concentration of an
element is a sum of the concentrations of both, neutral and the ionized species. By
adopting the above procedure, the composition of the 18K gold alloy is obtained as
75% Au, 21% Cu, 4% Ag, 19K gold alloy as 79% Au, 7% Cu, 12% Ag, 20K gold
alloy as 83% Au, 6% Cu, 11% Ag, and 22K gold alloy as 93% Au, 5% Cu, and 2%
Ag. The calibration free LIBS technique yields improved percentage compositions of
these gold alloys. The errors attached in this procedure are less than 10 %.
5.7. LIMITS OF DETECTION
The lines possessing much higher intensities are good candidates for the
determination of the limit of detection (LOD) as compared to the weaker emission
lines (Tawfik et al., 2008; Drogoff et al., 2001). To draw the calibration curves, the
line intensities of copper and silver are normalized by the intensity of the gold line at
312.28nm to reduce the effect of the instrumental signal fluctuations and matrix
effects. The calibration curves of copper and silver are drawn and the normalized
intensity versus the relative composition for the five gold alloys is presented in Fig:
5.10 (a, b).
The error bars show the calculated standard deviations of the signal intensities for
copper and silver. All the calibration curves are drawn for the data collected at 100mJ
laser energy operating at 532nm with 5ns pulse duration. The calibration curves show
good linearity, correlation factor i.e R2
for the linear fit of copper and silver is 0.99
within the experimental uncertainty. The calibration curve for gold is also drawn with
86
the intensities of the neutral gold lines at 479.26nm and at 523.02nm against the gold
concentration in Fig: 5.10. All the samples have been validated reasonably well with
the actual/certified concentrations in the gold alloys, which show the quality of the
fitting. The limit of detection is calculated using the equation (Ingle, 1988).
𝐿𝑂𝐷 =3𝜎
𝑏 (5.2)
Where, σ is the standard deviation of the background and b is the slope of the
calibration curve.
To calculate σ, more than hundred background noise values are taken on both the sides
of the peaks from the spectrum. The LOD values obtained for Cu, Ag and Au are
17.75ppm, 4.3ppm and 0.05ppm respectively, which are in good agreement with the
reported values in the literature (Giacomo et al., 2016; Tawfik et al., 2008; Drogoff et
al., 2001; Ingle, 1988). The high value of LOD in copper is attributed to the energy
transfer between the elements within the matrix (Tawfik et al., 2008).
87
Figure 5.10: Calibration curves of copper and silver obtained by drawing the
normalized line intensities against concentrations.
The calibration curves for gold are drawn using the neutral gold lines at 479.26nm and
at 523.02nm. The error bars show the standard deviations for the measured signal
intensities.
88
5.8 COMPOSITIONAL ANALYSIS USING LASER ABLATION TIME OF
FLIGHT MASS SPECTROMETER (LA-TOFMS)
Compositional analysis of all the Karats of gold alloys are also performed using
a locally fabricated laser ablation Time of Flight Mass Spectrometer.
Figure 5.11: Laser Ablation Time of Flight Mass spectra of 24K, 22K, 20K, 19K and
18K gold alloys at 5mJ Laser pulse energy
89
The spectra of all the samples acquired on the LA-TOF MS using Nd: YAG laser (532
nm) at laser fluence in the range of 0.1-1J/cm-2
and at 2 KV channeltron voltage is
shown in Fig. 5.11. The peaks appeared at m/z = 63, 107 and at 197 correspond to
copper, silver and gold respectively. The isotopic peaks of Cu (Cu63
, Cu65
) and silver
(Ag107
, Ag109
) are not clear in this figure, although we resolved these isotopic peaks in
a separate experiment. The dominating peak at m/z = 197 corresponds to gold, the
peak heights of copper and silver appear are in accordance to their compositions in the
samples.
Figure 5.12: Enlarge spectra with Lorentz fit to the experimental data points of the
laser ablation time of flight mass spectra of gold alloy samples
In Fig. 5.12, we present enlarged spectra showing variations in the line intensities of
the mass spectra with different Karats of gold with Lorentz fit to the experimental data
points. As 24K comprise of 99.99% gold therefore, the time of flight mass spectrum
shows only one peak, appeared at m/z = 197. The relative mass composition of copper,
90
silver and gold has been carried out by using the integrated line intensities. Using this
technique, the composition of 18K gold alloy is obtained as 17% copper, 7.1% silver
and 75.9% gold. The 19K gold alloy consists of 7.78% copper, 13.58% silver, and
78.64% gold. The 20K gold alloy contains about 6.48% copper, 11.42% silver and
82% gold. The 22K gold alloy consists of 5.7% copper, 1.6% silver and 92.7% gold
whereas, 24K pure gold having 99.99% purity and consequently no other peak of any
element is detected.
Figure 5.13: Bar graph showing the compositional analysis of all Karats of gold by
CF-LIBS and LA-TOF-MS
In Fig:5.13 we present a bar graph showing variations in the compositional
analysis results using the calibration free laser induced breakdown spectroscopic
technique (CF-LIBS) and the laser ablation time of flight mass spectroscopic technique
for the analysis of the 18K, 19K, 20K and 22K Karats of gold samples. From the bar
graph it is obvious that the compositions of Au by both the techniques are in excellent
agreement with the certified composition. About 2% deviations from the certified
values have been noticed in these samples.
91
CHAPTER 6
LASER ABLATION STUDIES OF BRASS ALLOY USING LIBS
AND LA-TOF-MS
Major part of this chapter has been published in the journal, “Laser Physics”. This
article is also selected for the “2018 Highlights Collections” of the journal. In this
contribution the author has quantitatively analyzed brass alloy and compared the
compositional results obtained from LA-TOF-MS with LIBS based techniques.
In this chapter, we present quantitative analysis of brass alloy using LIBS,
EDX and LA-TOF-MS. The neutral emission lines of copper and zinc are used to
calculate plasma parameters. The electron temperature is estimated using the
Boltzmann plot as (10000 ± 1000) K and the electron number density is calculated as
(2.0 ± 0.5) ×1017
cm-3
from the Stark broadened neutral copper line as well as using the
Saha-Boltzmann equation. The elemental composition is deduced by self-calibration
free (SCF-LIBS) (70% Cu and 30% Zn), internal reference line self-absorption
correction IRSAC (63.36% Cu and 36.64% Zn), EDX (61.75% Cu and 38.25% Zn),
and LA-TOF-MS (62% Cu and 38% Zn), whereas, the certified composition is (62%
Cu and 38% Zn). It is observed that the IRSAC method yields analytical results
comparable to that of EDX and LA-TOF-MS.
LIBS is a non-destructive and fast technique to analyze even a much smaller
quantity of precious samples and there is no need for any specific sample preparation.
Due to these advantages, LIBS has been used in several fields especially agriculture,
material processing, in environmental pollution monitoring and in medical. Again
several studies on the compositional analysis of brass alloys have been reported in the
92
literature (Grifoni et al., 2016; Achouri et al., 2015; Andrade et al., 2010; Shaltout et
al., 2010; Sheikh et al., 2008 ). The main objectives of the present work are to exploit
the LIBS techniques for the compositional of a brass alloy having known composition
of Cu and Zn as 62% and 38% respectively, and to compare the extracted results with
the certified compositions as well as with that determined using established analytical
techniques such as LA-TOF-MS and EDX.
