Session 3Session 3

Optical Spectroscopy:Optical Spectroscopy:Introduction/FundamentalsIntroduction/Fundamentals

Atomic and molecular Atomic and molecular spectroscopiesspectroscopiesInstrumentationInstrumentation

OverviewOverview

Physical basis of absorption and Physical basis of absorption and emissionemission Atomic spectraAtomic spectra Molecular spectraMolecular spectra

Instrumentation: components of Instrumentation: components of optical systems for spectrometersoptical systems for spectrometers

Common techniques in atomic Common techniques in atomic spectroscopy: AAS and ICP-OESspectroscopy: AAS and ICP-OES

CalibrationCalibration

33

Useful websites for Useful websites for spectroscopyspectroscopy

http://www.shsu.edu/~chm_tgc/sounhttp://www.shsu.edu/~chm_tgc/sounds/flashfiles/ICPwCCD.swfds/flashfiles/ICPwCCD.swf

http://www.thespectroscopynet.com/Ihttp://www.thespectroscopynet.com/Index.html?/ndex.html?/

http://http://teaching.shu.ac.uk/hwb/chemistry/tuteaching.shu.ac.uk/hwb/chemistry/tutorials/molspectorials/molspec//

http://www.chemguide.co.uk/http://www.chemguide.co.uk/analysis/uvvisiblemenu.htmlanalysis/uvvisiblemenu.htmlSee also individual citations on slides

44

Electromagnetic radiationElectromagnetic radiation

http://www.spectroscopynow.com/coi/cda/detail.cda?id=18411&type=EducationFeature&chId=7&page=1This primer also contains a wavelength-energy converter

E = h= hc

= frequency; = wavelength

SpectroscopySpectroscopy = interactions between light & matter = interactions between light & matter

55

FundamentalsFundamentals Absorption and emission of light by compounds is Absorption and emission of light by compounds is

generally associated with transitions of electrons generally associated with transitions of electrons between different energy levelsbetween different energy levels

E2

E1

E0

E = h= hc/

Emission: Sample (in an excited state) produces light/looses energy

Absorption: sample takes up energyConsumes light of appropriate wavelength

http://physics.nist.gov/PhysRefData/ASD/lines_form.html

Atomic spectra: line spectra provide specificity: each element Atomic spectra: line spectra provide specificity: each element has its own pattern, as each element has its own electronic has its own pattern, as each element has its own electronic configurationconfiguration

ground state

excited statesE2

E1

E2

E1

E0

E2

E1

66

FundamentalsFundamentals

The population of different states is The population of different states is given by the Boltzmann equation:given by the Boltzmann equation:

kTΔE

0

1

0

1 egg

NN

N0: number of atoms in ground stateN1: number of atoms in excited stateg1/g0 : weighting factors

Note: Equation contains temperature:Note: Equation contains temperature:

Excitation can be achieved by providing Excitation can be achieved by providing thermal energythermal energy

77

Atomic emission:Atomic emission:Flame spectroscopyFlame spectroscopy

ObservationObservation Caused by...Caused by...

Persistent golden-yellow Persistent golden-yellow flameflame

SodiumSodium

Violet (lilac) flameViolet (lilac) flame Potassium, cesiumPotassium, cesium

carmine-red flamecarmine-red flame LithiumLithium

Brick-red flameBrick-red flame CalciumCalcium

Crimson flameCrimson flame StrontiumStrontium

Yellowish-green flameYellowish-green flame barium, barium, molybdenummolybdenum

Green flameGreen flame Borates, copper, Borates, copper, thalliumthallium

Blue flame (wire slowly Blue flame (wire slowly corroded)corroded)

Lead, arsenic, Lead, arsenic, antimony, antimony, bismuth, copperbismuth, copper

Lithium

Cesium

SodiumQualitative method

88

A simple spectroscopeA simple spectroscope

Spectroscope: Device for Spectroscope: Device for qualitative assessment of a qualitative assessment of a samplesample

E.g. used in flame analysisE.g. used in flame analysis E.g. used in gemmologyE.g. used in gemmology

99

Atomic Spectroscopies - SynopsisAtomic Spectroscopies - Synopsis

Optical spectroscopies

Techniques for determining the elemental composition of an analyte by its electromagnetic or mass spectrum

Mass spectrometries

ICP-MS SIMSAASAES Fluoresc-

ence Spectros-

copy

Flame AAS GFAASICP-OESOthers

Seetable

Others(L. 6)

1010

Atomic spectroscopiesAtomic spectroscopies

TechniqueTechnique Atomisation/Atomisation/ExcitationExcitation

Sample etcSample etc

Arc/sparkArc/spark ee Electric arc/sparkElectric arc/spark Solid sample on carbon Solid sample on carbon electrodeelectrode

Laser microprobeLaser microprobe ee LaserLaser Solid sample on Solid sample on supportsupport

