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2. Introduction and principle
•Atomicabsorptionspectroscopy(AAS)isprobablythemostwidelyusedatomicspectroscopymethodbecauseofitssimplicity,effectivenessandrelativelylowcost.
•AASwasfirstintroducedcommerciallyinthe1950’sanditspopularityincreasedrapidlyafterthat.
•Itisbasedontheprinciplethatanelementwillabsorbradiationofaparticularwavelength.Themagnitudeofthisabsorptiondependsonthenumberofgroundstateatomsintheopticalpath.
•Theinstrumentratiosthetransmittedintensitiesinthepresenceandabsenceofsample.Thelightbeamemittedbythesourcepassesthroughtheflameinwhichtheelementislocatedintheatomicstate.Thebeamthentravelsthroughamonochromatorwhichselectsaverynarrowbandofwavelengthsbeforepassingthroughtothedetector.Iftherearenosuitableatomsinthesamplethentheintensityoflightrecordedbythedetectorwillbeidenticaltothefullintensityoflightemittedbythesource,Io.Ifatomsinthegroundstatearepresentthentheywillabsorbaportionofthelightinordertopromoteelectronstotheexcitedstate. Consequently,thelightreachingthedetectorwillbereduced,I.
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3. •Atomicabsorptionspectrophotometry(AAS)involvestheabsorptionofradiationbytheatomizedanalyteelementinthegroundstate.Theatomisationisachievedbythethermalenergyoftheflameorelectrothermallyinanelectricalfurnace.
•Thewavelength(s)oftheradiationabsorbedandtheextentoftheabsorptionformthebasisofthequalitativeandquantitativedeterminationsrespectively.
•Theatomicabsorptionmethodsusingflamearerapidandpreciseandareapplicabletoabout67elements. Electrothermalmethodsofanalysisontheotherhandareslowerandlessprecise;however,thesearemoresensitiveandneedmuchsmallersamples.
•Astheabsorptionofresonanceradiationishighlyselectiveandalsoverysensitive,thetechniqueofAAShasbecameapowerfulmethodofanalysis,whichisusedfortraceelementaldeterminationsinmostanalyticallaboratoriesforawidevarietyofapplications.
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4. Concentration dependence of Absorption
•AccordingtoBoltzmanndistributionlaw,thepopulationofthegroundstatei.e.,thenumberofspeciesinthegroundstate,ishighestanditkeepsondecreasingaswegotohigherenergylevels.Itcanbeshownthatformostelementsatmoderatetemperaturesprevailinginaflame,nearlyallatomsareingroundstateleavingonlyafewatomsinexcitedstate.TheabsorptionfollowsLambert-Beer’slawsothattheconcentrationofananalyteelementinthevaporsintheflamemaybedetermined.AccordingtoLambert-Beer’slaw,theextentofradiationabsorbedbytheabsorbingspeciesisafunctionofthepathlengthandtheconcentrationoftheabsorbingspecies.
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5. •Thus,absorbanceofthesampleisdirectlyproportionaltotheconcentrationoftheanalyte.
•Therefore,acalibrationplotofconcentrationofanalyteelementversusabsorbanceisdrawnfromthestandardsolutionsandtheconcentrationofelementinunknownsolutionisreaddirectlyfromthegraph.
•However,suchalinearrelationshipbetweentheabsorptionandtheconcentrationcanbeobservedonlyifallradiationpassingthroughthesampleisabsorbedtothesameextentbytheanalyteatoms.
•However,theexperimentalconcentrationversusintensitycalibrationcurveisobservedtobedeviatingfromthelinearityasaresultofthepresenceofnonabsorbedradiationandotherinterferences.
•Therefore,suitablemeasuresneedtobetakensoastominimizetheinterferencesandobtainthelinearityinthecalibrationcurves.
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6. Instrumentation
A typical atomic absorption spectrophotometer consists of the following components.
• Radiation source
• Atom reservoir
• Monochromator
• Detector
• Readout device
A block diagram showing the basic components of an atomic absorption spectrophotometer is given in Fig. 9.4.
