Analyses Concentrations of Metals in Solution 67 Elements ppb to % Levels Typically Better than 1% RSD Sample Preparation is Simple Instrument is Easy to Operate Technique is very specific

1. 1
Basic Atomic
Absorption
Spectroscopy
based on: Chris Mullins
Varian Optical Spectroscopy Instruments

2. 2
ATOMIC ABSORPTIONATOMIC ABSORPTION
CHARACTERISTICSCHARACTERISTICS
Analyses Concentrations of Metals in SolutionAnalyses Concentrations of Metals in Solution
6767ElementsElements
ppb to % Levelsppb to % Levels
Typically Better than 1% RSDTypically Better than 1% RSD
Sample Preparation is SimpleSample Preparation is Simple
Instrument is Easy to OperateInstrument is Easy to Operate
Technique is very specificTechnique is very specific

3. 3
Periodic TablePeriodic Table

4. 4
All atoms can absorb light . .All atoms can absorb light . .
Wavelength of light absorbed is specific for eachWavelength of light absorbed is specific for each
elementelement..

5. 5
Basic Principles of AASBasic Principles of AAS
Energy levels are quantisedEnergy levels are quantised
Only specific energy changes allowedOnly specific energy changes allowed
Therefore, each element has a number of energy levels which areTherefore, each element has a number of energy levels which are
unique to that elementunique to that element
Atoms can absorb discrete amounts of energy in the form ofAtoms can absorb discrete amounts of energy in the form of
electromagnetic radiationelectromagnetic radiation
Absorbed energy changes energy of atomAbsorbed energy changes energy of atom
Increased kinetic energyIncreased kinetic energy
Increased electronic energyIncreased electronic energy
In atomic absorption spectroscopy, we are only considering changesIn atomic absorption spectroscopy, we are only considering changes
to the energy of extranuclear electrons, caused by the absorptionto the energy of extranuclear electrons, caused by the absorption
of lightof light

6. 6
Energy Level DiagramEnergy Level Diagram

7. 7
The Atomic Absorption ProcessThe Atomic Absorption Process
SummarySummary
Ground state atom absorbs light of a specificGround state atom absorbs light of a specific
wavelengthwavelength
Energy of atom is elevated to an excited stateEnergy of atom is elevated to an excited state
Element does not change species ONLY gainsElement does not change species ONLY gains
energyenergy..

8. 8
Atomic EmissionAtomic Emission
Excited state atoms are unstableExcited state atoms are unstable
Rapidly revert to ground stateRapidly revert to ground state
Result from transitions from high energy to low energyResult from transitions from high energy to low energy
Wavelengths are exactly the same as for absorptionWavelengths are exactly the same as for absorption
Energy added to excite atomsEnergy added to excite atoms
**ThermalThermal
**LightLight
**ElectricalElectrical

9. 9
Emission ProcessEmission Process
Temperature dependentTemperature dependent
Low temperature = low intensity, few linesLow temperature = low intensity, few lines
Higher temperature = higher intensity, many linesHigher temperature = higher intensity, many lines
Light emitted proportional to energy spacingLight emitted proportional to energy spacing
Emitted lines unique for each element & for each ionEmitted lines unique for each element & for each ion..
Intensity of emitted line is proportional to concentration ofIntensity of emitted line is proportional to concentration of
element Compare with known standardselement Compare with known standards..

10. 10
Emission InstrumentsEmission Instruments
Most AAS can measure emissionMost AAS can measure emission
Flame source is relativelyFlame source is relatively LOWLOW temperaturetemperature
23002300--30003000oo
kk
Best for alkali metals (Li, Na, KBest for alkali metals (Li, Na, K((
Emission spectra more complex than absorptionEmission spectra more complex than absorption

11. Beer& Lambert LawBeer& Lambert Law
Lambert’s LawLambert’s Law
The portion of light absorbed by a transparentThe portion of light absorbed by a transparent
medium is independent of the intensity of themedium is independent of the intensity of the
incident lightincident light
Each successive unit of thickness of the mediumEach successive unit of thickness of the medium
absorbs an equal fraction of the light passing thoughabsorbs an equal fraction of the light passing though
itit
Beer’s LawBeer’s Law
The light absorption is proportional to the number ofThe light absorption is proportional to the number of
absorbing species in the sampleabsorbing species in the sample
11