Glow dischargeGlow discharge ee Glow discharge Glow discharge lamplamp

Solid sample discSolid sample disc

ICP-OESICP-OES ee Electromagnetic Electromagnetic inductioninduction

Liquid sample, sprayed Liquid sample, sprayed into gas plasmainto gas plasma

Flame photometry Flame photometry (atomic emission)(atomic emission)

ee FlameFlame Liquid sample, sprayed Liquid sample, sprayed into flameinto flame

AASAAS aa UV/Vis lightUV/Vis light Liquid sample, sprayed Liquid sample, sprayed into flame or furnaceinto flame or furnace

Atomic fluorescenceAtomic fluorescence fefe UV/Vis lightUV/Vis light Liquid sample, sprayed Liquid sample, sprayed into flame or furnaceinto flame or furnace

X-ray fluorescenceX-ray fluorescence fefe X-radiationX-radiation Solid or liquidSolid or liquid

ICP-MSICP-MS -- n/an/a Liquid sample, sprayed Liquid sample, sprayed into gas plasmainto gas plasma

1111

Atomic spectroscopiesAtomic spectroscopies

Common principles:Common principles: Sample introduction: Sample introduction:

Nebulisation, EvaporationNebulisation, Evaporation Atomisation (and excitation or Atomisation (and excitation or

ionisation) by flame, furnace, ionisation) by flame, furnace, or plasmaor plasma

Spectrometer components:Spectrometer components: Light source (can be sample Light source (can be sample

itself - Only AA requires itself - Only AA requires external light sourceexternal light source

Optical system (or mass Optical system (or mass spectrometer)spectrometer)

DetectorDetector

1212

Atomic spectra vs molecular Atomic spectra vs molecular spectra:spectra:

LinesLines BandsBands

(nm)

Typical atomic spectrum Two typical molecular spectra

Y axes: intensity of absorbed light. Under ideal conditions proportional to analyte concentration (I c; Beer’s law).

e.g. acquired by AAS Acquired by UV-Vis spectroscopy

1313

Origin of bands in molecular Origin of bands in molecular spectraspectra

Molecules have chemical bondsMolecules have chemical bonds Electrons are in molecular orbitalsElectrons are in molecular orbitals Absorption of light causes electron Absorption of light causes electron

transitions between HOMO and LUMOtransitions between HOMO and LUMO Molecules undergo bond rotations and Molecules undergo bond rotations and

vibrations: different energy sub-states vibrations: different energy sub-states occupied at RT and accessible through occupied at RT and accessible through absorption: many transitions possible:absorption: many transitions possible:

A band is the sum of many linesA band is the sum of many lines

Vibrational substates rotational substates

HOMO

LUMO

1414

Quantitative analysis by molecular Quantitative analysis by molecular absorption: Colorimetryabsorption: Colorimetry

Because absorption spectroscopy is widely applicable, Because absorption spectroscopy is widely applicable, sensitive (10sensitive (10-5-5-10-10-7-7 M), selective, accurate (0.1-3% M), selective, accurate (0.1-3% typically), and easy:typically), and easy: 95% of quantitative 95% of quantitative

analyses in field of healthanalyses in field of health performed with UV/Vis tests performed with UV/Vis tests

Hemoglobin in bloodHemoglobin in blood

First step in analysis: establish working conditionsFirst step in analysis: establish working conditions Select Select Selection, cleaning and handling of cells Selection, cleaning and handling of cells Calibration: determine relationship between absorbance and Calibration: determine relationship between absorbance and

concentrationconcentration

1515

Instrument componentsInstrument componentsAAS Spectrometer

ICP-OES Spectrometer

Monochro-mator

Sample = light

sourceDetector

Read-out/Data system

Light Source

Monochro-mator

Sample DetectorRead-

out/Data system

Light Source

Monochro-mator

Sample DetectorRead-

out/Data system

UV-Vis Spectrometer:

1616

UV-Vis spectrophotometer (dual UV-Vis spectrophotometer (dual beam)beam)

Diffraction grating

Slit

Mirror

Light sources

Slit

http://www.spectroscopynow.com/coi/cda/detail.cda?id=18412&type=EducationFeature&chId=7&page=1

Filter

Mirror

Half-Mirror

Sample

Reference

Detector

Monochromator

1717

Example for a dual beam Example for a dual beam spectrometerspectrometer

1818

Single Single beambeam

1919

UV-Vis spectroscopyUV-Vis spectroscopypracticalities: Referencingpracticalities: Referencing

Matrix (solvent, buffer etc) might also Matrix (solvent, buffer etc) might also have absorbance: Must be taken care ofhave absorbance: Must be taken care of

In dual beam:In dual beam: Simultaneous measurement of reference Simultaneous measurement of reference

cell eliminates absorbance of backgroundcell eliminates absorbance of background Recording of baseline recommendedRecording of baseline recommended

Single beam:Single beam: Requires measurement of reference Requires measurement of reference

spectrum, can be subtracted from sample spectrum, can be subtracted from sample spectrumspectrum

