儀器分析原理principles of instrumental analysis(6th,2007)4-2

1、_2HighP~..?gasflowItSamplesolutionsample~50...1..“.........?.........“...?...filmIr~~“...Highpressuregasflow(e)FIGURE811Typesofpneumaticnebulizers:(a)concentrictube(b)crossflow(c)frilleddisk(d)Babington.atomizer.Asmallli

2、quidsolidsampleisplacedonaconductsuchasacarbontubetantalumfilament.Anelectriccurrentthenevapatesthesamplerapidlycompletelyintotheargonflow.Incontrasttothenebulizerarrangementswehavejustconsideredanelectrothermalsystempro

3、ducesadiscretesignalratherthanacontinuousone.Thatisthesignalfromtheatomizedsampleincreasestoamaximumthendecreasestozeroasthesampleissweptthroughtheobservationregion.Peakheightsareasthenprovidethedesiredquantitativeinfmat

4、ion.HydrideGenerationTechniquesHydridegenerationtechniques6provideamethodfintroducingsamplescontainingarsenicantimonytinseleniumbismuthleadintoanatomizerasagas.Suchaprocedureenhancesthedetectionlimitsftheseelementsbyafac

5、tof10to100.Becauseseveralofthesespeciesarchighlytoxicdeterminingthemat“Fadetaileddiscussionofthesemethods.seeTNJkahara.inSampf“InlroduccioninAtomicSpecrroscopr.JSnt:ddon.ed..Chap10.1ewYk:Elsevier.19YO:JDidinaD.LTsak“H(ir

6、idlCl!fltfl1!imlAtomilSpectroscopy.NewYurk:Wiky199)lowconcentrationlevelsisquiteimptant.Thistoxicityalsodictatesthatgasesfromatomizationmustbedisposedofinasafeefficientmanner.Volatilehydridescanusuallybegeneratedbyadding

7、anacidifiedaqueoussolutionofthesampletoasmallvolumeofaI%aqueoussolutionofsodiumbohydridecontainedinaglassvessel.Atypicalreactionisobtainedbyintroducingsamplesolutionsbynebulization.Mostofthesetechniquesleadtoadiscreteana

8、lyticalsignalratherthanacontinuousone.DirectSampleionInthedirectsampleiontechnique.thesampleisphysicallyplacedintheatomizer.Fsolidsthesamplemaybegroundintoapowderwhichisthenplacedoninaprobethatiseddirectlyintotheatomizer

9、.Withelectricarcsparkatomizersmetalsamplesarefrequentlyintroducedasonebothelectrodesthatareusedtofmthearcspark.3BH(aq)3H(aq)4H]AsO](aq)3HBOJ(aq)4AsH](g)3Hp(l)Thevolatilehydrideinthiscasearsine(AsH))issweptintotheatomizat

10、ionchamberbyaninertgas.Thechamberisusuallyasilicatubeheatedtoseveralhundreddegreesinatubefurnaceinaflamewheredecompositionofthehydridetakesplaceleadingtofmationofatomsoftheanalyte.Theconcentrationoftheanalytcisthenmeasur

11、edbyabsptionemission.Thesignalhasapeakshapesimilartothatobtainedwithelectrothermalatomization.ElectrothennalVapizerrothermalvapizerswhichweredescribedbrieflyintheprevioussectionarealsousedfvarioustypesofsolidsamples.Thes

12、ampleisheatedconductivelyoninagraphitetantalumrodboat.Thevapizedsampleisthencarriedintotheatomizerbyaninertcarriergas.ArcSparkAblationElectricaldisgesofvarioustypesareoftenusedtointroducesolidsamplesintoatomizers.Thedisg

13、einteractswiththesurfaceofasolidsamplecreatesaplumeofaparticulatevapizedsamplethatisthentransptedintotheatomizerbytheflowofaninertgas.Thisprocessofsampleintroductioniscalledablation.Farcsparkablationtobesuccessfulthesamp

14、lemustbeelectricallyconductingitmustbemixedwithaconduct.Ablationisnmallycarriedoutinanine~tatmospheresuchasanargongasstream.Dependmelfl~lnd:0...llnta~eranJDW.Golightly:0..Chlp16.NeY“rk:VCH.l192VatercoolingFIGURE812Aglowd