6.1 OPTICAL EMISSION STUDIES
The laser produced plasma is generated using same high power Q-switched Nd:
YAG Laser as discussed earlier. A quartz lens (convex) of 20 cm focal length is used
to focus the laser beam on the target sample at atmospheric pressure. The measured
diameter of the focused laser beam spot was (0.10 ± 0.01) cm; the focal spot area about
7.85x10-3
cm2 corresponds to a maximum laser fluence of 64 J/cm
2. However, the
emission spectra are recorded at varied fluence values from 1-50 J/cm2. The laser
energy is measured by an energy meter (Nova-Quantal, France). The emitted radiation
is captured by a set of four spectrometers (Avantes, Holand) each having 10 µm slit
width and covering the wavelength range of 250 - 870 nm. To correct the emission
signal, the dark signal is subtracted from the observed signal using the LIBS software.
The same brass alloy is also quantitatively analyzed by using EDX and Laser-ablation
Time of Flight Mass Spectrometer.
In Fig 6.1 we present the optical emission spectrum of the laser produced brass
alloy covering the spectral region 463 – 527 nm. All the observed lines are identified
as belonging to neutral copper and zinc. The three copper lines around 510 -520 nm
are due to the 3d10
4p 2P3/2 → 3d
9 4s
2 2D5/2 at 510.55 nm, 4d
2D3/2 → 4p
2P1/2 at 515.32
93
nm and 4d 2D5/2 → 4p
2P3/2 at 521.82 nm transitions. The triplet around 460 - 480 nm
is identified as transitions from the 4s5s 3S1 upper level to the 4s4p
3P0,1,2 lower levels
in zinc. Interestingly, the relative intensities of these zinc lines are in accordance with
that expected in the LS coupling; proportional to the statistical weights of the
terminating levels. The line at 465.11 nm is due to the transition from the 3d94s5s
4D7/2
upper level to the 3d94s5p
4F9/2 level in copper.
Figure 6.1: Optical emission spectrum of the laser produced brass plasma, covering
the spectral region 463 – 527 nm.
The first step to determine the elemental composition from the observed optical
emission spectrum of the laser produced brass plasma is to measure the relative line
intensities and then construct a Boltzmann plot. We have used the measured intensities
and the relevant spectroscopic parameters of the copper (Cu I) lines at 261.83 nm,
282.43 nm, 296.11 nm, 319.40 nm, 427.51 nm, 458.69 nm, 465.11 nm, 510.55 nm,
94
515.32 nm, 521.82 nm, 529.25 nm, 578.21 nm and that of Zn I at 277.08 nm, 328.23
nm, 330.26 nm, 472.21 nm and 481.053 nm to draw the Boltzmann plot using the
relation (Brogia et al., 2000):
ln (𝐼𝜆
ℎ𝑐𝐴𝑘𝑔𝑘) = −
𝐸𝑖
𝐾𝐵𝑇+ ln (
𝑁0
𝑈(𝑇)) 6.1
Where I is the intensity of the emission line, 𝜆 is the transition wavelength, h is the
Planks constant, c is the velocity of light, 𝐴𝑘 is the transition probability, 𝑔𝑘 is the
statistical weight of the upper level, 𝐸𝑖 is the energy of the upper level, 𝐾𝐵 is the
Boltzmann constant, T is the excitation temperature, 𝑁0 is the total number density and
𝑈(𝑇) is the partition function.
Figure 6. 2: Typical Boltzmann Plots to estimate the plasma temperatures from the Cu
I and Zn I spectral lines
A plot of ln (Iλ
hcAkgk) versus the upper level energies yields a straight line. The
excitation temperature is calculated from its slope; 1
KBT. The Boltzmann plots based on
95
the Cu I and Zn I lines are presented in Fig. 6.2. The plasma temperatures are
estimated as (10000±1000) K for Cu I and (8500±1000) K for Zn I. In order to
determine the electron number density, we selected a well isolated and Stark
broadened Cu I line at 465.11 nm. The Stark width ΔλFWHM of this line is determined
by de-convoluting the observed line profile as a Voigt profile, which takes into account
the instrumental width, the Doppler width and the Stark broadening.
Figure 6.3: Stark broadened line profile of copper line at 465.01 nm along with the
Voigt fit.
In Fig.6.3, we present the experimental data points as dots and the line passing
through the data points is the Voigt fit. Eq. 1.22 from the chapter 1 is utilized for the
calculation of electron number density. The value of the Stark broadening parameter ωs
is reported in the literature as 4.1×10-3
nm (Konjevic et al., 1990). From the FWHM of
the line profile, the electron number density is estimated as (2.0 ± 0.5) ×1017
cm-3
. The
error in the electron number density is due to uncertainties in determining the FWHM
and that in the Stark broadening parameter. In order to use the LIBS technique for the
96
quantitative analysis, it is mandatory that the laser produced plasma is optically thin
and holds the Local Thermodynamic Equilibrium (LTE) condition. The validity of the
optically thin plasma can be checked by comparing the observed line intensities of two
lines of the same element and in the same charge state with that calculated from the
known atomic parameters (Unnikrishnan et al., 2012):
𝐼1
𝐼2=
𝜆𝑛𝑚
𝜆𝑘𝑖
𝐴𝑘𝑖
𝐴𝑛𝑚
𝑔𝑘
𝑔𝑛exp [−
(𝐸𝑘−𝐸𝑛)
𝑘𝐵𝑇] 6.3
Here I1 and I2 are the intensities of the lines at wavelength 𝜆𝑘𝑖, 𝜆𝑛𝑚 with their
corresponding transition probabilities from the upper levels to the lower levels
𝐴𝑘𝑖 ,𝐴𝑛𝑚 respectively, 𝐸𝑘,and 𝑔𝑘 is the energy and statistical weight of the upper level
corresponding to intensity I1 and 𝐸𝑛, 𝑔𝑛 is the energy and statistical weight of the
upper level corresponding to intensity I2. The left hand side of the equation depends on
the experimentally observed line intensities while the right hand side contains atomic
parameters which are tabulated in the NIST data base (NIST data base, 2016). In order
to check the condition of optically thin plasma, spectral lines having a common upper
energy level or very close lying energy levels are selected to minimize the temperature
dependence. After inserting the atomic parameters and the experimentally measured
line intensities of Zn I at 636.23 nm and 334.50 nm in Eq. 6.3 it yields the values
0.538 and 0.582 respectively which are in good agreement, differ only by 7%. Such
calculations for another pairs of lines: Cu I at (427.51 nm, 465.11 nm), Cu II at
(268.93 nm, 271.35 nm) and Zn II (250.19 nm, 255.79 nm) are also agree within the
experimental errors. The difference in the calculated values is associated to the errors
in measuring the line intensities and that in the reported transition probabilities.
97
Besides, the triplet due to transitions from the 4s5s 3S1 upper level to the 4s4p
3P0,1,2
lower levels in zinc also possesses intensities according to their statistical weights.
Thus, it supports our assertion that the plasma can be considered as optically thin.
The condition of the local thermodynamic equilibrium has been validated using the
McWhirter Criteria (Cristoforetti et al., 2010; Cristoforetti et al., 2010). The lower
limit of the electron number density at an elevated temperature and energy difference
between the transitions is calculated using Eq. 1.6 as 2.0×1013
cm-3
which is much
lower than that (2.0 ± 0.5) × 1017
cm-3
calculated from the Stark broadened line profile
of copper line at 29.26 nm. Thus the plasma in the present studies can be considered
very close to LTE.
