atomic absorption spectrophotometry cookbook原子吸收分光光度法食谱

'ATOMICABSORPTIONSPECTROPHOTOMETRYCOOKBOOKSection1BasicConditionsofAnalysisofAtomicAbsorptionSpectrophotometryAtomicAbsorptionSpectrophotometryCookbook-25- Section1CONTENTS1.PrincipalofAtomicAbsorptionSpectrophotometry11.1Whyatomsabsorblight11.2Relationbetweenlightabsorptionrateandatomicdensity21.3Sampleatomizationmethod3a)Flameatomicabsorption3b)Electro-thermalatomicabsorption52.BasicConditionforAnalysis92.1Conditionsofequipment9a)Analysisline9b)Slitwidth13c)Lampcurrentvalue142.2Analysisconditionsofflameatomicabsorption15a)Flameselection15b)Mixingratioofoxidantandfuelgas17c)Beampositioninflame172.3Analysisconditionsofelectro-thermalatomicabsorption18a)Dryingcondition18b)Ashingcondition19c)Atomizingcondition21d)Sampleinjectionquantity231.PrincipalofAtomicAbsorptionSpectrophotometry-25- 1.1WhyatomsabsorblightTheatomicabsorptionspectrometryusesabsorptionoflightofintrinsicwavelengthsbyatoms.Allatomsareclassifiedintothosehavinglowenergiesandthosehavinghighenergies.Thestatehavinglowenergiesiscalledthegroundstateandthestatehavinghighenergiesiscalledtheexcitedstate.Theatominthegroundstateabsorbsexternalenergiesandisputintheexcitedstate.Forexample,sodiumismainlyintwoexcitedstates,havinghigherenergiesby2.2eVand3.6eVrespectivelythaninthegroundstate,asshowninFig.1.1.(eVisaunittomeasureenergiesandiscalledanelectronvolt”.)When2.2eVenergyisgiventothesodiumatominthegroundstate,itmovesuptotheexcitedstatein(I)andwhen3.6eVenergyisgiven,itmovesuptotheexcitedstatein(II).Energyisgivenaslight,and2.2eVand3.6eVrespectivelycorrespondtoenergyoflightat589.9nmand330.3nmwavelength.Inthecaseofsodiuminthegroundstate,onlylightofthesewavelengthsareabsorbedandnootherwavelengthlightisabsorbedatall.Fig.1.1SodiumenergystatesThedifferencebetweenenergiesinthegroundstate,andintheexcitedstateisfixedbytheelementandwavelengthoflighttobeabsorbed.Atomicabsorptionspectrometryusesthehollowcathodelamp(HCL).TheHCLgivesofflightcharacteristictotheelementalwavelengthbeingmeasured.Thus,thelightabsorbedmeasurestheatomicdensity.1.2Relationbetweenlightabsorptionrateandatomicdensity-25- Whenlightofacertainintensityisgiventomanyatomsinthegroundstate,partofthislightisabsorbedbyatoms.Theabsorptionrateisdeterminedbytheatomicdensity.Fig.1.2PrincipleofatomicabsorptionWhenlightofIointensityisgiventodensityC,atomsspeedinlength1asshowninFig.1.2.ThelightisabsorbedanditsintensityisweakenedtoI.ThefollowingformulaisformedbetweenIandIo.I=Io´e-k•l•c(k:Proportionalconstant)Ioor–logI=k•l•cIoThisiscalledtheLambert-Beer"sLaw,and-logIvalueisabsorbance.Theaboveformulaindicatesthatabsorbanceisproportionaltoatomicdensity.Whenabsorbanceismeasuredonsamplesof1,2and3ppmforexampleandplotted,astraightlineisobtainedasshowninFig.1.3.Absorbanceandconcentrationrepresentedgraphicallyiscalledthecalibrationcurve.Whentheabsorbanceofanunknownsampleisobtained,theconcentrationcanbedeterminedfromthegraphasshown.