A Fundamental Study of Laser-Induced Breakdown

Spectroscopy Using Fiber Optics for Remote

Measurements of Trace Metals

Scott R. Goode and S. Michael AngelDepartment of Chemistry and Biochemistry

University of South Carolina

• Approach– Fiber optic technology– Wavelength resolution– Time resolution

• Accomplishments– Two operating instruments– Examining surface morphology– Studying matrix effects

• Future– Solutions and slurries

LIBS for Elemental Analysis

Laser-Induced Breakdown Spectroscopy

• Use laser to vaporize sample

• Laser electric field high enough to cause breakdown

• Monitor emission

• Fiber optics afford capability for remote analysis

Limiting Factor

• Discriminating analyte atomic emission from continuum background emission limits the analysis

– Time

– Wavelength

Time-Resolved LIBS Apparatus

Pulsed Lasermirror

focusinglens

Spectrographplasma

collectionlens

intensifieddetector

TimingControl

1064 nm

Pulsed laserLens

Delaygenerator

Controller

Detector

Computer

Lasertrigger

Spectrograph

Lens

Fiber-opticLIBS probe

Fiber-Optic LIBS System Configuration

SampleFocusing lens

Excitation Fiber

Collection Fiber

f/2 Lens Plasma

Fiber-Optic LIBS Probe Design

Lead in Paint Using Fiber-Optic LIBS Probe

Wavelength (nm)

Ti Ti Ti

140012001000

800

600

400

200

0406.0404.0402.0400.0398.0

Pb

Solder

Leaded Paint

Unleaded Paint

Inte

nsity

Leaded Paint Calibration Using Fiber-Optic Probe

200

150

100

50

0

Inte

nsity

0.100.080.060.040.020.00

Concentration of Lead (% w/w, Dry Basis)

L.O.D.= 0.014% Pb (wt/wt) Dry Basis

- 4 mJ/pulse, 2 Hz, 532 nm laser, avg. 5 replicate spectra

Fiber-Optic Transmission1201101009080706050403020100

Pow

er O

ut o

f fib

er (m

J)

150140130120110100908070605040302010Power into Fiber (mJ)

fiber breakdown

1 mm silica-clad 1 mm hard-clad800 m hard-clad600 m hard-clad

imaging fiberHe:Ne

Nd:YAG

Ar+

pellicle f/8

probe

b&w CCD

6x macrolens 10x imaging fiber

framegrabber

excitationfiber

ICCD

LIBS/Ramancollection

fiber

monitor

pulser

controller spectrograph

f/7 lens

10ximaging ex. w/GRIN

spectral excit.

Imaged region

Imaging fiber

GRIN lens

Filtered Ramanexcitation fiber(514.5 nm)

LIBS excitationfiber (1064 nm)(632 nm pointer)

Collection fiber(filtered for Raman)

Region of interest

Sample

Videocamera

Inte

nsity

16 x103

Inte

nsity

35x103

25

15

5

420416412408404

FeFe

Fe

Ca

FeFe

Fe

b

14

10

6

2

420416412408404

Sr Ca

Sr

d

Wavelength (nm)

5 mm

Region of InterestWavelength (nm)

a

c

1000800600400200

Darkfield image of TiO2 and Sr(NO3)2

on soil

Raman spectrum of Sr(NO3)2

Raman spectrum of TiO2200x103

150

100

50

0

Inte

nsity

Wavenumber (cm-1)

200x103

150

100

50

1600140012001000800Wavenumber (cm-1)

Inte

nsity

a

c

b

TiO2 @190 cm-1

Darkfield image of TiO2 and Sr(NO3)2 on soil

Raman Images

Sr(NO3 ) 2 @1055cm-1

a

c

b

Plasma Temperature Profile2500

0384382380378376374372370368366

Graph 7 (top of plasma)

2500

0384382380378376374372370368366

Graph 6

Graph 52500

0384382380378376374372370368366

Graph 22500

0384382380378376374372370368366

Graph 1 (bottom of plasma)2500

0384382380378376374372370368366

Graph 3

Graph 42500

0384382380378376374372370368366

2500

3843823803783763743723703683660

7

6

5

4

3

2

1

Observed plasm

a region

70006000Plasma temperature (K)