6.2 COMPOSITIONAL ANALYSIS USING SAC-LIBS AND IRSAC-LIBS
As we have established that the plasma is optically thin and also fulfils the
condition of LTE (see above), we therefore used the data for the qualitative analysis
based on the Calibration Free-LIBS and the Boltzmann Plot method. Here we have
used self-calibration free (SCF-LIBS) and internal reference line self-absorption
correction (IRSAC-LIBS) techniques for the compositional analysis as discussed in
chapter 3 section 3.3.2 and 3.3.3. As the laser produced plasma is close to LTE
condition, therefore the slops of the Boltzmann plots for each element are
approximately same and the intercepts differ according to the concentration of that
element in the sample. Boltzmann plots for Cu I and Zn I are presented in Fig. 6.2. The
concentrations of the neutral species are calculated from the intercepts along the Y-
axis. To calculate the concentration of the ionized species, the Saha-Boltzmann
98
equation is used that relates the number densities of the neutral as well as the singly
ionized species (Gomba et al., 2001) The procedure for SCF-LIBS is discussed in
chapter 3 yields the concentration of Cu as 70% and that of Zn as 30% containing
about 20% error. The results obtained by SCF-LIBS are compared with IRSAC-LIBS
technique. To correct the observed intensities of the spectral lines Sun et al., (2009)
proposed a relation to calculate the self-absorption coefficient using the following
relation as discussed in chapter 3. The self-absorption coefficient is calculated using
the lines of Cu I at 529.25 nm and Zn I at 328.23 nm. The intensities of the other lines
are corrected following the above mentioned procedure.
Figure 6.4: Typical Boltzmann Plots of copper and zinc after self-absorption
corrections to calculate the plasma temperatures.
The corrected line intensities are used to redraw the Boltzmann plots, shown in Fig
6.4. Some Cu I lines are not affected by self-absorption but the correction process
99
softly changed their intensities. On the other hand, the lines at 261.82 nm, 282.42 nm
are corrected by this process. Consequently, the data points are now more regular and
follow the fitted lines in the Boltzmann plot. After the intensity corrections by the
IRSAC method, the plasma temperatures calculated by the Boltzmann Plot (see Fig.
6.4) using the Cu I and Zn I lines approach comparable values. From the intercepts,
this method yields the concentration of Cu as 63.4% and that of Zn as 36.6% with
about 3% error. The results of the compositional analysis are more precise as
compared to that of the basic CF-LIBS.
6.3 QUANTITATIVE ANALYSIS USING LASER-ABLATION TIME OF
FLIGHT MASS SPECTROMETER (LA-TOF-MS)
Compositional analysis of the Cu-Zn alloy is also determined performed by the
Laser Ablation Time of Flight Mass Spectroscopy (LA-TOF-MS) and Energy
Dispersive X-ray Spectroscopy (EDX) techniques. The spectrum acquired with the
Time of Flight Mass Spectrometer, a one-meter linear system is shown in Fig. 6.5.
From the observed ion signals, the elemental composition has been determined by the
integrated line intensity as: Cu (62%) and Zn (38%). These values are in excellent
agreement with that of the certified compositions. The elemental analysis is also
achieved by the Energy Dispersive X-ray Spectroscopy (EDX). The analysis yields the
major elemental composition Cu (61.75%) and Zn (38.25%). A comparison of the
elemental compositions of the brass alloy, determined by the LIBS based techniques,
LA-TOFMS and EDX is presented in Table 6.1.
100
Figure 6.5: The mass spectrum of brass alloy measured by LA-TOF mass
spectrometer.
In SCF-LIBS, using the basic Boltzmann plot method without self-absorption
correction, the estimated error is greater than 10%. The IRSAC yields results of
compositional analysis comparable with that of LA-TOF-MS and EDX (error within
2%) as well as with the actual composition.
Table 6.1: Quantitative results for the copper–zinc based brass alloy
Element
(Certified)
LA-TOF-MS EDX
Weight (%) Relative standard error
SCF-LIBS IRSAC SCF-LIBS IRSAC
Copper (62%) 62% 61.75% 70% 63.36% 12% 2%
Zinc (38%) 38% 38.25% 30% 36.64% 21% 4%
101
In Fig: 6.6, we show a comparison of the quantitative analysis results of the
Laser Ablation Time of Flight Mass Spectroscopy, Calibration Free LIBS using basic
Boltzmann Plot method, the Internal Reference Line Self Absorption Correction
method and Energy Dispersive X-Ray Spectroscopy. It is evident from the histogram
that the IRSAC technique makes the CF-LIBS technique more reliable after
considering the self-absorption effects.
Figure 6.6: A histogram of the results of the composition of the copper–zinc based
brass alloy acquired using different analytical techniques.
102
CHAPTER 7
A COMPARATIVE STUDY OF COPPER NICKLE ALLOY USING
LIBS, LA-TOF-MS, EDX AND XRF
Major part of this chapter has been published in the journal, “Laser and particle
Beams”. In this contribution the author has quantitatively analyzed Cu-Ni and compared
the compositional results obtained from LA-TOF-MS with LIBS, EDS and XRF.
In this chapter, the LASER induced breakdown spectroscopy (LIBS) has
been used for the quantitative analysis of Cu-Ni Alloy of known composition (75%
Cu, 25% Ni) using one line calibration free Laser Induced Breakdown Spectroscopy
(OLCF-LIBS), self-calibration free LIBS (SCF-LIBS), algorithm based calibration
free LIBS (AB-CF-LIBS), Laser Ablation Time of Flight Mass Spectroscopy (LA-
TOF-MS), Energy dispersive X-ray spectroscopy (EDX) X-ray fluorescence
spectroscopic (XRF) technique. The plasma is generated by focusing the beam of a Q-
switched Nd: YAG laser (532 nm, pulse energy about 200 mJ, 5 ns pulse duration)
while the sample is placed in air at an atmospheric pressure. Plasma temperature about
(9500 ± 300) K is calculated by the Boltzmann plot method using the neutral lines of
Cu and Ni whereas the electron number density is calculated (2.0 ± 0.5)×1016
cm-3
from the Stark broadening of an isolated Cu line as well as using the relative intensities
of the neutral and singly ionized optically thin lines in the Saha-Boltzmann equation.
The elemental compositions have been determined by different techniques; OLCF-
LIBS (69% Cu and 31% Ni), SCF-LIBS (72% Cu and 28% Ni), AB-CF-LIBS (74%
Cu and 26% Ni), TOF (74% Cu and 26% Ni), EDX (75% Cu and 24.5% Ni) and XRF
(73% Cu and 24.7% Ni). It is demonstrated that the CF-LIBS technique gives
103
compositions comparable to that determined by LA-TOF, EDX or XRF which is also
in agreement with the certified reported composition. Copper (Cu) and nickel (Ni) are
adjacent elements in the Period Table. The Cu-Ni alloys are highly resistant to
corrosion therefore it is used in marine applications. A typical Cu-Ni alloy with 75%
copper and 25% nickel is used in new strewn coins. The main objectives of the present
work are to exploit the LIBS techniques for the quantitative analysis of the Cu-Ni alloy
which is used to make the Pakistani five rupee coin (2004) and to compare it with the
certified composition.
7.1 EMISSION STUDIES
The plasma on the surface of the sample is generated by focusing the beam of a
Nd: YAG laser at 532 nm, pulse energy 130mJ. As soon as the plasma is generated,
the plasma plume expands perpendicular to the target surface and after a few micro
seconds, it cools down.
Figure 7.1: Optical emission spectrum of the Laser produced Cu-Ni alloy plasma
covering the spectral region 295- 307 nm. The spectral lines of Cu-I and Ni I are
assigned in the blue and red colour respectively.
104
Figure 7.2: Optical emission spectrum of the Laser produced Cu-Ni alloy plasma
covering the spectral region 350 – 475 nm.
Figure 7.3: Optical emission spectrum of the Laser produced Cu-Ni alloy plasma
covering the spectral region 506 – 579 nm.