-25- Fig.1.3Calibrationcurve1.3SampleatomizationmethodTheprinciplementionedabovecanbeappliedtolightabsorptionoffreeatoms.A”freeatom”meansanatomnotcombinedwithotheratoms.However,elementsinthesampletobeanalyzedarenotinthefreestate,andarecombinedwithotherelementsinvariablytomakeaso-calledmolecule.Forexample,sodiuminseawatermainlycombineswithchlorinetoformaNaCl(Sodiumchloride)molecule.Absorptioncannotbedoneonsamplesinthemoleculestate,becausemoleculesdonotabsorblight.~~~~~~~~~~~Thecombinationmustbecutoffbysomemeanstofreetheatoms.Thisiscalledatomization.Themostpopularmethodofatomizationisdissociationbyheat-samplesareheatedtoahightemperaturesothatmoleculesareconvertedintofreeatoms.Thismethodisclassifiedintotheflamemethod,inwhichachemicalflameisusedastheheatsource;andaflamelessmethod,inwhichaverysmallelectricfurnaceisused.a)FlameatomicabsorptionTheflameisproducedbyaburnerforatomizationandthisisthemostpopularmethod.Itisstandardinalmostallatomicabsorptiondevicesavailableonthemarketatpresent.-25- Fig.1.4FlameatomicabsorptionAtypicaldiagramoftheburnerisshowninFig.1.4.Thisfigureexplainsmeasurementofcalciumcontainedinthesampleliquidascalciumchloride.Thesampleisatomizedbyanebulizeratfirst.Then,bigwaterdropsaredischargedtothedrain,andonlyafinemistismixedwithfuel,andoxidantintheatomizerchamberandsenttotheflame.Whentheygetintheflame,themistevaporatesinstantaneouslyandfineparticlesofcalciumchloridemoleculesareproduced.Whentheseparticlesfurtheradvanceintheflame,calciumchlorideisdissolvedbyheatandfreecalciumatomsandchlorideatomsareproduced.Ifabeamoflightatwavelength422.7nm(Ca)isintroducedthroughthispartoftheflame,atomicabsorptioncanbemeasured.Intheupperpartoftheflame,someofcalciumatomsarecombinedwithoxygentobecomecalciumoxideandsomearefurtherionized.Therefore,atomicabsorptiondoesnotshowsufficientsensitivityeveniflightisgiventosuchaposition.Manycombinationsofvariousgaseshavebeentestedastheflameforatomization.Inconsiderationofanalysissensitivity,safety,easyuse,costandotherpoints;therearefourstandardflamesused:air-acetylene,nitrousoxide-acetylene,air-hydrogenandargon-hydrogen.Theseflamesareusedforeachelementdependingonthetemperatureandgascharacteristics.b)Electro-thermalatomicabsorptionTheatomizationmethodusingaflameisstillpopularlyusedasthestandardatomizationmethodduetogoodreproducibilityofmeasuredvaluesandeasyuse.-25- However,amajordefectoftheflamemethodistheatomizationrateoutofallsamplequantityusedisabout1/10andtheremaining9/10isdischargedtothedrain.Therefore,ithasbeenpointedoutthatatomizationefficiencyislowandanalysissensitivityisnotsohigh.Electro-thermalatomicabsorption(flamelessmethod),usingagraphitetube,improvestheabovedefectstoelevatesensitivity10to200timesasmuch.ThismethodwasoriginatedbyDr.L"vovofRussia.Fig.1.5FlamelessatomizerIntheelectro-thermalatomicabsorptionmethod,thesampleisinjectedintheformedgraphitetubeandanelectriccurrentof300ampere(maximum)isappliedtothetube.Thegraphiteisheatedtoahightemperatureandtheelementsinthesampleareatomized.Iflightfromthelightsourceissentthroughthetube,lightisabsorbedwhentheyareatomized.Inanactualmeasurement,afterthesampleisinjectedinthetube,heatingisdoneinthreestagesasshowninFig.1.6.Thatis,inthedryingstage,thetubeisheatedtoabout100oCandwaterinthesampleevaporatescompletely.Then,intheashingstage,thetubeisheatedto400oCto1000oCandorganicmatterandothercoexistentmatterdissolveandevaporate.Lastly,intheatomizingstage,itisheatedto1400oCto3000oCandmetallicsaltsleftinthetubeareatomized.HeatingisusuallydonebychangingthetemperatureinstepsshownbythesolidlineinFig.1.6(stepheating).Dependingonthesample,whenthedecompositiontemperatureofcoexistentmatterisclosetoitsatomizationtemperature,heatingisdonebychangingtemperaturecontinuously(rampmodeheating).