12. 12
Beer Lambert LawBeer Lambert Law
ABSORBANCE = log10 I0 / It = a x b x c
WhereWhere::
IIoo = incident light intensity= incident light intensity
IItt = transmitted light intensity= transmitted light intensity
a = absorption coefficient (absorptivitya = absorption coefficient (absorptivity((
b = length of absorption pathb = length of absorption path
C = concentration of absorbing atomsC = concentration of absorbing atoms

13. 13
Deviations from Beers LawDeviations from Beers Law
Actual
Theoretical
A
B
S
CONC

14. Deviations from Beer’s LawDeviations from Beer’s Law
Stray lightStray light
Spectral effectsSpectral effects
Instrument designInstrument design
Greatest at higher concentrationsGreatest at higher concentrations
14

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Detection LimitDetection Limit::
That concentration or mass which is represented byThat concentration or mass which is represented by
three times the standard deviation of tenthree times the standard deviation of ten
measurements of the blankmeasurements of the blank..
This also corresponds to the MINIMUMThis also corresponds to the MINIMUM
concentration which can be determined with aconcentration which can be determined with a
99% confidence99% confidence..
Detection limit is a signal-to-noise ratio and isDetection limit is a signal-to-noise ratio and is
determined by all of the components in thedetermined by all of the components in the
instrumentinstrument..

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General Analytical Procedure for AASGeneral Analytical Procedure for AAS
SampleSample::
Dissolve sampleDissolve sample
Make up a solution which contains NOMake up a solution which contains NO analyteanalyte
BlankBlank::
Make up a series of solutions which contain knownMake up a series of solutions which contain known
amounts of analyteamounts of analyte
StandardsStandards::
Atomise blank and standardsAtomise blank and standards
Measure response for each and establish concentration/absorbanceMeasure response for each and establish concentration/absorbance
relationshiprelationship
Measure unknown sampleMeasure unknown sample

17. 17
Typical Calibration Curve in AASTypical Calibration Curve in AAS
0.600
0.000
0.00 5.50Cu Concentration, mg/L
A
B
S

18. 18
AAS ComponentsAAS Components
light-sensitive detector
electronic readout system
monochromator
light source - usually
a hollow cathode lamp
atomiser
(flame, furnace
or hydride
solution
(blank, standards
or sample)
Typical AAS Layout

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•**Light Source:Light Source:
--Hollow Cathode LampsHollow Cathode Lamps
-High Intensity Lamps-High Intensity Lamps
** AtomiserAtomiser
* Optical System* Optical System
** MonochromatorMonochromator
** Light sensitive detectorLight sensitive detector
** Electronics to process detector responseElectronics to process detector response

20. 20
Hollow Cathode Lamp DesignHollow Cathode Lamp Design
BaseBase
QuartzQuartz
End WindowEnd Window
Pyrex EnvelopePyrex Envelope
AnodeAnode
CathodeCathode
Electrical ContactsElectrical Contacts
Lamp ElementLamp Element
Code ContactsCode Contacts
Lamp CodeLamp Code
CircuitCircuit
GlassGlass
ShieldShield

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Operation of HC LampOperation of HC Lamp

22. Sputtering ProcessSputtering Process
22

23. Excitation ProcessExcitation Process
23

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Optical SystemsOptical Systems
**Lenses and MirrorsLenses and Mirrors
**GratingsGratings
**MonochromatorsMonochromators
**DetectorsDetectors
**Background Absorption and CorrectionBackground Absorption and Correction

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Ideal OpticsIdeal Optics
**Pass 100% of the source energy to the detectorPass 100% of the source energy to the detector
**Very high signal to noise ratioVery high signal to noise ratio
**Zero stray lightZero stray light
**Absolute selectivity of required wavelengthAbsolute selectivity of required wavelength
**Constant dispersion with wavelengthConstant dispersion with wavelength