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7. •Inatypicalflameatomicabsorptionspectrophotometricdetermination,theradiationfromahollowcathodelampismadetofallonthesampleoftheanalyteaspiratedintotheflame,whereapartofitisabsorbed.
•Thetransmittedradiationisthendispersedbyamonochromatorandsenttothedetector.Thedetectoroutputissuitablyprocessedandisdisplayedbyappropriatereadoutdevice.
•Thesesinglechannelinstrumentscanperformmeasurementsatasinglewavelengthonlyinonechannel.
•Nowadays,dual-channelinstrumentsarealsoavailablethatpermitsimultaneousmeasurementsattwodifferentwavelengths.
•Thesecontaintwoindependentmonochromatorsforthepurpose.Thus,thesecanbeusedforthesimultaneousdeterminationoftwoelements;onecanbetheanalytetobedeterminedandtheothermaybeareferenceelement
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8. 1.Lightsources:
•Thetwomostcommonlightsourcesusedinatomicabsorptionarethe‘‘hollowcathodelamp’’andthe‘‘electrodelessdischargelamp.’’
a.Hollowcathodelamp(HCL):Itconsistsofasealedcylindricalglasstubewithaquartzwindowatoneendandahollowedcylindricalcathodetogetherwithananodewiremadeoftungsten.
•Thecathodeisfabricatedfromtheanalyteelementandthelampisfilledwithaninertgassuchasargonorneonundervacuum(100-200Pa).
•Whenanelectricalpotentialisappliedbetweentheanodeandcathode, someofthefillgasatomsareionized.Thepositivelychargedfillgasionsacceleratethroughtheelectricalfieldtocollidewiththenegativelychargedcathodeanddislodgeindividualmetalatomsinaprocesscalled‘‘sputtering’’.Sputteredmetalatomsarethenexcitedtoanemissionstatethroughakineticenergytransferbyimpactwithfillgasions.Thusacharacteristicemissionlightisproduced.
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9. •Theemissionspectrumofthecathodematerialincludesanumberofintense,sharplinesduetotransitionsbetweenexcitedstatesandthegroundstate,oftencalledresonancelines.
•IntensityofresonancelinesfromanHCLincreaseswithincreasingcurrent.Asofnow,HCLforover60elementsareavailable.
•However,thesedaysmultielementcathodelampsaremoreinuseforroutinedeterminations,thoughtheirperformanceisnotveryreliable.
•Inthiscase,cathodeismadeupfromalloysofmetalshavingsimilarmeltingpointsuchasCa-Mg,Ag-Au,Cu-Fe,Zn-Cd,etc.
•Whenelementshavingdifferentmeltingpointsareused, morevolatileelementislostfirstresultingingradualweakeningofitsspectrumanddegenerationintooneelementlamp. B.K Uprety 9

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11. b.Electrodelessdischargelamp(EDL):
•Formostelements,thehollowcathodelampisacompletelysatisfactorysourceforatomicabsorption.
•Inafewcases,however,thequalityoftheanalysisisimpairedbylimitationsofthehollowcathodelamp.Theprimarycasesinvolvethemorevolatileelementswherelowintensityandshortlamplifeareaproblem.
•Theatomicabsorptiondeterminationoftheseelementscanoftenbedramaticallyimprovedwiththeuseofbrighter,morestablesourcessuchasthe‘‘electrodelessdischargelamp.
•Asmallamountofthemetalorsaltoftheelementforwhichthesourceistobeusedissealedinsideaquartzbulbtogetherwithneonorargon.
•Ontheapplicationofvoltage,dischargeisproducedandthegaseousatomsareexcitedbyapplicationofmicrowavefieldorradiofrequencyoftypicalfrequency.
•Astheexcitedatomsdecaytothegroundstateortootherlowenergylevels,characteristicradiationoftheatomisemitted.TheradiationsemittedbyEDLsareabout10-100timesmoreintensethanforthecorrespondingHCL.