The emission form the plasma plume contains characteristic spectral lines of
the constituent elements. The time delay of 2 µs between the laser pulse and the
105
detection system is opted to reduce the continuum contribution. In Figs. 7.1–7.3, the
emission spectra of the laser produced Cu-Ni alloy plasma are presented covering the
wavelength region from 200 to 700 nm. The major part of the Fig. 7.1-7.3 consists of
the spectral lines of copper and nickel as the alloy mainly contains these elements.
Besides a couple of lines attached to the singly ionize copper and nickel are also
observed.
7.2 DETERMINATION OF PLASMA TEMPERATURE
We have determined the plasma temperature from the relative intensities of the
emission lines of copper and nickel using the Boltzmann plot method (Cremers 2006;
Griem, 1997). The observed emission spectra contain spectral lines of Cu I at 296.11
nm, 450.93 nm, 453.96 nm, 458.69 nm, 510.55 nm, 515.32 nm, 570.02 nm, 578.21 nm
and 521.82 nm and that of Ni I at 490.44 nm, 300.25 nm, 301.20 nm, 305.08 nm,
339.30 nm, 344.62 nm, 345.85 nm, 346.16 nm and 356.64 nm which have been used to
construct the Boltzmann plot to extract the plasma temperature. We have selected the
optically thin lines in the Boltzmann plot that are free from self-absorption (Dong et
al., 2015; Sun et al., 2009). To validate the condition for optically thin plasma from
the observed emission spectrum, we used the experimentally observed intensity ratio
of various spectral lines of Cu I and Ni I and compared it with the ratio of their
transition probabilities (Unnikrishnan et al., 2012). Three transitions in Cu I are at
515.32nm and 521.82nm, 470.45 and 529.25, 510.55 and 515.32 and a pair of Ni II
lines at 254.66 and 251.163 were used to validate the condition of optically thin
plasma. The experimentally observed and the theoretically calculated values agree
106
within 10% uncertainty which supports the optically thin plasma assertion. The atomic
parameters of the selected lines are taken from the NIST database, (2016) that are
listed in Table-7.1.
Table 7.1: Spectroscopic parameters of copper and nickel lines taken from NIST
database.
𝛌(nm)
Transition Ak
(107s
-1)
Ek
(eV)
gk gi
Upper level Lower level
Cu
296.11
3d94s4p
2F7/2
3d94s
2 2D5/2
0.376
5.57
8
6
450.93 3d94s5s
4D1/2 3d
94s4p
4F3/2 2.75 7.99 2 4
453.97 3d94s5s
4D3/2 3d
94s4p
4F5/2 2.12 7.88 4 6
458.69 3d94s5s
4D5/2 3d
94s4p
4F7/2 3.20 7.80 6 8
510.55 3d10
4p 2P3/2 3d
94s
2 2D5/2 2.0 3.82 4 6
515.32 3d10
4d 2D3/2 3d
104p
2P1/2 6.0 6.19 4 2
521.82 3d10
4d 2D5/2 3d
104p
2P3/2 7.5 6.19 6 4
570.02 3d10
4p 2P3/2 3d
94s
2 2D3/2 0.024 3.82 4 4
578.21 3d10
4p 2P1/2 3d
94s
2 2D3/2 0.165 3.79 2 4
Ni
300.24 3d94s 3D3 3d
84s4p
3D3 8.0 4.15 7 7
301.20 3d94s
1D2 3d
84s4p
1D2 13.0 4.54 5 5
305.08
3d94s
3F4 3d
84s4p
3D3 6.0 4.09 9 7
339.30
3d94s
3D3 3d
94p
3F3 2.4 3.68 7 7
344.63 3d94s
3D2 3d
94p
3D2 4.4 3.71 5 5
345.85
3d94s
3F2 3d
94p
3D1 6.1 3.80 5 3
346.16
3d94s
5F4 3d
84s4p
3D3 2.7 3.61 9 7
356.64
3d94s
1D2 3d
94p
1D2 5.6 3.90 5 5
490.44
3d94p
2[1/2]1 3d
94d
3P2 6.2 6.07 3 5
107
The Boltzmann plots for copper and nickel are presented in Fig. 7.4. The plasma
temperatures have been extracted from the slopes of the straight lines, which yields the
value for copper as 9535 ± 500 K and for Nickel 9455 ± 500 K. The errors in the
deduced plasma temperatures mainly come from the uncertainties in the transition
probabilities and in the measurement of the line intensities. For the quantitative
analysis, we have used an average value of the plasma temperature 9500 ± 500 K.
Figure 7.4: Typical Boltzmann-Plots for estimating the plasma Temperature, emission
lines from singly ionized Cu and Ni are used for obtaining temperature.
7.3 DETERMINATION OF ELECTRON NUMBER DENSITY
One of the commonly used methods to calculate the electron number density is
from the measured Stark broadening of neutral or singly ionized spectral lines. The
electron number density (ne) is related to the full width at half maximum (FWHM) of
the Stark broadened line via the Eq. 1.22 discussed in Chapter 1.
108
Figure 7.5: Stark broadened profile of copper line at 510.55 nm along with the Voigt
fit FWHM 0.09 nm.
The Stark line widths S
FWHM of the spectral lines have been determined by
de-convoluting the observed line profiles as Voigt profile which takes into account the
instrumental width and Doppler broadening. The line profile of the optically thin line
of CuI at 510.55 nm is selected to calculate the electron number density using the
impact broadening parameter ω = 0.0139 nm listed in (Babina et al., 2003; Conjevic et
al., 1990). In Fig. 7.5 we show line experimental observed line profile, the dots, along
with the Voigt function fit, full line. The instrumental width of our spectrometer is
0.06 ± 0.01 nm and the Doppler width is estimated at an elevated temperature 9500 K
as 0.004 nm, which is very small and can be neglected. The electron number density is
calculated as (2.2 ± 0.5) ×1016
cm-3
.
109
7.4 NUMBER DENSITY USING SAHA-BOLTZMANN RELATION
The Saha-Boltzmann equation relates the number density of a particular
element in the two consecutive charged states Z and Z+1 as discussed in chapter 1.
The electron density is obtained using the intensity ratio of the neutral and singly
ionized spectral lines of Ni. The plasma temperature is taken as 0.82eV and the
ionization energy is 7.64 eV (Giacomo et al., 2001; Gomba et al., 2001). Substituting
the numerical values in Eq. 1.26, we have determined the value of ne from the two Ni
II lines at 251.09 nm and 254.66 nm whereas a number of neutral Ni I lines are used.
The estimated electron densities are determined in the range from 1.4 to 2.8 ×1016
cm-
3. However, an average value ne = (2.0 ± 0.3) × 10
16 has been used in the subsequent
calculations. The McWhirter criterion has also been validated for the Cu I line at
450.93 nm and Ni I line at 493.73 nm to check how close the plasma is to the local
thermodynamic equilibrium (LTE). The electron density is calculated as 1.1×1014
cm-3
which is much lower than that determined from the Stark broadened spectral lines of
copper. Thus our plasma is not far from LTE.
7.5 QUANTITATIVE ANALYSIS USING OL-CF-LIBS, SCF-LBS AND AB-
CF-LIBS TECHNIQUES
Here three calibrations free LIBS based techniques named OL-CF-LIBS, SCF-
LIBS and AB-CF-LIBS are utilized for the compositional analysis of Cu-Ni alloy
having certified composition. All these methods are discussed in the chapter 3 section
3.3.1, 3.3.2, and 3.3.4.