Preferentially in same cuvettePreferentially in same cuvette

2020

Wavelength(nm) 100 200 400 700 2000 4000 7000 10,000 20,000 40,000

Spectral region

VAC UV Visible Near IR IR Far IR

Light sourcesLight sources

Con

tin

uu

mLin

e

Ar lampXe lamp

D2 lamp

Tungsten lamp

Nernst glower (ZrO2 + Y2O3)

Nichrome wire

Lasers

Hollow cathode lamps

Globar (SiC)

2121

Example of a continuum source:Example of a continuum source:Output from Tungsten lampOutput from Tungsten lamp

Widely applied in UV-Vis spectrometers

2222

Hollow cathode lampHollow cathode lamp Used in AASUsed in AAS Filled with Ne or Ar at a pressure of 130-700 Pa Filled with Ne or Ar at a pressure of 130-700 Pa

(1-5 Torr).(1-5 Torr). When high voltage is applied between anode When high voltage is applied between anode

and cathode, filler gas becomes ionisedand cathode, filler gas becomes ionised Positive ions accelerated toward cathodePositive ions accelerated toward cathode Strike cathode with enough energy to "sputter" Strike cathode with enough energy to "sputter"

metal atoms from the cathode metal atoms from the cathode to yield cloud with to yield cloud with excited atoms excited atoms

• Atoms emit line spectraAtoms emit line spectra

2323

Example: Output from iron Example: Output from iron hollow cathode lamphollow cathode lamp

Small portion of spectrum Small portion of spectrum from Fe hollow cathode from Fe hollow cathode lamplamp

Shows sharp lines Shows sharp lines characteristic of gaseous characteristic of gaseous atomsatoms

Linewidths are artificially Linewidths are artificially broadened by broadened by monochromator monochromator (bandwidth = 0.08 nm) (bandwidth = 0.08 nm)

2424

Wavelength selectors: Wavelength selectors: dispersive elements and filtersdispersive elements and filters

Wavelength(nm) 100 200 400 700 2000 4000 7000 10,000 20,000 40,000

Spectral region

VAC UV Visible Near IR IR Far IR

Fluorite prism

Fused silica or quartz prism

Glass prism

NaCl prism

KBr Prism

Interference filters

Interference wedgeGlass filters

Continuous

Discontinuous

Gratings3000 lines/nm 50 lines/nm

2525

MonochromatorsMonochromators Consist of Consist of

Entrance slitEntrance slit Collimating lens or mirrorCollimating lens or mirror Dispersion element (prism or grating)Dispersion element (prism or grating) Focusing lens or mirrorFocusing lens or mirror Exit slitExit slit

Czerny-Turner grating Czerny-Turner grating monochromator:monochromator: Mirrors

Common in UV-Vis spectrometers

2626

DispersersDispersers

Separate polychromatic light into its Separate polychromatic light into its componentscomponents PrismPrism Diffraction grating: Diffraction grating: patterned surface which patterned surface which

diffracts lightdiffracts light

Blazed diffraction grating

Holographic grating

Prisms

2727

Echellette grating:Echellette grating:

Extra pathlength travelled by wave 2 Extra pathlength travelled by wave 2 must be multiple of must be multiple of for positive for positive interference:interference:

nn = d(sin = d(sin ii + sin + sin rr) ) for UV 1000-2000 lines/mm: d = 0.5-1 for UV 1000-2000 lines/mm: d = 0.5-1

mm

echelle: French for ladder

2828

Bandwidth of a Bandwidth of a monochromatormonochromator

Spectral bandwidth: range Spectral bandwidth: range of wavelengths exiting the of wavelengths exiting the monochromatormonochromator

Related to dispersion and Related to dispersion and slit widthsslit widths

Defines resolution of Defines resolution of spectra: 2 features can spectra: 2 features can only be distinguished if only be distinguished if effective bandwidth is less effective bandwidth is less than half the difference than half the difference between the between the of features of features

2929

Effect of slit width on peak Effect of slit width on peak heightsheights

3030

Components of optical system Components of optical system in an ICP-OES spectrometerin an ICP-OES spectrometer

spherical and cylindrical lensesspherical and cylindrical lenses flat and spherical mirrorsflat and spherical mirrors parallel planesparallel planes optical path under vacuum or optical path under vacuum or

controlled nitrogen atmospherecontrolled nitrogen atmosphere(necessary for wavelengths <200 (necessary for wavelengths <200 nm; air absorbs far UV light)nm; air absorbs far UV light)

Disperser(s)Disperser(s)

3131

Old models: Sequential type

Can only measure one wavelength at a given time: Slow

3232

Newer: Simultaneous typeNewer: Simultaneous type

Echelle cross disperser (polychromator): Consists of Echelle grating and prisms/ echellette: separates lights in 2 dimensions

CCD detector: 2D detector

This combination allows high-speed measurement, providing information on all 72 measurable elements within 1 to 2 minutes

3333

DetectorsDetectors

Wavelength(nm) 100 200 400 700 2000 4000 7000 10,000 20,000 40,000

Spectral region

VAC UV Visible Near IR IR Far IR

Photographic plate

Photomultiplier

Photocell

Phototube

Silicon diode

Charge-coupled device (170-1000)

