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Atomic absorption spectroscopy

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Atomic absorption spectroscopy

  1. 1. B.K Uprety 1
  2. 2. Introduction and principle •Atomicabsorptionspectroscopy(AAS)isprobablythemostwidelyusedatomicspectroscopymethodbecauseofitssimplicity,effectivenessandrelativelylowcost. •AASwasfirstintroducedcommerciallyinthe1950’sanditspopularityincreasedrapidlyafterthat. •Itisbasedontheprinciplethatanelementwillabsorbradiationofaparticularwavelength.Themagnitudeofthisabsorptiondependsonthenumberofgroundstateatomsintheopticalpath. •Theinstrumentratiosthetransmittedintensitiesinthepresenceandabsenceofsample.Thelightbeamemittedbythesourcepassesthroughtheflameinwhichtheelementislocatedintheatomicstate.Thebeamthentravelsthroughamonochromatorwhichselectsaverynarrowbandofwavelengthsbeforepassingthroughtothedetector.Iftherearenosuitableatomsinthesamplethentheintensityoflightrecordedbythedetectorwillbeidenticaltothefullintensityoflightemittedbythesource,Io.Ifatomsinthegroundstatearepresentthentheywillabsorbaportionofthelightinordertopromoteelectronstotheexcitedstate. Consequently,thelightreachingthedetectorwillbereduced,I. B.K Uprety 2
  3. 3. •Atomicabsorptionspectrophotometry(AAS)involvestheabsorptionofradiationbytheatomizedanalyteelementinthegroundstate.Theatomisationisachievedbythethermalenergyoftheflameorelectrothermallyinanelectricalfurnace. •Thewavelength(s)oftheradiationabsorbedandtheextentoftheabsorptionformthebasisofthequalitativeandquantitativedeterminationsrespectively. •Theatomicabsorptionmethodsusingflamearerapidandpreciseandareapplicabletoabout67elements. Electrothermalmethodsofanalysisontheotherhandareslowerandlessprecise;however,thesearemoresensitiveandneedmuchsmallersamples. •Astheabsorptionofresonanceradiationishighlyselectiveandalsoverysensitive,thetechniqueofAAShasbecameapowerfulmethodofanalysis,whichisusedfortraceelementaldeterminationsinmostanalyticallaboratoriesforawidevarietyofapplications. B.K Uprety 3
  4. 4. Concentration dependence of Absorption •AccordingtoBoltzmanndistributionlaw,thepopulationofthegroundstatei.e.,thenumberofspeciesinthegroundstate,ishighestanditkeepsondecreasingaswegotohigherenergylevels.Itcanbeshownthatformostelementsatmoderatetemperaturesprevailinginaflame,nearlyallatomsareingroundstateleavingonlyafewatomsinexcitedstate.TheabsorptionfollowsLambert-Beer’slawsothattheconcentrationofananalyteelementinthevaporsintheflamemaybedetermined.AccordingtoLambert-Beer’slaw,theextentofradiationabsorbedbytheabsorbingspeciesisafunctionofthepathlengthandtheconcentrationoftheabsorbingspecies. B.K Uprety 4
  5. 5. •Thus,absorbanceofthesampleisdirectlyproportionaltotheconcentrationoftheanalyte. •Therefore,acalibrationplotofconcentrationofanalyteelementversusabsorbanceisdrawnfromthestandardsolutionsandtheconcentrationofelementinunknownsolutionisreaddirectlyfromthegraph. •However,suchalinearrelationshipbetweentheabsorptionandtheconcentrationcanbeobservedonlyifallradiationpassingthroughthesampleisabsorbedtothesameextentbytheanalyteatoms. •However,theexperimentalconcentrationversusintensitycalibrationcurveisobservedtobedeviatingfromthelinearityasaresultofthepresenceofnonabsorbedradiationandotherinterferences. •Therefore,suitablemeasuresneedtobetakensoastominimizetheinterferencesandobtainthelinearityinthecalibrationcurves. B.K Uprety 5
  6. 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. 6 B.K Uprety
  7. 7. •Inatypicalflameatomicabsorptionspectrophotometricdetermination,theradiationfromahollowcathodelampismadetofallonthesampleoftheanalyteaspiratedintotheflame,whereapartofitisabsorbed. •Thetransmittedradiationisthendispersedbyamonochromatorandsenttothedetector.Thedetectoroutputissuitablyprocessedandisdisplayedbyappropriatereadoutdevice. •Thesesinglechannelinstrumentscanperformmeasurementsatasinglewavelengthonlyinonechannel. •Nowadays,dual-channelinstrumentsarealsoavailablethatpermitsimultaneousmeasurementsattwodifferentwavelengths. •Thesecontaintwoindependentmonochromatorsforthepurpose.Thus,thesecanbeusedforthesimultaneousdeterminationoftwoelements;onecanbetheanalytetobedeterminedandtheothermaybeareferenceelement B.K Uprety 7