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ATOMISATIONATOMISATION

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ATOMISATIONATOMISATION
**Process by Which Atoms are made Available forProcess by Which Atoms are made Available for
Absorption MeasurementAbsorption Measurement
**Create a Supply of FREE GROUND STATE ATOMSCreate a Supply of FREE GROUND STATE ATOMS
**Expose to Light of Characteristic Wavelength for thatExpose to Light of Characteristic Wavelength for that
ElementElement
MAJOR CLASSIFICATIONS OF ATOMISERSMAJOR CLASSIFICATIONS OF ATOMISERS::
--FlameFlame
--Graphite FurnaceGraphite Furnace

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FLAME ATOMISATIONFLAME ATOMISATION
Convert the Analyte Solution into Free Atoms inConvert the Analyte Solution into Free Atoms in
the Light Paththe Light Path
Primary AimPrimary Aim
Generate an AerosolGenerate an Aerosol
Introduce Aerosol into FlameIntroduce Aerosol into Flame
DOESNDOESN’’T Block NebulizerT Block Nebulizer
DonDon’’t Block Burnert Block Burner

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Atomisation ProcessAtomisation Process
light beam
atomisation
vaporisation
liquid melt
solid
aerosol
free atoms
compound
decomposition
desolvation
mixing
nebulisation
droplet
precipitation
solution

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NebuliserNebuliser
Draws solution through capillaryDraws solution through capillary
Shatters solution into dropletsShatters solution into droplets
Droplets and Oxidant passes through venturiDroplets and Oxidant passes through venturi
More uniform droplet sizeMore uniform droplet size
More smaller dropletsMore smaller droplets
Better sensitivityBetter sensitivity

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Atomisation ProcessAtomisation Process
Flame heat evaporates solventFlame heat evaporates solvent
Near base of flameNear base of flame
Converts aerosol intoConverts aerosol into VERY SMALLVERY SMALL solid dropletssolid droplets
Particles fuse or meltParticles fuse or melt
VaporisationVaporisation
Form moleculesForm molecules
Molecules dissociateMolecules dissociate
Form ground state atomsForm ground state atoms

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Position in Light PathPosition in Light Path
FlameFlame MUSTMUST be positioned to place maximumbe positioned to place maximum
atom population in light pathatom population in light path
Maximum atom population = maximum signalMaximum atom population = maximum signal
Type of Flames UsedType of Flames Used::
Air/AcetyleneAir/Acetylene
Nitrous Oxide AcetyleneNitrous Oxide Acetylene
Some Elements can use bothSome Elements can use both

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FlamesFlames
Air/CAir/C22HH22
30003000oo
CC
23002300oo
CC
NN220/C0/C22HH22

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Elements by Air/Acetylene FlameElements by Air/Acetylene Flame::
**Almost universally used for “easily atomisedAlmost universally used for “easily atomised
elements” (elements” (Cu, Pb, K, Na, etcCu, Pb, K, Na, etc.(.(
**Temperature of about 2300Temperature of about 2300 oo
CC
**Interferences negligibleInterferences negligible
**Chemical environment usuallyChemical environment usually NOTNOT criticalcritical
**Not fully effective because of interferencesNot fully effective because of interferences..

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Elements by Nitrous Oxide/Acetylene FlameElements by Nitrous Oxide/Acetylene Flame::
**Good for refractory oxides (Al, Si, W, etcGood for refractory oxides (Al, Si, W, etc.(.(
**Temperature 3000Temperature 3000oo
CC
**Chemical environment importantChemical environment important..
**Hotter nitrous oxide/acetylene flameHotter nitrous oxide/acetylene flame
**Minimise or remove interference compoundMinimise or remove interference compound..