Photoconductor

Thermocouple

Golay pneumatic cell

Pyroelectric cell

Photon detectors

Thermal detectors

3434

Photomultiplier: detects one Photomultiplier: detects one wavelength at a timewavelength at a time

Based on Based on photoelectric effectphotoelectric effect

Photocathode and Photocathode and series of dynodes in series of dynodes in an evacuated glass an evacuated glass enclosureenclosure

Photons strike cathode and electrons are emittedPhotons strike cathode and electrons are emitted Electrons are accelerated towards a series of Electrons are accelerated towards a series of

dynodes by increasing voltagesdynodes by increasing voltages Additional electrons are generated at each Additional electrons are generated at each

dynode dynode Amplified signal is finally collected and measured Amplified signal is finally collected and measured

at anodeat anode

3535

Photodiode arrays: measure Photodiode arrays: measure several wavelengths at onceseveral wavelengths at once

linear array of discrete photodiodes on an integrated circuit linear array of discrete photodiodes on an integrated circuit (IC) chip(IC) chip

Photodiode: Consists of 2 semiconductors (n-type and p-Photodiode: Consists of 2 semiconductors (n-type and p-type)type) Light promotes electrons into conducting band: generates electron-Light promotes electrons into conducting band: generates electron-

hole pairhole pair ““Concentration” of these electron-hole pairs directly proportional to Concentration” of these electron-hole pairs directly proportional to

incident lightincident light a voltage bias is present and the concentration of light-induced a voltage bias is present and the concentration of light-induced

electron-hole pairs determines the current through semiconductor electron-hole pairs determines the current through semiconductor

3636

Detection in simultaneous ICP-Detection in simultaneous ICP-OES:OES:

http://www.chemistry.adelaide.edu.au/external/soc-rel/content/ccd.htm

CCD:Charge-coupled device

•Also integrated-circuit chip•Contains an array of capacitors that store charge when light creates electron-hole pairs

•Accumulated charge is read out at given time interval

•Each wavelength is detected at a different spot

•Much more sensitive than photodiode array detectors

3737

Lecture 4Lecture 4

AAS and ICP-OESAAS and ICP-OESSample preparationSample preparation

InterferencesInterferences

CalibrationCalibration

3838

Crucial steps in atomic Crucial steps in atomic spectroscopies and other methodsspectroscopies and other methods

Adapted from www.spectroscopynow.com (Gary Hieftje)

Solid/liquid sample Solution

Molecules in gas phase

Sample preparation

Nebulisation

Atomisation=Dissociation

Vaporisation

Desolvation

Atoms in gas phase

IonsExcited Atoms

Laser ablation etc.

Sputtering, etc.

ICP-MS and other MS methods

AAS and AES, X-ray methods

IonisationExcitation

M+ X-

MX(g)

M(g) + X(g)

M+

3939

Sample Introduction: liquid Sample Introduction: liquid samplessamples

Often the largest source of noise Often the largest source of noise Sample is carried into flame or plasma as Sample is carried into flame or plasma as

aerosol, vapour or fine powderaerosol, vapour or fine powder Liquid samples introduced using nebuliserLiquid samples introduced using nebuliser

4040

Sample preparation for Sample preparation for analysis in solution: Digestion analysis in solution: Digestion

Digestion in conc. HNODigestion in conc. HNO33 and mixtures and mixtures thereof (e.g. thereof (e.g. aqua regiaaqua regia))

BrBr22 or H or H22OO22 can be added to conc. acids can be added to conc. acids to give a more oxidising medium and to give a more oxidising medium and increase solubilityincrease solubility

Certain materials require digestion in Certain materials require digestion in conc. HFconc. HF

Common to use microwave digestionCommon to use microwave digestion

4141

Microwave digestionMicrowave digestion

Supplied with dedicated vessels (e.g. PTFE) Closed vessel digestion minimises sample contaminationFaster, more reproducible, and safer than conventional methods

Rotor

4242

Sample preparation and sample Sample preparation and sample handling for trace analysishandling for trace analysis

As always – sample preparation is keyAs always – sample preparation is key Ultra-trace: Contaminations introduced during Ultra-trace: Contaminations introduced during

sample processing can seriously limit sample processing can seriously limit performance characteristicsperformance characteristics

Points to consider:Points to consider: Purity of reagentsPurity of reagents Chemical inertness of reaction vessels and any other Chemical inertness of reaction vessels and any other

material samples come into contact withmaterial samples come into contact with Working environmentWorking environment

Preparation of standards and blanks crucialPreparation of standards and blanks crucial Also measure a “process blank”:Also measure a “process blank”:

Important for determination of LOD and LOQImportant for determination of LOD and LOQ

4343

Common Units in trace Common Units in trace analysisanalysis

ppm, ppb, ppt, ppq…..: parts per million ppm, ppb, ppt, ppq…..: parts per million etc.etc.