  8. 8. 1.Lightsources: •Thetwomostcommonlightsourcesusedinatomicabsorptionarethe‘‘hollowcathodelamp’’andthe‘‘electrodelessdischargelamp.’’ a.Hollowcathodelamp(HCL):Itconsistsofasealedcylindricalglasstubewithaquartzwindowatoneendandahollowedcylindricalcathodetogetherwithananodewiremadeoftungsten. •Thecathodeisfabricatedfromtheanalyteelementandthelampisfilledwithaninertgassuchasargonorneonundervacuum(100-200Pa). •Whenanelectricalpotentialisappliedbetweentheanodeandcathode, someofthefillgasatomsareionized.Thepositivelychargedfillgasionsacceleratethroughtheelectricalfieldtocollidewiththenegativelychargedcathodeanddislodgeindividualmetalatomsinaprocesscalled‘‘sputtering’’.Sputteredmetalatomsarethenexcitedtoanemissionstatethroughakineticenergytransferbyimpactwithfillgasions.Thusacharacteristicemissionlightisproduced. B.K Uprety 8
  9. 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. 11. b.Electrodelessdischargelamp(EDL): •Formostelements,thehollowcathodelampisacompletelysatisfactorysourceforatomicabsorption. •Inafewcases,however,thequalityoftheanalysisisimpairedbylimitationsofthehollowcathodelamp.Theprimarycasesinvolvethemorevolatileelementswherelowintensityandshortlamplifeareaproblem. •Theatomicabsorptiondeterminationoftheseelementscanoftenbedramaticallyimprovedwiththeuseofbrighter,morestablesourcessuchasthe‘‘electrodelessdischargelamp. •Asmallamountofthemetalorsaltoftheelementforwhichthesourceistobeusedissealedinsideaquartzbulbtogetherwithneonorargon. •Ontheapplicationofvoltage,dischargeisproducedandthegaseousatomsareexcitedbyapplicationofmicrowavefieldorradiofrequencyoftypicalfrequency. •Astheexcitedatomsdecaytothegroundstateortootherlowenergylevels,characteristicradiationoftheatomisemitted.TheradiationsemittedbyEDLsareabout10-100timesmoreintensethanforthecorrespondingHCL. B.K Uprety 11
  12. 12. 2. Atomiseror atom reservoir Atomisers •Thepurposeofatomiseristoprovidearepresentativeportionoftheanalyteintheopticalpathandconvertitintofreeneutralgroundstateatoms.Inatomicabsorptionspectrophotometry,theflamesandfurnacesthatgenerateatemperatureintherangeof1500to3000ºCarethemostcommonmethodsofatomisation.Twocommontypesofatomisersusedforgeneratingatomicspeciesinthevapourphaseareflameatomisersandelectrothermalatomisers. •Inatypicalflameatomisationprocess,theanalytesolutionsaregenerallynebulisedwiththehelpofanebuliserintoaspraychamber.Theaerosolsoproducedalongwithamixtureofaburninggasandanoxidantisdirectedintoasuitableburner. •Flametemperaturedependsonfuel-oxidantratioandtherequisitetemperatureforanalysiscanbeobtainedbyvaryingthefuel-oxidantratio. Thefuel-oxidantcombinationscommonlyusedinAAS,thecorrespondingcombustionreactionsandtheflametemperaturesaregiveninTable9.1. B.K Uprety 12
  13. 13. •Whileanalyzingliquidsamples,flameisconsideredtobesuperiorintermsofperformancecharacteristicsandreproduciblebehaviourthoughsamplingefficiencyandsensitivityofothermethodsarebetter.Thisisbecauselargeamountofthesampleflowsdownthedrainandtheresidencetimeofindividualatomsinthepathlengthofflameisoftheorderof~0.1ms.Theregionofmaximumabsorptionisrestrictedtospecificareasoftheflame. B.K Uprety 13
  14. 14. •Burners:TwomajortypesofnebuliserburnersusedinAASarepremixnebuliser- burnersystemandtotalconsumptionburner. •Inpremixtypeburner,liquidissprayedintoamixingchamberwherethedropletsaremixedwiththecombustiongasandaresenttotheburner.Fig.9.6givesaschematicdrawingofsuchaburnerusedinAAS. •Ontheotherhand,inthetotalconsumptionburner,thenebuliserandburnerarecombined.Thisisalsocalledturbulentflowburner(Fig.7.1).Severalfactorsareinvolvedinthechoiceofaburner.Generallyspeaking,apremixburnerispreferredforatomicabsorptionwork,exceptwhenahighburning-velocityflamemustbeused. B.K Uprety 14
  15. 15. 2.Monochromators •Themonochromatorsarethedevicesthatcanselectivelyprovideradiationofadesiredwavelengthoutoftherangeofwavelengthsemittedbythesourceoremittedbyanalytesample. •InAAS,themonochromatorsselectagivenemissionlineandisolateitfromotherlinesduetomolecularbandemissionsandallnonabsorbedlines. •Someoftheselinesoriginatefromthefillergasinthehollowcathodelampwhilesomeothersarethespectralemissionsofvarioussamplecomponentsduringatomisation.MostcommercialAASinstrumentsusediffractiongratingsasmonochromators. B.K Uprety 15
  16. 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. B.K Uprety 16
  17. 17. Characteristic concentration or sensitivity B.K Uprety 17
  18. 18. Detection Limit B.K Uprety 18
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  20. 20. Interferences B.K Uprety 20
  21. 21. Chemical Interference B.K Uprety 21
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  23. 23. Ionization interferences B.K Uprety 23
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  25. 25. Matrix Interference B.K Uprety 25
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