-25- Heatingmustbedoneundertheconditions(temperature,heatingtime,andtemperatureraisingmethod),whichsuitthetypeofelementandcompositionofthesampletobemeasured.Ifheatingisstartedaftertheoptimumconditionsaresetontheequipmentinadvance,thetubeisautomaticallyheatedaccordingtothesettemperatureprogram.Fig.1.6Heatingprogramandabsorptioncurveaccordingtoelectro-thermalatomicabsorptionc)OtheratomicabsorptionmethodsMethodshavinghighersensitivitythannormalflameatomicabsorptionorelectro-thermalatomicabsorptionareoftenusedforspecialelementsincludingarsenic,seleniumandmercury.Theyusechemicalreactionsintheprocessofatomizationtovaporizeintheformofanatomorsimplemolecule.°HydridevaporgenerationtechniqueThehydridevaporgenerationtechniqueisusedtomakethesamplereactonsodiumborohydride.ItisacidifiedwithHCltoreducetheobjectmetal,andcombineitwiththehydrogeninordertoproduceagaseousmetalhydride.Thisgasissenttothehightemperatureatomizationunitformeasurement.As,Se,Sb,Sn,Te,Bi,Hgandothermetalsproduceametalhydridebythismethod.-25- Fig.1.7showstheblockdiagramofthehydridegeneratingequipment.Theperistalsisticpumpisusedtosendthesample,5Mhydrochloricacidand0.5%sodiumborohydridesolutiontothereactioncoil.Themetalhydrideisgeneratedinthereactioncoilandthegas-liquidseparatorisusedtoseparatethegasphaseandliquidphase.Argongasisusedasthecarriergas.Thegasphaseissenttotheabsorptioncell,whichisheatedbytheair-acetyleneflame,andthemetallicelementisatomized.Fig.1.7Blockdiagramofhydraulicgeneratingequipment-25- °ReductionvaporatomizationMercuryinsolutionisapositiveion.Whenitisreducedtoaneutralion,itvaporizesasafreeatomofmercury,atroomtemperature.Tin(II)chlorideisusedasareducingagentandmercuryatomsaresenttotheatomicabsorptionequipmentwithairasthecarriergas.Fig.1.8showstheblockdiagramofthemercuryanalysisequipment.200mlofthesampleisputinthereactionvessel,andtin(II)chlorideisaddedforreduction.Whenairissenttothegasflowcellthroughthedryingtube,atomicabsorptionbymercuryismeasured.Fig.1.8Blockdiagramofmercuryanalysisequipment-25- 2.BasicConditionforAnalysisTheequipmentmustbesetattheoptimumanalysisconditionstoobtainthebestmeasurementresults.Optimumconditionsgenerallyvarywiththeelementandwiththecompositionofthesample,evenifthesameelementsarecontained.Therefore,itisnecessarytofullystudythemeasuringconditionsinactualanalysis.2.1Conditionsofequipmenta)AnalysislineLightfromthehollowcathodelampshowsanumberofprimaryandsecondaryspectrumsofcathodeelementsandfillergas.Theyarecomplicatedparticularlywith4,5,6,7and8familiesinthemiddleoftheperiodictable,showingseveralthousandspectrums.Partsofmanyspectrallinescontributetoatomicabsorption.Theatomicabsorptionanalysisselectsandusesthespectrallineofthebiggestatomicabsorbance.Thespectrallinehavingabsorptionsensitivitysuitablefortheanalysismaybeused.Thisdependsontheconcentrationrangewheretheelementsinthesamplearemeasured.AnelementmayhavetwoormorespectrallinesshowingatomicabsorptionasinTable2.1.Itisdesirabletocheckabsorptionsensitivityandemissionintensityofthesespectrallines.Alsostudytheconcentrationrangeinwhicheachwavelengthismeasuredinordertoavoidthedilutionerrorwhentheconcentrationishighasinthemaincomponentanalysis.