For OL-CF-LIIBS technique the partition functions of Cu and Ni are U(I) Cu =
3.93, U(II)Cu = 1.58 and U(I)Ni = 40.08, U(II)Ni = 18.48 (NIST database) are deduced at
110
an average value of the plasma temperature 0.82 eV. An average value of electron
density is deduced as: ne = (2.0 ± 0.5) × 1016
cm-3
. The concentration of neutral atoms
Cz is calculated from the Boltzmann plot equation and concentration of ionized atoms
Cz+1 is calculated using Saha–Boltzmann equation. Total concentration of Cu and Ni is
presented as: 𝐶𝑡𝑐𝑢 = 𝐶𝑧
𝐶𝑢 + 𝐶𝑧+1𝐶𝑢 , 𝐶𝑡
𝑁𝑖 = 𝐶𝑧𝑁𝑖 + 𝐶𝑧+1
𝑁𝑖 . To calculate the percentage
compositions, we used the following relations:
𝐶𝑁𝑖% = 𝑛𝑡𝑜𝑡
𝑁𝑖 ∗58.69
𝑛𝑡𝑜𝑡𝑁𝑖 ∗58.69+ 𝑛𝑡𝑜𝑡
𝐶𝑢 ∗63.54∗ 100 7.1
𝐶𝑁𝑖% = 𝑛𝑡𝑜𝑡
𝐶𝑢 ∗63.54
𝑛𝑡𝑜𝑡𝑁𝑖 ∗58.69+ 𝑛𝑡𝑜𝑡
𝐶𝑢 ∗63.54∗ 100 7.2
This procedure yields the concentration of Cu as 69% and that of Ni as 31% with about
6% error.
For the quantitative analysis of the Cu-Ni alloy the self-calibrated free (SCF-
LIBS) method is also used. Here the Boltzmann plots are drawn for each element Cu
and Ni separately. Initially the intercepts are determined from the Boltzmann plots of
Ni and Cu and an average value of the electron temperature is deduced as 0.82 eV as
shown in Fig.7.4. The neutral lines of Ni and Cu are used to estimate the FCIs values
for each species and the values of FCNiI
(or nNiI
) and FCCuI
(or nCuI
) are deduced. Due
to the insufficient number of observed lines of Ni II and Cu II, it was not possible to
draw the Boltzmann plot for the singly ionized species, separately. Thus, the Saha-
Boltzmann equation is used for estimating the values of F𝐶𝑁𝑖 𝐼𝐼(or nNiII
) and F𝐶𝐶𝑢 𝐼𝐼(or
nCuII
). Finally, by adopting the procedure discussed in chapter 3, the composition of the
Cu-Ni Alloy is estimated as Cu = 72% and Ni= 28% with about 3% error. The results
111
are listed in Table 7.2. The agreement between the derived and the actual
concentrations is evident.
Table 7.2: Quantitative calculation by self-calibration free (SCF-LIBS) method
To apply the algorithm based calibration free (AB-CF-LIBS) average values of
plasma temperature and electron number density are considered. The density ratios of
the species of the same element are calculated and the value of ne is obtained from the
Saha-Boltzmann equation. Four optically thin lines of Cu I and Ni II are used to
calculate the nCuI
/nNiII
ratio and an average value is deduced as 0.37 (see Table 7.3).
Table 7.3: The density ratio (ncu I/nNi-II) for the calibration free quantitative
analysis
Cu I
𝛌
I' (10-2
)
Cu
P(I)
Cu
Ek(eV)
Cu
Ni II
𝛌
I'(10-2
)
Ni II
P(II)
Ni II
Ek(eV)
Ni II T(eV)
Average
ncu I
/nNi-II
515.32 8.51 3.93 5.52 254.66 1.80 18.48 6.73 0.82
0.37 521.82 0.32 3.93 6.87 254.66 1.80 18.48 6.73 0.82
515.32 8.51 3.93 5.52 251.16 4.74 18.48 6.62 0.82
521.82 0.32 3.93 6.87 251.16 0.47 18.48 6.62 0.82
Parameters Cu Ni
qs 28.23 24.80
U(I) 3.93 40.08
U(II) 1.58 18.48
𝜒 7.726 7.640
ni 7.14×10
12 2.36×10
12
nii 5.40×10
13 2.28×10
13
nt 6.12×10
13 2.52×10
13
112
Substituting the average value of ne we obtained the experimental values of nNiII
/ nNiI
and nCuII
/ nCuI
as 9.10 and 7.14 respectively. To calculate the theoretical values of ne
and the ratio of number densities of the same elements nNiII
/ nNiI
and nCuII
/ nCuI
as well
as the ratio of the number densities of different elements ncu I
/nNi-II ,
we have used an
algorithm (Unnikrishnan et al., 2012; Gomba et al., 2001) MATLAB program. In brief
we have used the estimated plasma temperature T (eV), assumed initial values of nt,Cu ,
nt,Ni and ne in this algorithm. The algorithm is stopped where the theoretical value of ne
converges. Again using the converged value of ne, the converged values of nt,Cu , nt,Ni
are calculated.
In the next step converged values of ne, nt,Cu , nt,Ni are used to calculate the density
ratios of nNiII
/ nNiI
, nCuII
/ nCuI
and nCuI
/nNiII
. If these theoretical ratios do not match with
the experimentally found ratios then use the above converged value of ne and vary the
values of nt,Cu , nt,Ni until the theoretical value of ne and also the ratios nNiII
/ nNiI
, nCuII
/
nCuI
and nCuI
/nNiII
matches with the experimentally found values as shown in table 7.4.
In Table-7.4, we enlist the experimental as well as the theoretical values of these ratios
showing a good agreement.
Table 7.4: Comparison of the experimentally and theoretically values derived at
0.82 eV plasma temperature.
Experimental values Theoretical values
ne (cm-3
) 2.00×1016
2.04×1016
NNiII
/nNiI
9.1 8.93
NCuI
/nNiII
0.37 0.36
NCuII
/nCuI
7.1 7.0
113
The optimized value of 𝑛𝑡𝑜𝑡𝑁𝑖 = 1.0 × 1016 and 𝑛𝑡𝑜𝑡
𝐶𝑢 = 2.6 × 1016 are deduced. Using
these density values, the weighted concentrations are estimated as: CNi
= 26% and CCu
= 74% with 1% error.
7.6 QUANTITATIVE ANALYSIS BY LA-TOF-MS, EDX AND XRF
Composition of the Cu-Ni alloy is also determined by the Laser ablation/
ionization Time of Flight Mass Spectroscopy, Energy dispersive X-ray Spectroscopy
(EDX) and by the X-ray Fluorescence (XRF) technique. The spectrum acquired with a
homemade one meter linear time of flight mass spectrometer is shown in Fig. 7.6.
From the observed ion signal, the elemental composition has been determined as: Cu
(74 ± 1%) and Ni (26 ± 1%). Incidentally, these values are in excellent agreement with
the certified compositions.
Figure 7.6: Time of Flight Mass Spectrum of the Cu-Ni alloy.
The Energy Dispersive X-ray (EDX) Spectrum of the Cu-Ni ally is reproduced
in Fig. 7.7. The presence of the constituent major elements in the sample is evident
showing Cu (75%) and Ni (24.5%). The analysis also yields the presence of a very
114
small amount of Mn (0.4%) and Si (0.1%) in the EDX spectrum which might be
impurities on the surface of the sample.
Figure 7.7: Energy Dispersive X-ray spectrum of the Cu-Ni alloy.
The elemental analysis has also been performed by the X-ray Fluorescenec
Spectroscopic Technique (XRF). The analysis yields the major elements present in the
sample with composition of Cu (73%) and Ni (24.7%).
