Basic TheoryAtomic Absorption SpectroscopyMimi Wong

Bohr model of ATOMIf higher energy is absorbed by the atom,the electron can be completely dissociated from the atom Positively charged ion

Bohr model of ATOMEnergy Level Diagram

Theory of AA

Light SourceHollow Cathode Lamp (HCL)are a type of discharge lamp that produce narrow emission from atomic species electric discharge ionizes rare gas atoms, which are accelerated into the cathode and sputter metal atoms into the gas phase. Collisions with gas atoms or electrons excite the metal atoms to higher energy levels, which decay to lower levels by emitting light

AtomizerThe analyte atoms be in the gas phase. Ions or atoms in a sample must undergo desolvation and vaporization in a high-temperature source FAAS: Air-Acetylene / Nitrous oxide-Acetylene analyzes solution onlyGFAAS: electro-thermal furnace accepts solution, slurries and solid samplesMXM(H2O)m,X-(MX)nDesolvationVaporizationAtomizationM*E (measured by detector)MExcitationE (=h) from HCL

Advantages of GFAAS vs. FAASmuch more efficient atomizer than flame can accept directly very small absolute quantities of sampleprovides a reducing environment for easily oxidized elementssamples are placed directly in the graphite furnace and the furnace is electrically heated in several steps to dry the sample, ash organic matter, and vaporize the analyte atoms

What happen inside atomizerLight (energy) from HCL is absorbed

Monochromatorconsists of a diffraction grating (dispersing element), slits, and spherical mirrors. The light source emits a broad spectrum of radiation as represented by the multi-colored line from the lamp to the grating Schematic of a Czerny-Turner monochromator

Detectorconvert photons to an electrical signal consists of a photocathode and a series of dynodes in an evacuated glass enclosure. When a photon of sufficient energy strikes the photocathode, it ejects a photoelectron due to the photoelectric effect The amplification depends on the number of dynodes and the accelerating voltage. This amplified electrical signal is collected at an anode at ground potential, which can be measured. Schematic of a PMT

SummaryAtomic Absorption AAhProcesses take place for a nebulized sample droplet in AA burner headHCL emits unique energy which can only be absorbed by a particular element

single element analysisa = coefficient of absorptionb = length of burner headc = element concentrationI = intensity, By Beers Law

E = (E E) = abc c = E /ab c I

Theory of ICP-AESMXM(H2O)m,X-(MX)nDesolvationVaporizationIonizationMExcitationM+DecayExcitationDecayAtomizationProcesses take place for a nebulized sample droplet in ICP dischargeatom emissionh1M*M+ * h2Ion Excited StateIon Ground StateGround StateExcited Statesion emission4321atomic /ionic emissionMeasure the emitted light during the excited atoms decay back to ground state

multiple element analysis = coefficient of emissionb = length of burner headc = element concentrationIi = intensity, Eii= 1,2,3,4, . nBy Beers Law

Ei = hi = bc c = Ei / b c I ienergy

Detection Limits

AA vs. ICP-AES : Detection LimitsRemarks:For comparing the typical ICP-AES, FAAS and GFAAS in the market* Can reach to108 with range extension** precision improves with the use of internal standards105 10 6*103103

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ICP-AESFlame AASGFAAS

Level of measurement

Detection LimitVery good for most elementsVery good for some elementsExcellent for some elements

Typical Linear Dynamic Range

Precision

short term0.3 - 2%**0.1 - 1%1 - 5%

long term (>4 hrs)< 5%

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AA vs. ICP-AES : Matrix & InterferencesRemarks:For comparing the typical ICP-AES, FAAS and GFAAS in the market* Can reach to108 with range extension** precision improves with the use of internal standards

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ICP-AESFlame AASGFAAS

Interferences

Spectralcommonalmost nonefew

Chemical (matrix)almost nonemanymany

Ionizationminimalsomeminimal

Dissolved solids (max tolerable conc)2 - 30%0.5 - 3%> 20%

Semi-quantitative analysisyes & easynono

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AA vs. ICP-AES : Speed & CostRemarks:For comparing the typical ICP-AES, FAAS and GFAAS in the market

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ICP-AESFlame AASGFAAS

No. of elements> 75> 68> 50

Sample throughput5-30 elements /min/sample15 sec/elements /sample4 min/elements /sample

Sample UsagehighVery highVery low

Routine operationeasyeasyeasy

Method developmentskill requiredeasyskill required

Unattended operationyesnoyes

Combustible gasesnoyesno

Hollow Cathode Lamp / Electrodeless Dicharge Lamp (EDL)noyesyes

Capital costhighlowmedium/high

Sample running costaffortablelowhigh

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Interference & Solutions

ApplicationChoosing a Technique for the Determination of Copper in an Electroplating BathSource: Choosing The Right Trace Element Technique, R. Thomas, Todays Chemist at Work, Vol 8, No. 10, 42-48

ApplicationChoosing a Technique for the Determination of Lead and Cadmium in Blood Source: Choosing The Right Trace Element Technique, R. Thomas, Todays Chemist at Work, Vol 8, No. 10, 42-48

Other ApplicationsPb poisoning : detection of Zn and Pb in human hair Se and Hg analysis in sea food, such as fish tissue, mussels, oyster & etc.

Advantages of AA over ICP-AESWide dynamic range measure from ppm to sub-ppb levels (FAAS + GFAAS)Low running cost Gas supply : Acetylene vs. ArgonLabour skill : Less in AASTraining time : Less in AASMethod development time : Less in AASMaintenance : Less in AAS Spectral Interference Zero

AAS is a very mature technology