-25- Table2.1Analysislinesandabsorptionsensitivities(CharacteristicsofhollowcathodelampandhandlingmethodHamamatsuPhotonics)Ele-mentAnalysislinewavelength(nm)AbsorptionsensitivityFlametypeCs852.1110Air-C2H2Cu324.75327.40217.89218.17222.57104.71.21.00.6Air-C2H2Dy404.59421.17418.68108.98.0N2O-C2H2Er400.79415.11386.28105.95.5N2O-C2H2Eu459.40462.72466.19108.77N2O-C2H2Fe248.33271.90371.99385.99102.70.90.6Air-C2H2Ga294.36287.42403.30108.24.2Air-C2H2Gd407.89422.59378.31101010N2O-C2H2Ge265.16270.96269.13104.83.0N2O-C2H2Hf286.64307.29289.83109.35.0N2O-C2H2Hg253.6510ReductionvaporizationEle-mentAnalysislinewavelength(nm)AbsorptionsensitivityFlametypeAg328.07338.29105.3Air-C2H2üýþAl309.27396.15237.13237.30108.62.0N2O-C2H2As193.70197.20189.00106.25.0Ar-H2Au242.80267.59105.5Air-C2H2üýþB249.68249.77208.89108.2N2O-C2H2Ba553.55350.11100.01N2O-C2H2Be234.8610N2O-C2H2Bi223.06222.83306.77103.02.5Air-C2H2Ca422.67239.86100.05Air-C2H2Cd228.80326.11100.02Air-C2H2Co240.73251.98243.58346.58104.41.30.5Air-C2H2Cr357.87425.44427.88428.97104.42.71.0Air-C2H2-25- Ele-mentAnalysislinewavelength(nm)AbsorptionsensitivityFlametypeNd492.45463.42100.8N2O-C2H2Ni232.00341.48352.45231.10351.50105.15.02.00.9Air-C2H2Os290.90305.86263.71330.16104.54.02.0N2O-C2H2Pb217.00283.33261.41202.20103.90.20.1Air-C2H2Pd244.79247.64276.31340.46106.82.21.5Air-C2H2Pr495.13513.34504.55106.92.5N2O-C2H2Pt265.95292.98102.0Air-C2H2Rb780.02794.76104.6Air-C2H2Re346.05346.47345.19105.33.5N2O-C2H2Rh343.49339.69328.09102.80.2Air-C2H2Ru349.8910Air-C2H2Ele-mentAnalysislinewavelength(nm)AbsorptionsensitivityFlametypeHo410.38416.30105.8N2O-C2H2In303.94325.61410.48109.44.0Air-C2H2Ir208.88266.47284.97102.61.5Air-C2H2K766.49769.90404.41102.50.03Air-C2H2La550.13403.72357.44364.95102.30.80.5N2O-C2H2Li670.78323.26100.06Air-C2H2Lu331.21328.17107.1N2O-C2H2Mg285.21202.58100.9Air-C2H2Mn279.48280.11403.08104.71.1Air-C2H2Mo313.26319.40320.88104.70.8Air-C2H2üýþNa589.00589.59330.23330.30104.80.02Air-C2H2Nb334.91405.89108N2O-C2H2-25- Ele-mentAnalysislinewavelength(nm)AbsorptionsensitivityFlametypeSb217.58206.83231.15212.74107.03.61.0Air-C2H2Sc391.18390.74402.37402.04326.99107.67.05.03.0N2O-C2H2Se196.03203.99102.0Ar-H2Si251.61250.69251.43252.41288.16103.03.02.50.7N2O-C2H2Sm429.67484.17108.2N2O-C2H2Sn224.61286.33233.48106.26.0Air-C2H2Sr460.73407.77100.6Air-C2H2Ele-mentAnalysislinewavelength(nm)AbsorptionsensitivityFlametypeTe214.27225.90101.0Air-C2H2Ti364.27365.35398.98109.04.0N2O-C2H2Tl276.78377.57104.2Air-C2H2Tm371.79410.58374.41106.56N2O-C2H2V318.40306.64305.63103.83.0N2O-C2H2W255.14400.87407.44103.60.1N2O-C2H2Y410.23412.83407.74108.58N2O-C2H2Yb398.79346.43246.45103.22.0N2O-C2H2Zn213.86307.59100.002Air-C2H2Zr360.1210N2O-C2H2Ta271.47264.75275.83105.92.6N2O-C2H2Tb432.64431.88390.14108.56.0N2O-C2H2-25- b)SlitwidthConcerningspectrallinesemittedfromthehollowcathodelamp,