In Table-7.5 we enlist a comparison of the elemental compositions of the Cu-Ni alloy,
which has been used to make the Pakistani five rupee coin, determined by all the five
techniques; OLCF-LIBS, SC-LIBS, CF-LIBS, LA-TOF, EDX and XRF. The errors in
the measured elemental compositions using LA-TOF, EDX, XRF and CF-LIBS are
comparable (within 2%).
115
Table 7.5: Compositional analysis using different techniques.
Composition OLCF-LIBS SCF-LIBS AB-CF-LIBS EDX XRF LA-TOF-MS
Cu% 69 72 74 75 73 74
Ni% 31 28 26 24.5 24.7 26
To summarize all the dada analyses, we present a comparison of different techniques
in the form of a histogram in Fig. 7.8
Figure 7.8: Histogram across different techniques vs composition of Cu-Ni alloy
116
CHAPTER 8
ON THE ELEMENTAL ANALYSIS OF DIFFERENT CIGARETTE
BRANDS USING LIBS LA-TOF-MS
Major part of this chapter has been published in the journal, “Spectrochimica Acta
Part B”. In this contribution the author has quantitatively analyzed different cigarette
brands available in Pakistan using LA-TOF-MS and LIBS.
In this chapter, we present qualitative and quantitative analysis of the major
and trace elements present in different brands of tobacco available in Pakistan using
laser induced breakdown spectroscopy (LIBS) and Laser ablation Time of Flight Mass
Spectrometer (LA-TOFMS). The compositional analysis using calibration free LIBS
technique is based on the observed emission spectra of the laser produced plasma
plume whereas the elemental composition analysis using LA-TOFMS is based on the
mass spectra of the ions produced by laser ablation. The optical emission spectra of
these samples contain spectral lines of calcium, magnesium, sodium, potassium,
silicon, strontium, barium, lithium and aluminum with varying intensities. The
corresponding mass spectra of the elements are detected in LA-TOF-MS with their
composition concentration. The analysis of different brands of cigarettes demonstrates
that LIBS coupled with a LA-TOF-MS is a powerful technique for the elemental
analysis of the trace elements in any solid sample.
In Pakistan, a large percentage of people smoke and “Smoking is injurious to
health” is a slogan printed on every cigarette packet as an awareness campaign.
However, there is very little perception among the population about dangers of tobacco
consumption. The leaves of Tobacco plants contain trace elements being absorbed
117
from the local soil where it is cultivated. The use of Tobacco products are notoriously
known as a major cause of dangerous diseases such as cancer, cardiovascular and
mortality in the world. In the smoking process, some of the metal contents remain in
the ash as well as in the smoke. Harmful gasses in the tobacco smoke as well as the
metallic contents present in tobacco severely hamper the human health. Excessive
deficiency or imbalance of any metal in the human body is the major source of
diseases
8.1 OPTICAL EMISSION STUDIES
The primary focus of the present work is to determine the compositions of
metallic contents in different cigarette brands available in Pakistan using different
analytical techniques and to compare the compatibility of the determined elemental
compositions. For this purpose the plasma on the surface of the tobacco is generated
by focusing the beam of a Nd:YAG laser at 532 nm, pulse energy 130 mJ and 2 µs
delay between the laser pulse and the detection system. The laser energy is measured
by an energy meter (Nova-Quantel, France). A quartz lens (convex) of 10 cm focal
length is used to focus the laser beam on the target sample placed in air at an
atmospheric pressure. Four spectrometers (Avantes, Holand) each having the split
width of 10 µm in the detection system are utilized. In Figs. 8.1(a, b), the emission
spectra of the one cigarette brand (Kisan) is presented covering wavelength region
from 250 to 870 nm. The observed spectra are analyzed by identifying the spectral
lines of the constituent elements with the help of the NIST database, (2016). At least
two or three lines of each element are cross checked to confirm the presence of that
element in the sample.
118
Figure 8.1: Optical emission spectrum of the Laser produced Kisan Cigarette tobacco
plasma covering the spectral region 250- 870nm.
The major part of the Figs. 8.1(a,b) consists of the lines of calcium, magnesium,
sodium, potassium, silicon, strontium, barium, lithium and aluminum. The dominating
lines of Ca, Mg, and Na are present along with the traces of Li, Si, Al, K, Ba and Sr.
The emission lines of oxygen and nitrogen are also detected. In Fig 8.2 (a, b, c, d) we
119
show an enlarged sections of the observed spectra covering the wavelength ranges: 280
nm-324 nm, 400 nm-500 nm, 580 nm-590 nm and 650 nm-780 nm respectively.
120
Figure 8.2: Optical emission spectrum of the Laser produced tobacco plasma covering
the spectral region from (a) (280nm-324nm), (b) (400nm-4700nm), (c) (490nm-
590nm) and (d) (650nm-780nm).
In Fig. 8.3 we present the variations in the intensity of Cal I line at 527.03 nm
measured at different delay times from 0 to 40 µs between the laser fire and the
121
opening of the detection system. The observed line intensities decay exponentially and
the observed data points fit very well with an exponentially decaying function. The
profiles of the observed calcium line at different delay times are presented as an inset
in Fig. 8.3. We have calculated the time resolved spectrum from the time integrated
spectrum using the technique as described by (Ahmed et al., 2016).
Figure 8.3: Variation of Intensity of emission line of Ca I at 527.03nm at different
delay times between laser pulse and acquisition time of Kisan brand.
8.2 DETERMINATION OF PLASMA TEMPERATURE
The plasma temperature is estimated from the relative intensities of the
emission lines of Ca I and Ca II using the Saha-Boltzmann plot method, assuming that
the plasma is optically thin and also is close to the local thermo-dynamical equilibrium
(LTE) (McWhirter, 1965). This assertion is based on the selection of optically thin
122
lines in the observed spectra which are free from self-absorption (Sun et al., 2009).
The observed lines of Ca I at 458.15, 487.813nm, 558.197nm, 559.849nm, 585.745nm,
612.22nm, 616.956nm, 644.98nm, 647.16nm and 649.96m and that of Ca II at
315.88nm, 317.93nm, 370.60nm, 373.69nm, 849.80 and 854.21nm are used to
estimate the plasma temperature. The spectroscopic data of these lines are taken from
the NIST database, (2016) and listed in Table 8.1. Fig. 8.4 (a) shows the Boltzmann
plots for various cigarette brands based on the emission lines of Ca II.
Table-8.1: Spectroscopic parameters of the emission lines of Ca I and Ca II (NIST
data base, 2016) to construct the Boltzmann plot.