ppm: mg/kg; often also used as mg/Lppm: mg/kg; often also used as mg/L ppb: ppb: g/kgg/kg ppt: ng/kgppt: ng/kg ppq: pg/kg ppq: pg/kg

4444

Atomic absorption Atomic absorption spectroscopyspectroscopy

4545

Atomic Absorption SpectroscopyAtomic Absorption Spectroscopy Flame AAS has been the most widely used of all atomic Flame AAS has been the most widely used of all atomic

methods due to its simplicity, effectiveness and low costmethods due to its simplicity, effectiveness and low cost First introduced in 1955, commercially available since First introduced in 1955, commercially available since

19591959 Qualitative and quantitative analysis of >70 elementsQualitative and quantitative analysis of >70 elements

Quantitative: Can detect ppm, ppb or even lessQuantitative: Can detect ppm, ppb or even less Rapid, convenient, selective, inexpensiveRapid, convenient, selective, inexpensive

H

Li

Na

K

Rb

Cs

Fr

Be

Mg

Ca

Sr

Ba

Ra

Sc

Y

La

Ac

Ti

Zr

Hf

V

Nb

Ta

Cr

Mb

W

Mn

Tc

Re

Fe

Ru

Os

Co

Rh

Ir

Ni

Pd

Pt

Cu

Ag

Au

B

Al

Ga

In

Tl

C

Si

Ge

Sn

Pb

N

P

As

Sb

Bi

O

S

Se

Te

Po

F

Cl

Br

I

At

Ne

Ar

Kr

Xe

Rn

He

Zn

Cd

Hg

H

Li

Na

K

Rb

Cs

Fr

Be

Mg

Ca

Sr

Ba

Ra

Sc

Y

La

Ac

Ti

Zr

Hf

V

Nb

Ta

Cr

Mb

W

Mn

Tc

Re

Fe

Ru

Os

Co

Rh

Ir

Ni

Pd

Pt

Cu

Ag

Au

B

Al

Ga

In

Tl

C

Si

Ge

Sn

Pb

N

P

As

Sb

Bi

O

S

Se

Te

Po

F

Cl

Br

I

At

Ne

Ar

Kr

Xe

Rn

He

Zn

Cd

Hg

4646

Hollow cathode lamps with characteristic emissions

Hollow cathode lamps available for over 70 elementsHollow cathode lamps available for over 70 elementsCan get lamps containing > 1 element for determination Can get lamps containing > 1 element for determination of multiple speciesof multiple species

Nebuliser and Spray chamber

Flame fuelled by (e.g.) acetylene and air

Burner

Flame AA Spectrometer

4747

Schematic

Light Source Monochromator Detector Amplifier

E.g. Hollow cathode lamp

Analyte solution

Atomiser Fuel (e.g. acetylene)Air

I0 It

Nebuliser, spray chamber, and burner

4848

Flame atomisation:Flame atomisation:Laminar flow burner - componentsLaminar flow burner - components

NebuliserNebuliser:: converts sample solution into aerosol converts sample solution into aerosol Spray chamberSpray chamber:: Aerosol mixed with fuel, oxidant and Aerosol mixed with fuel, oxidant and

burned in 5-10 cm flameburned in 5-10 cm flame Fuel: Acetylene or nitrous oxideFuel: Acetylene or nitrous oxide Oxidant: Air or oxygen Oxidant: Air or oxygen Burner head:Burner head:

Laminar flow: quiet Laminar flow: quiet flame and long path-flame and long path-lengthlength

But: poor sensitivity But: poor sensitivity (not very efficient (not very efficient method, most of method, most of sample lost)sample lost)

from: Skoog

4949

Structure of a flameStructure of a flame

Relative size of regions varies with fuel, oxidant and their ratio

5050

Electrothermal atomisation: Electrothermal atomisation: GFAASGFAAS

Provides enhanced sensitivity Provides enhanced sensitivity entire sample atomised in very entire sample atomised in very

short timeshort time atoms in optical path for a second atoms in optical path for a second

or more or more (flame 10(flame 10-4-4

s) s)

Device: Device: Graphite furnaceGraphite furnace

5151

Sensitivity and detection Sensitivity and detection limits in AASlimits in AAS

Sensitivity:Sensitivity: number of ppm of an element to give number of ppm of an element to give 1% absorption.1% absorption.