theirwavelengthisanindependentlineorcomplicatednearbylinedependingontheelement.CalciumandmagnesiumhavenootherspectrallinesneartheobjectanalysislineasshowninFig.2.1.Incaseofsuchanalysislines,slitwidthissetconsiderablygreatertoobtainsufficientenergy.Fig.2.1LampspectrumsNickelhasmanyspectrallinesneartheobjectanalysislineof232.0nm(2320A).Becauselightofthesenearbywavelengthsishardlyabsorbedwithnickelatoms,theresolvingpowerspectroscopemustbeincreased(slitwidthisnarrowed)toseparateonly232.0nmlight.Ifmeasurementismadeinthelowresolvingpowercondition,themeasurementsensitivitygrowsworseandatthesametime,linearityofthecalibrationcurvebecomesdeteriorated.(Fig.2.2)Cobalt(Co),iron(Fe),manganese(Mn)andsilicon(Si)showcomplicatedspectrumslikenickel.Theresolvingpowerofthespectroscopemustbebelow2Atomeasuretheseelementsaccurately.-25- Fig.2.2Slitwidthandcalibrationcurvec)LampcurrentvalueIfthehollowcathodelampoperatingconditionsarenotproper,thespectrallinecausesaDopplerbroadeningorbroadeningduetoself-absorption,toaffectthemeasuredvalue.Dopplerbroadeningiscausedbythetemperatureofthehollowcathodelampspace,whichdoesnotcontributetolampemission.Asthehollowcathodelampcurrentincreases,luminanceincreases;thusthespectrallinesbroadencausingabsorptionsensitivitytodropasshowninFig.2.3.Thelifeofthehollowcathodelampisgenerallyindicatedbyampere-hour(A.Hr).Therefore,thelifeisshortenedifthecurrentvalueisincreased.Suchbeingthecase,alowcathodelamplightingcurrentvalueisdesirablebutluminancedropsifitistoolow.Detectorsensitivitymustbeincreased,butnoiseresultsfromit.Thelampcurrentvalueisdeterminedbythreefactors:luminance(noise)oftheabovelamp,absorptionsensitivity,andlamplife.-25- Fig.2.3Sensitivitybychangingthehollowcathodelampcurrentvalue2.2Analysisconditionsofflameatomicabsorptiona)FlameselectionAir-acetylene,air-hydrogen,argon-hydrogen,andnitrousoxide-acetylenearethestandardtypesofflamesusedinatomicabsorptionanalysis.Theseflamesvaryintemperature,reducibilityandtransmissioncharacteristics.Theoptimumflamemustbeselectedaccordingtotheelementbeinganalyzed,andpropertiesofthesample.Air-acetyleneflame(AIR-C2H2)Thisflameismostpopularlyusedandabout30elementscanbeanalyzedbythis.Nitrousoxide-acetyleneflame(N2O-C2H2)Thisflamehasthehighesttemperatureamongflamesusedforatomicabsorption.Aluminum,vanadium,titanium,etc.combinestronglywithoxygenintheair-acetyleneflameandotherrelativelylowtemperatureflames.Freeatomsdecreaseandmakemeasurementdifficult.However,suchelementsarehardtocombinewithoxygenduetohightemperatureinthenitrousoxide-acetyleneflamemakingsatisfactorymeasurementpossible.Thenitrousoxide-acetyleneflamecanalsobesubstitutedfortheelementsanalyzedbytheair-acetyleneflame.Thehightemperatureofthenitrousoxide-acetyleneflamehasverysmallinterferences.-25- Air-hydrogenflame(Air-H2)andargon-hydrogenflame(Ar-H2)Thehydrogenflameabsorbsverylittlelightfromthecathodelamp,onlyintheshortwavelengthregion.