session 3 optical spectroscopy: introduction/fundamentals atomic and molecular spectroscopies...
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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
6666
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
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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
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