Limit of detection:Limit of detection: dependent upon signal:noise dependent upon signal:noise ratio:ratio:

S/N Light intensity reaching detector

S/N=3.2

5252

Interferences in AASInterferences in AAS Broadening of a spectral line, which can occur due to Broadening of a spectral line, which can occur due to

a number of factors (a number of factors (PhysicalPhysical)) SpectralSpectral: emission line of another element or : emission line of another element or

compound, or generalcompound, or general background radiation background radiation from the from the flame, solvent, or analytical sampleflame, solvent, or analytical sample Background correction can be applied Background correction can be applied

ChemicalChemical: Formation of compounds that do not : Formation of compounds that do not dissociate in the flame dissociate in the flame

IonisationIonisation of the analyte can reduce the signal of the analyte can reduce the signal MatrixMatrix interferences due to differences between interferences due to differences between

surface tension and viscosity of test solutions and surface tension and viscosity of test solutions and standardsstandards

Another caveat: Non-linear response common in AASNon-linear response common in AAS

5353

Physical interferences:Physical interferences:Atomic line widths/ line shapesAtomic line widths/ line shapes

Very important in atomic spectroscopyVery important in atomic spectroscopy Narrow lines increase precision, Narrow lines increase precision,

decrease spectral interferencesdecrease spectral interferences Lines are broadened Lines are broadened

by several mechanisms:by several mechanisms: Natural broadeningNatural broadening Doppler effectDoppler effect Pressure broadening Pressure broadening

Figure taken from http://www.cem.msu.edu/~cem333/Week03.pdf

5454

Natural linewidthsNatural linewidths

Width of an atomic spectral line is Width of an atomic spectral line is determined by the lifetime of the determined by the lifetime of the excited stateexcited state

Consequence of the Heisenberg Consequence of the Heisenberg uncertainty principleuncertainty principle

For example, lifetime of 10For example, lifetime of 10-8-8 seconds seconds (10 ns) yields peak widths of 10(10 ns) yields peak widths of 10-5-5 nm nm

5555

Doppler EffectDoppler Effect

Due to rapid motion of atoms in gas phaseDue to rapid motion of atoms in gas phase Atom moving toward the detector absorbs / emits Atom moving toward the detector absorbs / emits

radiation radiation of shorter of shorter than atom moving perpendicular to than atom moving perpendicular to detector.detector.

Atom moving away from the detector absorbs / emits Atom moving away from the detector absorbs / emits radiation of longer radiation of longer : detector perceives fewer : detector perceives fewer oscillations oscillations

Photon detector

5656

Pressure broadeningPressure broadening Results from collisions of absorbing/emitting Results from collisions of absorbing/emitting

speciesspecies With analyte atoms or combustion products of fuelWith analyte atoms or combustion products of fuel Deactivates the excited state – shorter lifetime - Deactivates the excited state – shorter lifetime -

broader spectral linesbroader spectral lines Increases with concentration and temperature Increases with concentration and temperature E.g. in flame, Na absorbance lines broadened up to 10E.g. in flame, Na absorbance lines broadened up to 10--

33 nm. nm.

Doppler and pressure effects broaden atomic Doppler and pressure effects broaden atomic lines by 1-2 orders of magnitude as compared lines by 1-2 orders of magnitude as compared with their natural linewidthswith their natural linewidths

5757

Background correction in AASBackground correction in AAS particularly important in GFAASparticularly important in GFAAS UseUse beam chopper beam chopper to distinguish the signal due to to distinguish the signal due to

flame from desired atomic line at the same flame from desired atomic line at the same wavelength (old method)wavelength (old method)

Lamp and flame emissionreach detector

Only flame emissionreaches detector

Resulting signal

5858

Background correction in AASBackground correction in AAS

High energy Deuterium background High energy Deuterium background correctorcorrector

Deuterium lamp

Hollow cathode lamp

Beam combiner

Sample

Detector

Lamps are pulsed out of phase with each other

5959

Minimising the effect of Minimising the effect of Matrix InterferencesMatrix Interferences

The term "matrix" refers to the sum of all compositional The term "matrix" refers to the sum of all compositional characteristics of a solution, including its acid characteristics of a solution, including its acid compositioncomposition

Calibration standards Calibration standards and samples must be and samples must be matrix-matched matrix-matched in in terms of composition,terms of composition,total dissolved solids, total dissolved solids, and acid concentration and acid concentration of the solutionof the solution

Also advisable for Also advisable for ICP-OES and -MSICP-OES and -MS

Effect on K concentration on measured Sr

6060

Specialised applications in Specialised applications in AAS: Flameless cold vapour AAS: Flameless cold vapour

methodsmethods Mercury: has sufficient vapour pressure at RTMercury: has sufficient vapour pressure at RT Hydride generationHydride generation technique for technique for

determination of As, Sb, Bi, Se, Te, Ge, Pb, determination of As, Sb, Bi, Se, Te, Ge, Pb, and Sn and Sn Generation of volatile metal hydrides (As, Sb, Bi, Generation of volatile metal hydrides (As, Sb, Bi,

Se, Te, Ge, Pb, and Sn)Se, Te, Ge, Pb, and Sn) Reduction by NaBHReduction by NaBH44 to form volatile hydride (e.g. to form volatile hydride (e.g.