(RefertoFig.2.4).Therefore,measurementcanbedonewithasmallerbackgroundnoise,inthisshortwavelengthregion,thanwiththeair-acetyleneflame.ThosewavelengthelementsareAs,Se,Zn,Pb,Cd,Sn,etc.Sincetheargon-hydrogenflameabsorbsthesmallestamountoflightfrom200nmandbelow,itistypicallyused.Thedisadvantageofusingahydrogentypeflameisthatitissusceptibletointerferencesduetoit’slowtemperature.Fig.2.4LightabsorbanceofvariousflamesTable2.2showsthemaximumtemperatureofeachflame.Table2.3showselementsandtypesofflamesused.Table2.2FlametemperatureFlametypeMaximumtemperatureArgon-hydrogenAir-hydrogenAir-acetyleneNitrousoxide-acetylene1577oC2045oC2300oC2955oC-25- Table2.3Elementsandflamesusedformeasurementb)MixingratioofoxidantandfuelgasThemixingratioofoxidantandfuelgasisoneofthemostimportantitemsamongmeasurementconditionsofatomicabsorptionanalysis.Themixingratioaffectsflametemperatureandenvironment,anddeterminesgeneratingconditionsofgroundstateatoms.Therefore,theflametypeaswellasthebeampositionintheflamedescribedinthenextparagraph,control80to90percentofabsorptionsensitivityandstability(reproducibility).Cu,Ca,Mg,etc.increasesensitivityintheoxidizingflamecontainingmoreoxidant(fuelleanflame)andSn,Cr,Mo,etc.increasesensitivityinthereducingflamecontainingmorefuelgas(fuelrichflame).Becauseextremelyfuelleanorfuelrichmaycauseinstability,itmustbesetattheoptimumvaluedependingonthetargetobject.Absorptionvaluesbychangingtheacetyleneflowaremeasuredwithconstantairflowandtheconditionshowingthemaximumabsorptionvalueisobtained.Becausetheabovestudyisconcernedwiththeburnerpositiondescribedinthenextparagraph,acetyleneflowandburnerheightareadjustedtodecidetheoptimummixingratio.c)BeampositioninflameDistributionofgroundstateatomsgeneratedintheflamearenotuniformdependingontheelement,butvariesdependingontheflamemixingratio.Fig.2.5showsdistributionofgroundstateatomswhenthegasmixingratioischangedinthemeasurementofchromium.Itindicatesthatatomdistributionanddensitychange-25- whenthemixingratioischanged.Becauseabsorptionsensitivitychangeswiththebeampositionintheflame,theburnerpositionissetsothatthebeampassestheoptimumposition.Fig.2.5Distributionofchromiumatomsinair-acetyleneflame(Atomicabsorptionspectroscopy,W,salvin)2.3Analysisconditionsofelectro-thermal(flameless)atomicabsorptionElectro-thermal(flameless)atomicabsorptionconductsheatinginthreebasicstagesforsampleatomization.ThefirststepistheDryingStage,whichevaporatesthesolvent.ThesecondstepistheAshingStage;todissolveorganicmatterinthesampleandevaporatethesalts.ThethirdstepistheAtomizationStage.Ifneeded,aCleaningStagecanbeset.Thefollowingdescribeseachconditionsetting.a)DryingconditionThisstageistoevaporatethesolvent.Theheatingtemperatureandtimearesetdependingonthetypeandquantityofthesolventusedformeasurement.Thestandardheatingtemperatureforevaporatingthesolventis60oCto150oCforwater-typesamples,or50oCto100oCfororganic-typesamples.Theheatingtimeisbasedon1secondper1mlofthesample.Theheatingtemperatureandtimearesetsothatthesolventisevaporatedcompletely.Ifthedryingconditionisnotperfect,afizzle(bumping)isheardorsmokeblowsthroughthegraphitetubeholewhenthenextstageisentered.Toclearlyexamine,setthemeasurementmodetothedeuteriumlampmode,andcheckiftheabsorptionpeakisexactlyzero.Theaboveisthejudgmentcriteria.