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12. 2. Atomiseror atom reservoir
Atomisers
•Thepurposeofatomiseristoprovidearepresentativeportionoftheanalyteintheopticalpathandconvertitintofreeneutralgroundstateatoms.Inatomicabsorptionspectrophotometry,theflamesandfurnacesthatgenerateatemperatureintherangeof1500to3000ºCarethemostcommonmethodsofatomisation.Twocommontypesofatomisersusedforgeneratingatomicspeciesinthevapourphaseareflameatomisersandelectrothermalatomisers.
•Inatypicalflameatomisationprocess,theanalytesolutionsaregenerallynebulisedwiththehelpofanebuliserintoaspraychamber.Theaerosolsoproducedalongwithamixtureofaburninggasandanoxidantisdirectedintoasuitableburner.
•Flametemperaturedependsonfuel-oxidantratioandtherequisitetemperatureforanalysiscanbeobtainedbyvaryingthefuel-oxidantratio. Thefuel-oxidantcombinationscommonlyusedinAAS,thecorrespondingcombustionreactionsandtheflametemperaturesaregiveninTable9.1.
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13. •Whileanalyzingliquidsamples,flameisconsideredtobesuperiorintermsofperformancecharacteristicsandreproduciblebehaviourthoughsamplingefficiencyandsensitivityofothermethodsarebetter.Thisisbecauselargeamountofthesampleflowsdownthedrainandtheresidencetimeofindividualatomsinthepathlengthofflameisoftheorderof~0.1ms.Theregionofmaximumabsorptionisrestrictedtospecificareasoftheflame.
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14. •Burners:TwomajortypesofnebuliserburnersusedinAASarepremixnebuliser- burnersystemandtotalconsumptionburner.
•Inpremixtypeburner,liquidissprayedintoamixingchamberwherethedropletsaremixedwiththecombustiongasandaresenttotheburner.Fig.9.6givesaschematicdrawingofsuchaburnerusedinAAS.
•Ontheotherhand,inthetotalconsumptionburner,thenebuliserandburnerarecombined.Thisisalsocalledturbulentflowburner(Fig.7.1).Severalfactorsareinvolvedinthechoiceofaburner.Generallyspeaking,apremixburnerispreferredforatomicabsorptionwork,exceptwhenahighburning-velocityflamemustbeused.
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15. 2.Monochromators
•Themonochromatorsarethedevicesthatcanselectivelyprovideradiationofadesiredwavelengthoutoftherangeofwavelengthsemittedbythesourceoremittedbyanalytesample.
•InAAS,themonochromatorsselectagivenemissionlineandisolateitfromotherlinesduetomolecularbandemissionsandallnonabsorbedlines.
•Someoftheselinesoriginatefromthefillergasinthehollowcathodelampwhilesomeothersarethespectralemissionsofvarioussamplecomponentsduringatomisation.MostcommercialAASinstrumentsusediffractiongratingsasmonochromators.
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16. 3.Detectors
•AsthewavelengthsofresonancelinesfallinUVregion,themostcommonlyuseddetectorinatomicabsorptionspectrophotometryisphotomultiplier(PM)tubewhoseoutputisfedtoareadoutsystem.
•Theradiationreceivedbythedetectormayoriginatenotonlyfromtheselectedresonancelinesbutalsofromtheemissionwithintheflame.
•Therefore,inadditiontoabsorptionsignalintensityIA,thedetectormayreceivesignalintensityof(IA+S)whereSistheintensityofemittedradiationfromflame.
•Actuallyonerequiresonlythesignalintensityduetoabsorption,itistherefore, importanttoeliminateeffectsduetoflameemission.
•Thisisachievedbymodulatingtheemissionfromtheemissionlinesourceusingamechanicalchopperdevice.
4.ReadoutDevices
Thereadoutsystemsincludemeters,chartrecordersanddigitaldisplaymeters.These
days,however,microprocessorcontrolledsystemsarecommerciallyavailablewhere
everythingcanbedonebytouchofabutton.Moderninstrumentsprovideafast
displayoftheexperimentalconditions,absorbancedata,statisticalvaluesand
calibrationcurves,etc.
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17. Characteristic concentration or sensitivity
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18. Detection Limit
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20. Interferences
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21. Chemical Interference
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23. Ionization interferences
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25. Matrix Interference
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