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InterferencesInterferences
Sometimes the response of the analyte in the sampleSometimes the response of the analyte in the sample
is different to its response in the standards, This isis different to its response in the standards, This is
called an interferencecalled an interference..
There areThere are four Main Categoriesfour Main Categories of Interferenceof Interference::
PhysicalPhysical
Viscosity and surface tension in the flame.Viscosity and surface tension in the flame.
Few in GFAASFew in GFAAS

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IonisationIonisation
Occurs with easily ionisable elements in the presenceOccurs with easily ionisable elements in the presence
of other easily ionisable elementsof other easily ionisable elements
ChemicalChemical
Too many to listToo many to list
OpticalOptical
Background and spectralBackground and spectral

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Effect of IonisationEffect of Ionisation

39. 39
Flame Atomisation SummaryFlame Atomisation Summary
AdvantagesAdvantages
Cheap to purchase andCheap to purchase and
operateoperate
Analyses 67 metalsAnalyses 67 metals
Simple methodSimple method
developmentdevelopment
Simple to operateSimple to operate
FastFast
Good precision >1% RSDGood precision >1% RSD
Few spectral interferencesFew spectral interferences
DisadvantagesDisadvantages
Sensitivity is limited to mg/lSensitivity is limited to mg/l
Sample volume required isSample volume required is
mlml’’ss
Use of flammable gasesUse of flammable gases
Sample must be a solutionSample must be a solution
with a viscosity similar towith a viscosity similar to
waterwater

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Limitations of Flame AtomisationLimitations of Flame Atomisation
Due toDue to::
Inefficient samplingInefficient sampling
~~10%10%reaches flamereaches flame
Large dilution factorLarge dilution factor
~~11to 10,000to 10,000
Short residence time for atoms in the light pathShort residence time for atoms in the light path
~~1010--44
secondsseconds
Limited minimum concentration measurementsLimited minimum concentration measurements
Low ppm rangeLow ppm range

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GRAPHITEGRAPHITE
FURNACEFURNACE
ATOMISATIONATOMISATION

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Benefits of Graphite FurnaceBenefits of Graphite Furnace
AtomisationAtomisation
Entire sample is atomised atEntire sample is atomised at
one timeone time
Free atoms remain in theFree atoms remain in the
optical path longeroptical path longer
Enhanced sensitivityEnhanced sensitivity

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Principles of Graphite FurnacePrinciples of Graphite Furnace
AtomisationAtomisation
**Flame replaced by graphite tube in argon chamberFlame replaced by graphite tube in argon chamber..
**Functions of argonFunctions of argon::
--Protect graphite from oxidationProtect graphite from oxidation
--Remove interfering species during early thermal stageRemove interfering species during early thermal stage
Small volume of sample dispensed directly into pyrolytically coatedSmall volume of sample dispensed directly into pyrolytically coated
graphite tubegraphite tube
**Function of pyrolyticallyFunction of pyrolytically::
coated graphitecoated graphite
Make tube resistant to oxidationMake tube resistant to oxidation
Prevent liquids from soaking into tubePrevent liquids from soaking into tube
Prevent atomic vapour from penetrating tubePrevent atomic vapour from penetrating tube
Improved sensitivity and reproducibilityImproved sensitivity and reproducibility

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Advantages of Graphite FurnaceAdvantages of Graphite Furnace
AtomisationAtomisation
All analyte in tube is atomisedAll analyte in tube is atomised
Atoms retained in tube (light path) slightly longer than in flameAtoms retained in tube (light path) slightly longer than in flame
AtomsAtoms NOTNOT diluted By flame gases or matrixdiluted By flame gases or matrix
Improved sensitivityImproved sensitivity
Improved detection limitsImproved detection limits
Ground state atom interferencesGround state atom interferences
Still existStill exist
Different than those found in flameDifferent than those found in flame
Amenable to controlAmenable to control
Choice of analytical conditionsChoice of analytical conditions
Choice of matrix modifierChoice of matrix modifier
Direct analysis of a wide variety of samplesDirect analysis of a wide variety of samples
Minimise sample preparation errorsMinimise sample preparation errors
Lends itself to unattended automationLends itself to unattended automation

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Know your gas cylinders andKnow your gas cylinders and
plumbingplumbing

46. 46
THANK YOUTHANK YOU