Wavelength
𝛌(nm)
Transition Ak
(107 s
-1)
Ek
(eV) gk
Upper level Lower level
Ca I
458.15 3p64s4f
3F2 3p
63d4s
3D2 0.35 5.23 5
487.81 3p64s4f
1F3 3p
63d4s
1D2 1.88 5.25 7
558.88 3p63d4p
3D3 3p
63d4s
3D3 4.90 4.74 7
559.85 3p63d4p
3D1 3p
63d4s
3D1 4.30 4.74 3
585.75 3p64p
21D2 3p
64s4p
1P1 6.60 5.05 5
612.22 3p64s5s
3S1 3p
64s4p
3P1 2.87 3.91 3
616.13 3p64s5p
3P2 3p
63d4s
3D2 0.33 4.53 5
644.98 3p63d4p
1D2 3p
63d4s
3D1 0.90 4.44 5
647.17 3p63d4p
3F3 3p
63d4s
3D3 0.59 4.44 7
649.38 3p63d4p
3F2 3p
63d4s
3D1 4.40 4.43 5
Ca II
315.887 3p64d
2D3/2 3p
64p
2P1/2 31.0 7.05 4
317.933 3p64d
2D5/2 3p
64p
2P3/2 36.0 7.05 6
370.603 3p65s
2S1/2 3p
64p
2P1/2 8.8 6.47 2
373.69 3p65s
2S1/2 3p
64p
2P3/2 17.0 6.47 2
849.802 3p64p
2P3/2 3p
63d
2D3/2 0.11 3.15 4
854.209 3p64p
2P3/2 3p
63d
2D5/2 0.99 3.15 4
123
The Saha-Boltzmann plot for one of the cigarette brand (Kisan) along with the
Boltzmann plots of Ca I and Ca II lines are shown in Fig. 8.4 (b). The average
temperatures for the Big tobacco, Small tobacco, Rope tobacco, Bridge, Capstan
original, Chance, Diplomat, Gold Flake, Gold Leaf Special, Gold Leaf, Gold Street,
Morven and Kisan are estimated as (8500±500)K, (9000±500)K, (8500±500)K,
(9000±500)K, (10000±500)K, (9500±500)K, (9000±500)K, (9000±500)K,
(10000±500)K, (10000±500)K, (9600±500)K, (10000±500)K and (9600±500)K
respectively. The average values of the plasma temperatures are used to estimate the
elemental composition of the trace elements present in the various tobacco brands.
Figure 8.4: (a) Boltzmann plots of all the tobacco brands using Ca II spectral lines. (b)
Shows the Saha Boltzmann plot for Ca along with an inset showing the Boltzmann
plots of Kisan cigarette brand.
8.3 DETERMINATION OF ELECTRON NUMBER DENSITY:
We validated the condition for the optically thin plasma from the emission
spectrum by comparing the observed intensity ratio with that calculated from the
known spectroscopic parameters that agree within 10% (Unnikrishnan et al., 2012). As
124
the plasma is established as optically thin, the electron number density is obtained
using the intensity ratios of the neutral and singly ionized lines of Mg and Ca using the
Saha-Boltzmann equation as discussed in the Chapter 1. The lines of Mg I at 383.82nm
and Mg II at 279.55nm and 280.27nm are used for the estimation of electron number
density. Similarly the neutral and singly ionized lines of Ca at 428.93, 429.89, 431.86
nm and 315.88, 370.6, 373.69, 854.20, 279.55, 280.27nm respectively are used to
calculate the number density. A good agreement among the number densities derived
from the Mg and Ca lines are observed. An average value of the number density has
been used to determine the compositions of the samples. In Fig. 8.5, we present the
electron number densities calculated for different tobacco brands. Interestingly, all the
derived number densities lie in the range (1.0 - 6.8) × 1017
cm-3
.
Figure 8.5: Bar graph showing the variation of number densities in the emission
spectra of different cigarette brands.
To validate whether the plasma is close to the local thermo-dynamical
equilibrium (LTE), the McWhirter’s criterion is used to determine the lower limit of
the number density. We have used the optically thin lines of Ca I, Ca II, Mg I and Mg
125
II having largest ∆E gap between the adjacent levels. For a stationary and
homogeneous plasma in which the collisional mechanism is dominating over the
radiative process, the lower limit of the electron density for which the plasma may
satisfy the LTE condition (Unnikrishnan et al., 2012) is calculated, using Eq. 1.6, in
the range of about 1014
cm-3
which is much lower than that of determined from the
Saha-Boltzmann equation which is about 1017
cm-3
. The above condition is necessary
but not sufficient to declare that the plasma follow LTE. Therefore, the condition of
LTE in inhomogeneous plasma is also validated using the emission lines of Ca and
Mg. The diffusion length is calculated using Eq. 1.7. The calculated characteristic
variation length “d” is found to be much larger than the diffusion length which is in
accordance with the required criteria 10𝐷𝜆<d . In the light of both above the
mentioned conditions it is concluded that the plasma is close to LTE.
8.4 COMPOSITIONAL ANALYSIS USING OL-CF-LIBS
After calculating the plasma temperature and the electron number density, the
one line calibration free laser induced breakdown spectroscopic method (OL-CF-
LIBS) is used to estimate the composition of elements in different cigarette brands as
discussed in the chapter 3 section 3.3.1. All the spectroscopic parameters used for the
compositional analysis are taken from the NIST database. Average values of electron
densities and plasma temperatures are used for the analysis. Adopting this procedure,
nine elements with different concentrations are detected in all the cigarette brands.
After calculating the concentrations of the metallic contents in all the cigarette brands,
126
the compositional analysis is carried out using the laser ablation time of flight mass
spectrometer to reconfirm the CF-LIBS results.
8.5 ELEMENTAL ANALYSIS BY LASER ABLATION TIME OF FLIGHT
MASS SPECTROMETER
Compositional analysis of the Pakistani Cigarette brands is also performed
using a locally fabricated Time of Flight Mass Spectroscopy (LA-TOFMS). Pallets of
the samples are prepared and the corresponding ion/mass spectra are recorded at
different laser energies. In Fig. 8.6 we show, as an example, only one of the spectra of
a cigarette brand. The peaks corresponding to Li, Mg, Al, Si, K, Na, Ca, Sr, and Ba are
quite evident. The dominating signals correspond to calcium which is also observed
quite strong in the emission spectra of the plasma plume. The relative abundance of the
metallic contents is determined using the integrated line intensities.
Figure 8.6: Laser Ablation Time of Flight Mass spectrum of Kisan Tobacco.
The results obtained by LA-TOFMS confirmed the quantitative results
obtained from CF-LIBS. The compositional results of all the cigarette brands obtained
127
by LA-TOFMS are in excellent agreement with that of CF-LIBS (within 2% error).
Therefore, an average value of the concentration of each element is considered that is
listed in Table 8.2.
Table 8.2: Average elemental composition of Pakistani Cigarette Brands
Sr.
No
Element Percent
Big
tobacco
small
tobacco
Rope
tobacco
Bridge
Capstan
original
Chance
Diplomat
Gold
Flake
Gold
Leaf
special
Gold
Leaf
Gold
Street
Morven
Kisan
1 Li (±2)% 1.1 0.3 1.0 1.3 1.4 1.6 1.7 1.6 1.7 1.4 1.7 2.3 1.7
2 Na (±2)% 1.7 3.0 2.1 1.6 2.6 3.1 3.0 2.8 3.3 2.2 3.0 4.5 1.7
3 Mg (±2)% 13.6 17.8 21.2 14.1 23.9 20.1 15.9 20.0 20.2 17.8 24.7 20.1 16.5
4 Al (±2)% 0.2 0.7 0.3 0.6 1.5 1.1 1.0 1.1 0.9 1.4 0.5 1.2 1.3
5 Si (±2)% 0.5 1.3 0.6 0.8 2.5 3.1 1.6 1.9 1.7 2.6 0.8 3.0 1.6
6 K (±2)% 35.6 36.4 47.5 24.5 5.4 6.8 21.6 9.4 8.3 26.2 7.1 14.7 11.9
7 Ca (±2)% 45.9 39.2 26.3 54.9 61.1 63.8 53.6 60.6 63.1 46.5 60.9 52.8 62.9
8 Sr (±2)% 0.1 0.1 0.1 0.3 0.2 0.0 0.1 0.3 0.1 0.2 0.0 0.1 0.4
9 Ba (±2)% 1.3 1.2 0.8 1.9 1.5 0.4 1.5 2.2 0.6 1.6 1.4 1.2 2.1
In Fig. 8.7 we present a bar chart of the cigarette brands versus the average
composition percentage (%) showing variations of the concentrations of the detected
elements in different tobacco brands. From Table: 8.2 and Fig. 8.7, it is clear that the
compositions of the health hazardous elements such as lithium, aluminum, silicon,
potassium, barium and strontium are high in some cigarette brands such as Morven,
Kisan, Gold Flake, Gold Leaf, Chance and Diplomat which may trigger dangerous
diseases. Metallic lithium reacts with oxygen and water vapor in air and makes lithium
hydroxide (Li-OH) and lithium nitride (Li3N). Both are potentially hazardous because
of their extremely corrosive nature. Excessive intake of Li can also cause hypertonia,
hypothermia, cyanosis and ECG changes. Aluminum and silicon are mostly found in
the animal and plant tissues and in the natural water everywhere. The excess of
128
aluminum and silicon in the human body causes cancer and Alzheimer’s disease
(Rondeau et al., 2000).