-25- Therearetwoheatingmethods:StepandRampmodes.Inthestepmode,thefurnaceisdirectlyheatedtothetargettemperature,atthebeginningofthestage,andmaintainedataconstanttemperatureuntiltheendofthestage.Intherampmode,heatingisperformedataconstantratesothatthetargettemperatureisreachedbytheendofthestage.Thesampleinjectedinthegraphitetubediffuses(spreads)inthetube.Iftoomuchsampleisinjectedorsampleviscosityishigh,thesamplemaystayonthesurfaceofthegraphitetube.Ifsharpheatingisdone,thesamplebubblesorbumps.Whenbubblingorbumpingoccurs,thesamplefliesofffromthefillerportanddiffusesatrandominthetube,makingreproducibilityworse.Insuchacase,itiseffectivetomakeheatingbystepmodeataslightlylowertemperaturethanthesolventevaporatingtemperature.However,rampmodeheatingiseasiertosetthecondition.Rampmodeheatingandstepmodeheatingmaybecombinedtoincreasethedryingefficiency.Thepyrolyticgraphitetubehassmallfiltrationduetoitsfinesurface.Therefore,specialcareisnecessary.Spreadingconditionsofthesampleintothetubevarieswiththegraphitetubetemperatureandsampleinjectiontoworsenreproducibility.So,itisdesirabletoinjectthesampleundertheconstanttemperatureof10to15oChigherthanroomtemperature.b)AshingconditionIforganicmatter,orsalts,existintheatomizationstage,backgroundabsorption(chemicalinterference)occursgivinganerrorintheanalysisvalue.Therefore,organicmatterandsaltsareevaporatedintheashingstagewherepossible.Itisdesirabletoincreasetheashingtemperatureashighaspossibletoremoveorganicmatterandsalts.However,iftheashingtemperatureisincreased,evaporationofthetargetmetalhappensanderrorsintheanalysisvaluesoccur.Therefore,itmusthavealimit.Thevolatilization(evaporation)temperatureofthetargetmetalischeckedinadvancetodecidetheashingtemperature.Fig.2.6showstherelationbetweentheashingtemperatureandabsorptionsensitivityofaleadsolutionwithnitricacid.Theashingtemperatureandabsorptionsensitivityevery100oCsuggestthatvolatilizationoccursfrom500oCinthecaseoflead.-25- Theconditionisstudiedonleadnitrate,butthevolatilizingtemperaturemustbecheckedonthesamechemicalspeciesasthesampletobemeasured.Thatisbecausethevolatilizingtemperaturevarieswiththechemicalspeciesofthetargetmetalgeneratedintheashingstage.Fig.2.6RelationbetweenleadashingtemperatureandsensitivityBackgroundabsorptiondecreasesastheashingtemperaturerises.Fig.2.7showsbackgroundabsorptionattheleadwavelengthof1/10dilutedwholebloodsolution,asoneexample,toshowbackgroundtendency.Astheashingtemperaturerisesto300,400and500oC,backgroundabsorptiondecreasesbutisnotlostcompletely.Therefore,ahigherashingtemperatureisdesirable.Itisassumedthatleadstartsvoltilizationattheashingtemperature500oCasshowninFig.2.6andtheashingtemperatureofleadcannotberaisedabove500oC.-25- Fig.2.7RelationbetweenashingtemperatureandbackgroundabsorptionOnemeanstodecreasebackgroundabsorptionistodilutethesample,butitcannotbeappliedwhendensityofthetargetmetalisverylow.Amatrixmodifierisusedinsuchacase.Palladium(II)nitrateandnickelnitrateareusedasthematrixmodifier.Theyhavetheeffectofincreasingthevolatilizingtemperatureofthetargetmetalasmentionedin5.3.Thatis,becausetheashingtemperaturecanberaised,backgroundabsorptioncanbedecreasedandabsorptionsensitivitycanbeincreased.Stepmodeheatingandrampmodeheatingareavailableastheheatingmethodinthesamewayasdrying.Instepmodeheating,saltsinthegraphitetubemayblowoutfromthesamplefillerportaftercompletionofdrying.Generally,themethodcombinedwithrampmodeheatingandstepmodeheating;rampheatingisdonefromdryingtemperaturetoashingtemperature,taking10to20secondsandthentheashingtemperatureiskeptforthespecifiedtime.Heatingtimeintheashingstagevarieswiththequantityofsalt,ororganicmattercontainedinthesample,andisgenerally30to60seconds.Whetherashingisperfectornotforthisheatingtimecanbecheckedbymagnitudeofbackgroundabsorption.Thedeuteriumlampmodeissetasthemeasuringmodeandabsorptionpeakintheatomizingstageismeasured.Thetimewhenabsorptionmagnitudedoesnotchange,eveniftheashingtimeisextended,isthesettingtime.c)AtomizingconditionThisstepistoatomizethetargetmetal.Heatingmaybemadeforabout5secondsataslightlyhighertemperaturethantheatomizingtemperatureofthetargetmetal.-25- Absorptionsensitivity,whentheatomizingtemperatureischanged,ischeckedtodecidetheatomizingtemperature.Fig.12.8showstherelationbetweentheatomizingtemperatureandabsorptionsensitivity.Itindicatesthatheatingmaybedoneat2500oCorabove.Fig.2.8RelationbetweenaluminumatomizingtemperatureandsensitivityIftheatomizingtemperatureissettoohighformetalsoflowmeltingpointsincludingcadmiumandlead,theatomstayingtimeinthetubebecomesextremelyshortandsensitivitymaydrop.Metalsincludingboron,molybdenumandcalciumareeasilymaintainedinthegraphitetube.Therefore,atomizationisdoneatatemperatureashighaspossibleorpyrolyticgraphitetubeisused.About1l/minofargonisrunthroughthegraphitetubeinthedryingandashingstages.Ifargongasisrunintheatomizingstage,sensitivitydropssharply.Therefore,argonisstopped.Sensitivitycanbeadjustedfivetimesasmuchbychangingargonflowfrom0to1.5l/mintoadjustabsorptionsensitivity.Stepheatingisgenerallyused.Whenbackgroundabsorptionattheatomizationstageisbig,atomicabsorption,backgroundabsorption,andmeasurementshouldbemadebyrampheating.Theheatingtimeissetsothattheatomicabsorptionpeakreturnsto0levelwithintheheatingtime.However,whenthemetaliseasytostayinthegraphitetubeorbackgroundabsorptionisbiganddoesnotreturnto0level,thetimewhenthepeakreturnstothespecifiedlevelissetasheatingtime,andcleaningisdonethereafter.-25- Cleaningisdonetoevaporatemetalandsalt,whichremainsinthegraphitetube,attheendoftheatomizingstage.Heatingcanbedonesufficientlyfor2to3secondsatthemaximumtemperatureof3000oCbutlowertemperatureisdesirablewherepossible.Thestandardcleaningtemperatureistheatomizationtemperatureplus200oC.Cleaningisdoneatabout2500oCforcadmiumandlead,whichhavelowatomizationtemperatures.d)SampleinjectionquantityProportionalrelationsdonotworkbetweenthesamplequantityinjectedinthegraphitetubeandabsorptionsensitivity.Thisisbecausethediffusionareainthetubeandfiltrationdepthvarywithsampleinjectionquantity.Therefore,thecalibrationlinecanbepreparedbychangingtheinjectionquantityofthestandardsolutionfromthespecifieddensity.Solutionsofdifferentdensitiesareinjectedinthespecifiedquantityatonetime.Theinjectionquantityofthestandardsampleisnaturallythesameasthatofthesample.Themaximumsampleinjectionquantityis50mlbutdiffusionandfilteringdepthvarywithadifferenceinphysicalpropertiesofthesample.Itspreadstothelowtemperaturepart,oroverflowstothefillerportoftendroppinganalysisaccuracy.Therefore,10to20mlisideal.-25-'