(5.3)-4.3原子分光光度计的结构

4.3StructureofatomicabsorptionspectrophotometerAtomicabsorptionspectrometrymoduleofⅠ.StructureofatomicabsorptionspectrophotometerFornearlyhalfacentury,AtomicAbsorptionSpectrometry(AAS)isananalyticalmethodthathasbeenwidelyusedforquantitativemeasurementanddeterminationofindividualelementsinsamples.Therearetwomaintypesofatomicabsorptionspectrometer,namelysingle-beamanddouble-beamtype.ItsbasicconstructionprincipleisshowninFigure3-1.Ascanbeseenfromthediagram,iftheatomizerisregardedastheabsorptionpoolinthespectrophotometer,thestructureprincipleoftheinstrumentissimilartothegeneralspectrophotometerthatis,generallycomposedoffourmainparts:lightsource,atomizationsystem,opticalsystemanddetectionsystem.Figure4.3.1BasicStructureofAtomicAbsorptionSpectrometer2LightsourceAtomizationsystemMonochromatorSpectroscopicsystemPhotomultiplierInspectionsignalprocessingDisplaysignalprocessingMonochromatorSignalProcessingI.Structureofatomicabsorptionspectrophotometer-lightsourceTherequirementsofatomicabsorptionmethodforitslightsourceareasfollows:

Thehalfwidthoftheemissionspectrumlineshouldbenarrowerthanthatoftheabsorptionspectrumline.

Theintensityoftheemissionspectrumlineshouldbelargeenoughtoensurethatthereisenoughsignal-to-noiseratio;

Theintensityoftheemissionspectrumlineshouldbestableandthereshouldbenobackgroundemissionorsmallbackgroundemission.

Lowworkingvoltage,longservicelife.3I.Structureofatomicabsorptionspectrophotometer-lightsourceHollowCathodeLampisaspecialglowdischargetube,withitsdischargeandluminescencemechanismthesameascommonglowdischargetube,andspecialfeaturesinthematerialselectionofthelampcathodeandanodeandthedeviceinstallationmethod,whichisforthepurposeofmeetingtheneedtoproducesharplinelight.Thestructureofhollowcathodelampmainlyincludes:ananodeandahollowcylindricalcathode.Thetwoelectrodesaresealedinaglasscasewithanopticalwindow(glassorquartzwindow)filledwithalow-pressure(2~10mmHg)inertgas.ThestructureisshowninFigure3-2.Figure4.3.2StructureofHollowCathodeLamp4GlasscaseHollowcathodeAnodeQuartzwindowI.Structureofatomicabsorptionspectrophotometer-lightsourceTheworkingprincipleofhollowcathodelampisthatwhenaproperDCvoltage(usually300~500V)isappliedbetweenthecathodeandanode(glowinganddischargingstart),veryfewcationsexistinginthegasbetweenthepolesmovetowardsthecathodeundertheactionofanelectricfield,andthecathodesurfaceisbombardedwhichcausedtheelectronsacquiredenergyescapingfromthesurface(i.e.,leavethecathode),andtheescapedelectronsareacceleratedbytheelectricfieldandruntowardtheanode.Inthecourseoftherushtotheanode,thecollisionwiththeencounteredatomsionizedthelattertoproducenegativeanionandpositiveanion,andthepositiveanionrushestothecathodeundertheactionofanelectricfieldandbombardedthesurfaceofthecathode.Inthisway,oncethedischargeistriggeredatahighervoltage(glowingstartingvoltage),thedischargecanbemaintainedatalowervoltageandthelightwillglow(lightglowing).5Ⅰ.ThestructureoftheatomicAbsorptionSpectrophotometer-AtomizationsystemTheroleoftheatomizationsystemistoconverttheundertestelementsinthesampleintoagaseousground-stateatom(theprocessiscalledatomizationofthesample)andsenditintothelightpathtoabsorbtheresonance(characteristic)radiationprovidedbythehollowlamp.(1)FlameatomizationdeviceThedeviceusedinflameatomizationmethod(calledflameatomizer)ismainlycomposedofthreeparts:atomizer,fogchamberandburner.

Thefunctionoftheatomizeristodispersethetestliquidintoverysmalldroplets(5~70μmindiameter),whoseperformancehasasignificanteffectonthesensitivity,accuracy,precisionandchemicalinterferenceforthemeasurementanddetermination,andisthecorecomponentoftheatomicabsorptionspectrometer.6ThrottlingpipeBoosterTestliquidImpactballCapillariesⅠ.ThestructureoftheatomicAbsorptionSpectrophotometer-Atomizationsystem(1)Flameatomizationdevice

Burner:Afteratomizingthetestliquidintothepre-mixingchamber(alsoknownasfogchamber,whoseeffecthastwo,oneistomakethefogdropdiametereven,theotheristomakefuelgas,combustion-supportinggasandsmallfogdropevenlymixed),gas-solisformedandintotheflame,andthelargerfogdropcondensesinthewall,andisdischargedthroughthewasteliquidpipebelow.Figure4.3.3PremixedBurners1.Flame2.Burner3.Impactball4.Capillary5.Atomizer6.Testsolution7.Wasteliquid8.Pre-mixingchamber7Ⅰ.ThestructureoftheatomicAbsorptionSpectrophotometer-Atomizationsystem(1)Flameatomizationdevice