15、isgeatomizer.(FromD.S.GoughP.HannafdR.M.LoweAnal.Chern.1989611652.Figure1(a)p.1653.Copyright1989AmericanChemicalSociety.)whereablationtakesplacetoconvertthesampleintoaplumeofvappartiCUlatematterthatisthensweptintotheatom

16、izer.Laserablationisapplicabletobothconductingnonconductingsolidsinganicganicsamplespowdermetallicmaterials.Inadditiontobulkanalysisafocusedlaserbeampermitsanalysisofsmallareasonthesurfaceofsolids.Severalinstrumentmakers

17、offerlasersamplers.TheGlowDisgeTechniqueAglowdisge(GD)deviceisaversatilesourcethatperfmsbothsampleintroductionsampleatomizationsimultaneously(seeFigure812).Aglowdisgetakesplaceinalowpressureatmosphere(Ito10tr)ofargongasb

18、etweenapairofelectrodesmaintainedatadevoltageof250to1000V.Theappliedvoltagecausestheargongastobreakdownintopositivelygedargonionselectrons.Theelectricfieldacceleratestheargonionstothecathodesurfacethatcontainsthesample.N

19、eutralsampleatomsarethenejectedbyaprocesscalledsputtering.Therateofsputteringmaybeashighas100figmin.Theatomicvapproducedinaglowdisgeconsistsofamixtureofatomsionsthatcanbedeterminedbyatomicabsptionfluescencebymassspectrom

20、etry.Inaddition.afractionoftheatomizedSeeR.K.~t3rcus1.R.Harville.Y.~lei.C.R.Shick.Anal.Chern..199460.)02AV.WHarrison.C.M.Barshick1.A.KingleTP.H.RatliffY.I.1cl.Anal.Chem..l990.62943A:R.K.1Iarcus.Spearopy.1992.:“j.“il.12:G

21、I(H~[)i~dJ.1rgeSpectroscupiesR.K.~l::ncllsed.NewYk:PlenumPress.IYJ3AtomicAbsptionAtomicFluescenceSpectrometryInthischapterweconsidertwo(lpesofopticalatomicspectrometricmethodsthatusesimilartechniquesfsampleintroductionat

22、omization.Thefirstisatomicabsptionspectrometry(.4AS)whichfnearlyhalfocenturyhasbeenthemostwidelyusedmethodft~determinationofsingleelementsinarwlytjcalsamp!!!.Thesecondisatomicfluescencespectrometry(4FS)whichsincethe1960s

23、hasEeenstudiedextensilev.Byc(trasttotheabsp1ionfIleTlOdatomicfluescencehasnotgainedwidespreadgeneralusefroutineelementalanalysishowever.Thusalthoughseveralinstrumentmakershaveinrecentyearsbeguntoofferspecialpurposeatomic

24、fluescencespectrometerstheoastmajityofinstrumentsarestilloftheatomicabsptiontype.Becauseofthi.~differenceinuwgewedelotethebulkofthischaptertoAASconfineourdeionofAFStoabriefsectionattheend.r71Throughoutthischapterthislogo

25、indicateslQ.janopptunityfonlineselfstudyatwww.chemistryskooglinkingyoutointeractivetutialssimulationsexercises.PritodiscussingAASIindetaiLwefirstpresentanoverviewofthetypesofatomizersusedinbothAASAFSEx.citationAtomicions

26、.loniz.ationtExcitedire“ersiblclIonsAtoms.DissociationIExcited(reversible)atomsGaseuus:“.moleculesThetwomostcommonmethodsofsampleatomizationencounteredinAASAFSflameatomizationelectrothermalatomizationarefirstdescribed.We

27、thenturntothreespecializedatomizationproceduresusedinbothtypesofspectrometry.Volatilizationt.....~~..::Solidgas.::~.w:l.aerosol.(~.~.Desohatlont“.Spray)~:.Nebulizationt~.:Inaflameatomizerasolutionofthesampleisnebulizedby

28、aflowofgaseousoxidantmixedwithagaseousfuelcarriedintoaflamewhereatomizationoccurs.AsshowninFigure91acomplexsetofinterconnectedprocessesthenoccurintheflame.Thefirstisdesalvationinwhichthesolventevapatestoproduceafinelydiv