Top

Bottom

Reg

ions

7

6

5

4

3

2

1

LIBS Imaging Spectrometer

sample

ICCD

lens

beam stopAOTF

RF generator

collimating lens

plasma

laser

1064 nmmirror

1064 nmmirror

Background Subtracted

722.8 nm LeadEmission + Continuum

715.2 nmContinuumBackground

2 .64 mm

Repetition Rate: 2 Hz, 2000 Shots, 2.5 s Delay

Background Subtracted Lead Emission

Temporal Dependence of Lead Emission

50 ns 675 ns 1. 3 s 1. 9 s 2. 5 s

Pb emission at 722.8 nm

Continuum background

Background subtracted

2.5 mm

2.5 mm

Lead Crater Depth and Plasma Height

0.38 mm0.38 mm 0.50 mm

0.63mm1.42mm

2.75 mm

100 shots 2400 shots960 shots

Plasma Height vs. Number of Laser Shots

2500

2000

1500

1000

200015001000500Number of Laser Shots

Plas

ma

Heig

ht (m

icron

s)

2.5 s delay1.0 s delay

Rep Rate: 2 Hz

Using High Wavelength Resolution

If the major source of noise is the continuum background

– Eliminate the background by time resolution

– Use wavelength resolution to distinguish the atomic lines from the continuum background

Echelle Spectrometer

Matrix effects

• Use binary alloy (brass samples)

• Examine signals from zinc (volatile) and copper (nonvolatile)

• Vary laser power

• Vary focal depth

Studying selective volatilization

• Measure zinc and copper emission from brass standards

• Perform measurements while varying laser power (Q-switch delay)

• See if ratio is independent of power and proportional to concentration

Effect of Laser Power2.86% Zn

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

105 115 125 135 145 155 165 175 185 195

Q-switch delay/ s

Zn/C

u em

issi

on r

atio

5/5/985/14/98

Effect of Laser Power4.18 % Zn

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

105 115 125 135 145 155 165 175 185 195

Q-switch delay/ s

Zn/C

u em

issi

on ra

tio

5/5/985/8/98

Effect of Laser Power24.8 % Zn

0.40.6

0.81

1.2

1.41.61.8

2

105 115 125 135 145 155 165 175 185 195

Q-switch delay/ s

Zn/C

u em

issi

on ra

tio

5/7/985/8/98

Effect of Laser Power34.6 % Zn

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

2.2

2.3

2.4

105 115 125 135 145 155 165 175 185 195

Q-switch delay/s

Cu/

Zn e

mis

sion

ratio

5/7/985/8/98

Effect of Laser Power39.7 % Zn

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

105 115 125 135 145 155 165 175 185 195

Q-switch delay/ s

Zn/C

u em

issi

on ra

tio

5/7/985/8/98

Calibration Plot

Brass CRMs

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8

Zn/Cu Concentration Ratio

Zn/C

u E

mis

sion

Rat

io

110 s delay

180 s delay

Effect of focus

• Measure Zn-to-Cu emission ratio

– As a function of composition

– As a function of focal point

• Negative: focal point below surface

• Zero: at surface

• Positive: above surface

Zn-to-Cu ratio as a function of focal point 2.86% Zn

0.14

0.16

0.18

0.2

0.22

0.24

0.26

0.28

-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11

z/ mm

Zn/C

u em

issi

on ra

tio

Zn-to-Cu ratio as a function of focal point 4.18 % Zn

0.2

0.22

0.24

0.26

0.28

0.3

0.32

0.34

0.36

0.38

-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11

z/ mm

Zn/C

u em

issi

on ra

tio

Zn-to-Cu ratio as a function of focal point 8.48 % Zn

0.3

0.32

0.34

0.36

0.38

0.4

0.42

0.44

0.46

0.48

-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11

z/ mm

Zn/C

u em

issi

on ra

tio

Zn-to-Cu ratio as a function of focal point 24.8 % Zn

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11

z/ mm

Zn/C

u em

issi

on ra

tio

Zn-to-Cu ratio as a function of focal point 34.6 % Zn

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11

z/ mm

Zn/C

u em

issi

on ra

tio

Zn-to-Cu ratio as a function of focal point 39.7 % Zn

0.95

1.05

1.15

1.25

1.35

1.45

1.55

1.65

-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11

z/ mm

Zn/C

u em

issi

on ra

tio

Conclusions

• LIBS is more complex than originally thought.

• Much of the data are consistent with a low-power heating mechanism and a high power dielectric vaporization mechanism.