SnHSnH44)) Hydrides carried into light path by argon gasHydrides carried into light path by argon gas Decomposed into elemental vapour by injection Decomposed into elemental vapour by injection

into (electrothermally) heated silica cellinto (electrothermally) heated silica cell

6161

Calibration – some Calibration – some practical aspectspractical aspects

6262

PrinciplesPrinciples

Recap: Measured quantity must change with analyte Recap: Measured quantity must change with analyte concentration in systematic and defined wayconcentration in systematic and defined way

Can be determined by calibration, using defined Can be determined by calibration, using defined standardsstandards

Stock solutions of standards can either be prepared Stock solutions of standards can either be prepared or purchasedor purchased

Working solutions are best prepared by Working solutions are best prepared by weighingweighing the the amounts of stock solution and matrix (rather than amounts of stock solution and matrix (rather than using volumetric ware)using volumetric ware)

NEVER extrapolate: concentration of sample must be NEVER extrapolate: concentration of sample must be in same range as standardsin same range as standards

6363

Calibration in AASCalibration in AAS In theory, Beer’s law In theory, Beer’s law

applies for dilute solutionsapplies for dilute solutions In practice, deviation from In practice, deviation from

linearity is usuallinearity is usual Small dynamic rangeSmall dynamic range Possible to use non-linear Possible to use non-linear

curve fitting for calibrationcurve fitting for calibration Reasons: Self-absorption: Reasons: Self-absorption:

excited atoms emit light that excited atoms emit light that can also be absorbed instead can also be absorbed instead of that of source: of that of source: on on average, less light per average, less light per number of atoms is absorbednumber of atoms is absorbed

Linear range

6464

Alternative to matrix-Alternative to matrix-matching:matching:

Method of standard Method of standard additionsadditions Extensively used in absorption Extensively used in absorption

spectroscopy, accounts for matrix effectsspectroscopy, accounts for matrix effects Several aliquots of sample Several aliquots of sample

Sample (1): diluted to volume directlySample (1): diluted to volume directly Samples (2,3,4,5…): known amounts of Samples (2,3,4,5…): known amounts of

analyte added before dilution to volumeanalyte added before dilution to volume BUT: Only makes sense if the added

standard closely matches the analyte present in the samples chemically and physically

if simple, dissolved ions are analysed

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Method of standard Method of standard additionsadditions

If linear relationship exists between measured quantity If linear relationship exists between measured quantity and concentration (must be verified experimentally) and concentration (must be verified experimentally) then:then:

VVxx, C, Cxx:: volume and concentration of analyte volume and concentration of analyte VVss: variable volume of added standard: variable volume of added standard CCss: concentration of added standard: concentration of added standard VVTT: total volume of volumetric flask: total volume of volumetric flask kk: proportionality constant (= : proportionality constant (= єєll)) AAxx, A, ATT: absorbances of standard alone and sample + standard : absorbances of standard alone and sample + standard

addition, respectively.addition, respectively.

T

ss

T

xxT V

ckV

V

ckVA

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Method of standard Method of standard additionsadditions

slope = m = (єlcs) / VT

intercept = b = (єlVxcx) / VT

Graphical evaluation

Limitations • The calibration graph must be substantially linear since

accurate regression cannot be obtained from non-linear calibration points.

• Caution: The fact that the measured part of the graph is linear does not always mean that linear extrapolation will produce the correct results

• It is also essential to obtain an accurate baseline from a suitable reagent blank

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Most simple version of Most simple version of standard addition: spikingstandard addition: spiking

Spiking means deliberately adding analyte to an Spiking means deliberately adding analyte to an unknown sampleunknown sample

Involves:Involves: preparation of sample and measurement of preparation of sample and measurement of

absorbanceabsorbance Addition of standard with known concentration, Addition of standard with known concentration,

measurement of absorbancemeasurement of absorbance From difference in absorbance, calculate From difference in absorbance, calculate From reading of sample alone, calculate amount of From reading of sample alone, calculate amount of

analyteanalyte (use Beer’s law for calculations)(use Beer’s law for calculations)

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Other uses for spikingOther uses for spiking

Add spike at beginning of sample Add spike at beginning of sample preparationpreparation

Process sample with and without Process sample with and without spikespike

Difference should correspond to Difference should correspond to amount spikedamount spiked

Deviation allows to calculate recovery Deviation allows to calculate recovery factorfactor

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Atomic emission Atomic emission spectroscopyspectroscopy

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Atomic emission spectroscopyAtomic emission spectroscopy

Historically, many techniques based on Historically, many techniques based on emission have been used (See Table on emission have been used (See Table on p. 4)p. 4)

Flame and electrothermal methods now Flame and electrothermal methods now widely superseded by widely superseded by Inductively-Inductively-Coupled Plasma (ICP)Coupled Plasma (ICP) method method Developed in the 1970sDeveloped in the 1970s Higher energy sources than flame or Higher energy sources than flame or

electrothermal methodselectrothermal methods

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ICP-AES/OESICP-AES/OES

Offer several advantages over Offer several advantages over flame/electrothermal:flame/electrothermal: Lower inter-element interference (higher Lower inter-element interference (higher

temperatures)temperatures) With a single set of conditions signals for dozens of With a single set of conditions signals for dozens of

elements can be recorded simultaneouslyelements can be recorded simultaneously Lower LOD for elements resistant to decomposition Lower LOD for elements resistant to decomposition Permit determination of non-metals (Cl, Br, I, S)Permit determination of non-metals (Cl, Br, I, S) Can analyse concentration ranges over several Can analyse concentration ranges over several

decades (vs 1 or 2 decades for other methods)decades (vs 1 or 2 decades for other methods) Disadvantages:Disadvantages:

More complicated and expensive to runMore complicated and expensive to run Require higher degree of operator skillRequire higher degree of operator skill

Inductively coupled plasma-atomic emission spectroscopy(or optical emission spectroscopy)

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Modern ICP-OES spectrometerModern ICP-OES spectrometer

Over 70 elements (in principle simultaneously)Over 70 elements (in principle simultaneously) Including non-metals such as sulfur, phosphorus, Including non-metals such as sulfur, phosphorus,

and halogens (not possible with AAS)and halogens (not possible with AAS) ppm to ppb rangeppm to ppb range Principle: Argon plasma generates excited atoms Principle: Argon plasma generates excited atoms

and ionsand ions; these emit characteristic radiation; these emit characteristic radiation

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ICP-AES InstrumentationICP-AES Instrumentation

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Components for sample Components for sample injection and the ICP torchinjection and the ICP torch

www.cleanwatertesting.com/news_NR149.htm

www.midwestrefineries.com/refiningandassaying.htm

Up to 7000°C

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Meinhard nebuliserMeinhard nebuliser

Caution: The capillary is easy to block and difficult to unblock

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ICP torchICP torch

d=2.5 cm

water cooled induction coil powered by RF generator (2 kW power at 27 MHz)

concentric quartz tubes

11-17 L/min

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Torch Ignition SequenceTorch Ignition Sequence

Ionisation of Ionisation of Argon initiated Argon initiated by spark from by spark from Tesla coilTesla coil

After leaving injector, sample moves at high velocityPunches hole in centre of plasma

Switch on RF powerStart gas flow

Plasma generated

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Atomisation / IonisationAtomisation / Ionisation

In plasma, sample moves through several In plasma, sample moves through several zoneszones Preheating zone (PHZ): temp = 8000 K: Preheating zone (PHZ): temp = 8000 K:

Desolvation/evaporationDesolvation/evaporation Initial radiation zone (IRZ): 6500-7500 K: Initial radiation zone (IRZ): 6500-7500 K:

Vaporisation, AtomisationVaporisation, Atomisation Normal analytical zone (NAZ): 6000-6500 K: Normal analytical zone (NAZ): 6000-6500 K:

IonisationIonisation

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Advantages of plasmaAdvantages of plasma

Prior to observation, atoms spend ~ 2 sec at Prior to observation, atoms spend ~ 2 sec at 4000-8000 K (about 2-3 times that of hottest 4000-8000 K (about 2-3 times that of hottest combustion flame)combustion flame) Atomisation and ionisation is more completeAtomisation and ionisation is more complete Fewer chemical interferencesFewer chemical interferences

Chemically inert environment for atomisationChemically inert environment for atomisation Prevents side-product (e.g. oxide) formationPrevents side-product (e.g. oxide) formation

Temperature cross-section is uniform (no Temperature cross-section is uniform (no cool spots)cool spots) Prevents self-absorptionPrevents self-absorption Get linear calibration curves over several orders of Get linear calibration curves over several orders of

magnitudemagnitude

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Radial and axial observationRadial and axial observation

http://las.perkinelmer.com/content/relatedmaterials/brochures/bro_atomicspectroscopytechniqueguide.pdf

Axial Radial. Can achieve higher sensitivity

Combined viewing expands dynamic range

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ApplicationsApplications

ICP-OES used for quantitative analysis of: ICP-OES used for quantitative analysis of: Soil, sediment, rocks, minerals, airSoil, sediment, rocks, minerals, air

GeochemistryGeochemistry MineralogyMineralogy AgricultureAgriculture ForestryForestry FornensicsFornensics Environmental sciences Environmental sciences Food industryFood industry

Elements not accessible using AAS Elements not accessible using AAS Sulfur, Boron, Phosphorus, Titanium, and ZirconiumSulfur, Boron, Phosphorus, Titanium, and Zirconium

8282

Homework for revisionHomework for revision

Read Read http://las.perkinelmer.com/content/relatedmaterials/brochures/bro_atomicspectroscopytechniqueguide.pdf

8383

Lab Experiment 3Lab Experiment 3 Analyse a Chromium complex for [Cr] in three Analyse a Chromium complex for [Cr] in three

ways:ways: UV (absorbance & extinction coefficient)UV (absorbance & extinction coefficient) Titration (moles Cr and charge)Titration (moles Cr and charge) AAS (Cr standard curve and unknown concentration)AAS (Cr standard curve and unknown concentration)

AAS data analysisAAS data analysis Fit standards to quadratic equationFit standards to quadratic equation

A=A=aa[Cr][Cr]22 + + bb[Cr] + [Cr] + cc Use Use aa, , bb, and , and cc to calculate unknown concentration to calculate unknown concentration