29、idedsolidmolecularaerosol.Theaerosolisthenvolatilizedtofmgaseousmolecules.Dissociationofmostofthesemoleculesproducesanatomicgas.Someoftheatomsinthegasionizetofmcationselectrons.Othermoleculesatomsareproducedintheflameasa

30、resultofinteractionsofthefuelwiththeoxidantwiththevariousspeciesinthesample.AsindicatedinFigure91afractionofthemoleculesatomsionsarealsoexcitedbytheheatoftheflametoyieldatomicionicmolecularemissionspectra.Withsomanycompl

31、exprocessesoccurringitisnotsurprisingthatatomizationisthemostcriticalstepinflamespectroscopytheonethatlimitstheprecisionofsuchmethods.Becauseofthecriticalnatureoftheatomizationstepitisimptanttoundersttheacteristicsofflam

32、esthevariablesthataffecttheseacteristics.AnalytesolutionFIGURE91Processesoccurringduringatomization.thattemperaturesof17000eto2400eoccurwiththevariousfuelswhenairistheoxidant.Atthesetemperatures.onlyeasilydecomposedsampl

33、esareatomizedsooxygennitrousoxidemustheusedastheoxidantfmerefractysamples.Theseoxidantsproducetemperaturesof25000eto31oooewiththecommonfuels.TypesofFlamesTable91liststhecommonfuelsoxidantsusedinflamespectroscopytheapprox

34、imaterangeoftemperaturesrealizedwitheachofthesemixtures.NoteMaximumBurningTemperatureVelocityFuelOxidantCcms1NaturalgasAir170019003943NaturalgasOxygen27002800370390HydrogenAir20002100300440HvdrogenOxygen255027009001400Ac

35、etyleneAir21002400158266AcetyleneOxygen3050315011002480AcetyleneNitrous26002800285oxideGeneralreferencesonatomicahsptionspectrometryincludeLH.JLljunenP.PcramakiSpectrochemicalAnalnisbyAwmieAbmrptioflF.missiol2ndcd.Camhri

36、dgc:RoyalSocietyofChemistry.2004:J.A.CRroekaertAnllyticalAromicSpfCIfOmelryl~ilhFlamesPillSmasVeinheimGermany:WileyveH.2002RMagyar.GuideLinestoPlanningAtomicSpectrometricAnalysis.NewYk:Elsevier19HZ:1.0IngkJr.S.R.Crouch.S

37、pectrochnnicalAnalysis.Chap.10.EnglewoodCliffs.NJ:PrenticeHaiL188~M.SperlingB.Welz.AtomicAhsptlonSpeetrometn.3rded..Nc:Yk:VCHPublishers.1999:N.HBill~s.A.Bogaerts.JA.CHroekaert.Anal.Chern..2004.70..1113Secondarycombustion

38、zoheburningvelocitieslistedinthefourthcolumnofTable91areimptantbecauseflamesarestableonlyincertainrangesofgasflowrates.Ifthegasflowratedoesnotexceedtheburningvelocitytheflamepropagatesbackintotheburnergivingflashback.Ast

39、heflowrateincreases.theflamerisesuntilitreachesapointahovetheburnerwheretheflowvelocitytheburningvelocityareequal.Thisregioniswheretheflameisstable.Athigherflowratestheflameriseseventuallyreachesapointwhereitblowsofftheb

40、urner.Withthesefactsinminditiseasytoseewhyitissoimptanttocontroltheflowrateofthefueloxidantmixture.Thisflowrateverymuchdependsonthetypeoffueloxidantbeingused.FlameStructureAsshowninFigure92.imptantregionsofaflameincludet

41、heprimarycombustionzonetheinterzonalregionthesecondarycombustionzone.Theappearancerelativesizeoftheseregionsvaryconsiderablywiththefueltooxidantratioaswellaswiththetypeoffueloxidant.Theprimarycombustionzoneinahydrocarbon

42、flameisrecognizablebyitsblueluminescencearisingfromthebemissionofC“.CH.otherradicals.Thermalequilibriumisusuallynotachievedinthisregionitis.thereferarelyusedfflamespectroscopy.Theinterzonalarea.whichisrelativelynarrowins