Potassium causes stomach upset, nausea, diarrhea, vomiting, intestinal gas, and other
side effects. Barium is relatively abundant in nature and is found in plants and animal
tissues. A daily intake of barium by human is about 750 µg mainly coming from diet.
However, its surfeit results in perioral aresthesia and severe diarrhea and can cause
troubled breathing, tiredness or weakness and stiffness (Rondeau et al., 2000; Harvey
et al., 1999; Goyer et al., 1996; Morosashi et al., 1994).
Figure 8.7: Bar graph showing the compositions of metals in different cigarette
brands.
129
CHAPTER 9
SUMMARY
9.1 CONCLUSIONS
First aim of this study was to design and fabricate a linear time of flight mass
spectrometer coupled with a Q-switched Nd-YAG laser at 532 nm. Second aim was to
compare the results obtained from different calibration free LIBS techniques with that
of laser ablation time of flight mass spectrometer (LA-TOF-MS). We have
successfully designed and fabricated a modified version of a linear time of flight mass
spectrometer which yields improved mass spectra of different isotopes. There are two
problems with linear time of flight mass spectrometers regarding the resolution. The
problem related to the spatial distribution of ions along the axis of the flight tube was
minimized by introducing a multistage higher accelerating voltages, whereas, the time
lagging due to different directions of motion of the charged particles was removed by
inserting a magnetic lens after the extraction region. It is observed that the accelerating
voltage must be adjusted much larger than the initial kinetic energies of the ions. The
mass resolution can be further improved by using low laser fluences. After improving
the resolution, two isotopes of lithium, two isotopes of copper, eight isotopes of
cadmium and four isotopes of lead have been resolved in accordance to their natural
abundances, reflecting a good performance of our locally developed system. The
compositional analysis of a brass alloy was performed revealing good agreement with
the certified composition (62% Cu, 38% Zn).
130
This thesis is organized in eight chapters; first and second chapter is related to
the introduction and review of the literature. In the third chapter we have presented
details about the techniques used for compositional analysis. In the fourth chapter we
have discussed in detail about the improvement of resolution of linear time of flight
mass spectrometer and their results.
In Chapter 5 we have presented the internal reference line self-absorption
correction calibration free laser induced breakdown spectroscopy (IRSAC-LIBS)
combined with a laser ablation time of flight mass spectrometric technique (LA-TOF-
MS), for the quantitative analysis of different gold alloys that yields results with higher
accuracy and precision as compared to the other traditional methods. The other
traditional methods require additional information about the Karats prior to the
analysis. The conspicuous advantage of our technique is that there is no need to have
the preliminary information about the gold content of the sample or about the gold
Karats. The linearity of the calibration curves demonstrates a good agreement between
the gold Karats and their relative intensities. This method yields a good limit of
detection for gold about 0.05ppm, silver and copper as 4.3 ppm and 17.75 ppm
respectively.
In the sixth chapter we have reported the elemental composition of the copper-
zinc based brass alloy of certified compositions (62% Cu and 38% Zn) using the LIBS
techniques as well as with other established analytical techniques. The two LIBS
techniques; self-calibration free (SCF-LIBS) and an internal reference line self-
absorption correction method (IRSAC-LIBS) have been employed for the
compositional analysis. The errors estimated in the SCF-LIBS was around 10%
131
whereas, the IRSAC method takes in to account the self-absorption effect which
reduces the errors to about 5%. The compositions extracted using the Laser Ablation
linear Time of Flight Mass Spectrometer and EDX contain much smaller errors (about
1%) which are also in excellent agreement with that of the certified compositions.
In the second last chapter we have presented quantitative analysis of the Cu-Ni
alloy (Pakistani Five Rupee Coin of year 2004) using three LIBS based techniques and
three other standard analytical techniques. The one line calibration free OLCF-LIBS
method yield results containing about 6 % error. The self-calibration free (SCF-LIBS)
method which is based on the Boltzmann Plot method contains about 3% error. The
algorithm based (AB-CF-LIBS) method contains about 2 % error. The elemental
analysis using the LA-TOF, EDX and XRF techniques yields much better results, with
1% error. The results of the CF-LIBS based methods are comparable with that of LA-
TOF, EDX and XRF revealing the importance of LIBS.
In the last chapter we present a detailed study using OLCF-LIBS technique and
LA-TOF-MS on the identification and compositional analysis of different Pakistani
cigarette brands. Results revealed that tobacco brands available in Pakistan mainly
contain of calcium, magnesium, sodium, potassium, silicon, strontium, barium, lithium
and aluminum. It is observed that the concentrations of Li, Al, Si, K, Ba and Sr are
high in Morven, Kisan, Gold Flake, Gold Leaf, Chance and Diplomat and their use can
trigger dangerous diseases. The addicted use of low quality brands (Morven, Kisan,
Gold Flake, Gold Leaf, Chance, Gold Leaf Special and Diplomat) is even more
dangerous which can cause serious diseases such as cancer, troubled breathing,
tiredness, weakness, stiffness and anesthesia.
132
From the present study it is demonstrated that algorithm based AB-CF-LIBS
(uncertainty about 1%) is one of the best analytical technique for the elemental
compositional analysis of samples having only two elements. It became complex for
the samples having more than two elements. Self-calibration free SCF-LIBS
(uncertainty about 8%) and internal reference line self-absorption correction (IRSAC-
LIBS) (uncertainty about 5%) techniques are the best techniques for the quantitative
analysis of samples having appropriate emission lines of the detected elements to draw
the Boltzmann plots for all the elements. One line calibration-free OL-CF-LIBS
(uncertainty about 7 %) technique can be applied for any type of the environmental or
industrially important samples as it requires only one optically thin spectral lines for
all the elements present in the emission spectra of the sample and there is no need to
draw the Boltzmann plots for all the elements. All these techniques are compared with
the LA-TOF-MS which validates the present methods for a fast and precise
determination of composition in environmental, industrially important samples and
gold in jewellery without any specific sample preparation. From the present study it is
concluded that LA-TOF-MS and CF-LIBS techniques are complementary to each
other and can be used efficiently for quality control of different cigarette brands, food
industry, mining industries, steel industries, soil filtering and crop yields.
133
9.2 FUTURE RECOMMENDATIONS
The present studies using LIBS and LA-TOF-MS can be extended further to
other environmental as well as industrial important samples. Resolution of the laser
ablation time of flight mass spectrometer (LA-TOF-MS) can be further enhanced using
long (two meter) flight tube and by using differential pumping. In this way this system
can also be capable for analyzing the biological samples. In the present studies, we
have performed all the experiments using a CCD detector that is why we could do only
the time integrated and specially resolved experiments. However, compositional
results may be improved using ICCD detector by performing the time resolved
experiments and by studying the plasma dynamics. Due to our budgetary constraints
we were unable to purchase the ICCD detector and differential pumped LA-TOF-MS
system which we intend to acquire in the future.
134
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