• Can design experiments to decouple excitation and vaporization.

Segregate excitation effects from vaporization effects

• Brass samples, known composition

• Laser ablation into solution

• Dissolution

• Chemical analysis by ICP-MS

• Determine if materials vaporized in proportion to concentration

• Determine factors that affect selective and nonselective vaporization

Spectrometer

• High Spectral Resolution (7500)

• High Time Resolution (5 ns)

• Delivery?

Alternative Excitation

• Use laser system to vaporize solid sample.

• Direct vapor into microwave-excited plasma.

• Use emission from microwave plasma for chemical analysis.

Pulsed Nd:YAG

Controller

1064nmmirror

plasmasample

Pulsed Nd:YAG

TimingControl

Spectrograph

lens

ICCD

lens

Pulser

Optical Fiber

Colinear Dual-Pulse LIBS Configuration

25x103

20

15

10

5

Inte

nsity

(arb

uni

ts)

530525520515510505500

Wavelength (nm)

0 s between lasers

1 s between lasers

1064 nm Laser 1 (100 mJ) Laser 2 (180 mJ)

Colinear Dual-Pulse LIBS Enhancement for Copper

Sign

al-to

-Bkg

Optimum Delay Between Lasers for Copper Enhancement

16

14

12

10

8

6

4

2

5004003002001000

Time Between Lasers (s)

Laser 1 = 100 mJLaser 2 = 180 mJ

Colinear Dual-Pulse LIBS

0.38 mm

20 s T

Cu S/B 15

0.38 mm

1 s T

Cu S/B 14

0.38 mm

0 s T

Cu S/B 3

Copper Craters from Colinear Dual-Pulse LIBS

100

Optimum Timing Between Lasers for Lead Enhancement

4.0

3.5

3.0

2.5

806040200

Pb S

BR

Time Between Lasers (s) T

Colinear Dual-Pulse LIBS

Comparison of Lead Craters (colinear geometry)

0.60 mm 0.60 mm

Zero s T One s T

Pb S/B 6Pb S/B 2.5

Orthogonal Dual-Pulse LIBS

Orthogonal Dual-Pulse LIBS

Controller

Nd:YAG

plasma

Nd:YAG

TimingControl Spectrograph

ICCD

Pulser

10

8

6

4

2

0

Inte

nsity

530525520515510505500Wavelength (nm)

0 s between lasers -1 s between lasers

Orthogonal Dual-Pulse LIBS Enhancement for Cu

14

12

10

8

6

4

2

0

Cu

Sig

-to-b

kg

-5 -4 -3 -2 -1 0Time between lasers (s)

Enhancement of Copper Emission Using Non-Ablating Prespark

150 m 150 m 176 m

Orthogonal Dual-Pulse LIBS GeometrySEM Craters for Copper

2.86% Zinc at Low Power

36.4120.8

144.456.3 141.2

2.86% Zinc at High Power

86.6

259.9111.8

110.3

4.18% Zinc at Low Power

88.9133.9

124.9101.2

90.5

95.0

4.18% Zinc at High Power

89.1 91.0

97.8

60.6

93.8

71.7

57.8

24.8% Zinc at Low Power

130.0 7.862.0

75.4 88.0

24.8% Zinc at High Power

101.3

89.157.9

93.3

100.0106.7

100.8

35.6% Zinc at Low Power

70.9

92.5

90.2

79.1

101.6

34.6% Zinc at High Power

108.8 109.6

85.4

173.9126.3 119.6

119.1

84.4

34.6% Zinc at High Power Surface Effect

110.5

99.4

Targeted DOE Needs

• ID No: SR99-3025: Monitoring Technologies for Effectiveness of Solidification and Stabilization Systems

ID No: SR99-1003: Improvements to Physical, Chemical, and Radionuclide Quantification of Solid Waste

ID No: SR99-1004: Need for Continuous Emissions Monitors for Measurement of Hazardous Compound Concentrations in Incinerator Stack Gas

Targeted DOE Needs

ID No. RL-SS06 Improved, Real-Time, In-Situ Detection of Hexavalent Chromium in Groundwater

ID No. RL-DD038: Liquids Characterization for CDI

ID No. RL-SS15: Improved, In Situ Characterization to Determine the Extent of Soil Contamination of One or More of the Following Heavy Metals: Hexavalent Chromium, Mercury, and Lead