Atomic absorption spectroscopy is a widely used technique for elemental analysis. It works by measuring the absorption of light by ground state atoms of the element of interest. The sample is atomized in a flame or furnace and the absorption of light of a specific wavelength is measured. This allows for quantitative determination of elemental concentration. The technique provides sensitive and selective analysis but can be subject to various interferences that must be addressed, such as chemical, ionization, and matrix effects.

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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.
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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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