Flameanditstype:whatatomicabsorptionspectrometrymeasuresistheground-stateatom,whileflameprovidesenergyfortheatomizationofthesample.Undertheactionofflametemperature,compoundsundergoaseriesofcomplexprocesses,suchasevaporation,drying,gasification,dissociationandevenexcitation,ionizationandcoalescence.Figure4.4.4PremixedBurners1.Flame2.Burner3.Impactball4.Capillary5.Atomizer6.Testsolution7.Wasteliquid8.Pre-mixingchamber8Ⅰ.ThestructureoftheatomicAbsorptionSpectrophotometer-AtomizationsystemFlameisformedbytheburningoffuelgas(reducingagent)andcombustion-supportinggas(oxidizingagent)afteraviolentchemicalreaction.First,anormalflameisalsocalledaneutralflame.Itisalsocalled"chemometryflame"becausetheratiooffuelgasandcombustion-supportinggasiscalculatedaccordingtothechemicalreactionbetweenthem.Theflamehasthecharacteristicsofhightemperature,littleinterference,stabilityandlowbackground.Second,therichburningflame,i.e.theflameformedwhentheproportionoffuelgasishigherthantheratioofchemometry.Theflamehasrichsemi-decompositionproductsandstrongreducingatmosphere.Itsflametemperatureisslightlylowerthantheformer,buttheregionsuitableforatomizationislarger.9Ⅰ.ThestructureoftheatomicAbsorptionSpectrophotometer-AtomizationsystemThird,thepoorburningflame,i.e.theflameformedwhentheproportionoffuelgasislowerthantheproportionofchemometry.Inthiskindofflame,becausealotofcoldcombustion-supportinggastakesawayheatfromtheflame,sothetemperatureisrelativelylow.Thetwotypesofflamemostcommonlyusedarehydrocarbonflameandhydrogenflame.Air-acetyleneflameandlaughinggas-acetyleneflamebelongtothefirstkindofflame.Thesetwokindsofflamehavebasicallymettherequirementsofatomicabsorptionanalysisanddeterminationofmostelements,sotheyarealsothemostcommonlyusedflameindailyanalysis.Flamelevelsproducedbypre-mixedburnersareclearandcanberoughlydividedintofourdistinctareas:IntermediatethinareaSecondaryreactionareaFirstreactionareaBurnerseam10Ⅰ.ThestructureoftheatomicAbsorptionSpectrophotometer-AtomizationsystemCompositionandPropertiesofCommonFlame11Table3~1CompositionandPropertiesofCommonFlamesFlamenameAiracetyleneAir-richacetyleneLaughinggas-acetyleneAir-pooracetyleneBurningspeedSlightly<2300Maximumflametemperature/℃Emissionbackgroundorvoice.FlameatmosphereOxidabilityStrongoxidabilityReducibilityStrongreducibilityThelowemissionbackgroundhasstrongabsorptionabilityintheshortwaveregiontomakestrongernoise.Loweremissionbackground,lowerCNandNHpeaksStrongemissionbackgroundandnoiseStrongemissionbackgroundandnoiseCNandNHpeakincreaseHalfoftheelementscanbedeterminedtohavealowsensitivitytowardselementseasytoformrefractoryoxidelikeW,Mo,V,etc.Applicabletoelementsthatarevolatileordonotreadilyformoxidessuchasorganicsolventsprayoralkalis.Applicabletoelementseasytoformoxides.HighsensitivitytoW,Mo,V,etc.Applicabletoelementseasytoformrefractoryoxide(e.g.AL,Be,Si,Ti,W,VandRareEarth)ApplicationsituationAuxiliarygasFlametypeAuxiliarygasFlowRateRatioⅠ.ThestructureoftheatomicAbsorptionSpectrophotometer-AtomizationsystemGassourcedeviceandusesafetymeasuresIntheflameatomicabsorptionmethod,therearemanykindsoffuelgasandcombustion-supportinggasthatmaybeused.Sourcesofvariousgasarenotthesame,andnotallofthemaresuppliedbyinstruments.①Airismostlysuppliedbyaircompressor.Theaircompressorhasthedistinctionofpistontypeandthemembranetype.Generally,thepressureoftheformeris6×105Pa(6atm),whichshouldbeusedafterdecompression,stabilizationandpurification(removingwatervaporfromcompressedairandoilbroughtbyaircompressor);themaximumpressureofthelatteris3×105Pa(3atm),andthepressurecanbeadjustedtotherequiredvaluebythesafetyvalve.Theservicepressureofairisgenerally2×105Pa(2atm)andtheflowrateis10~20L/min.12Ⅰ.ThestructureoftheatomicAbsorptionSpectrophotometer-Atomizationsystem②Acetyleneismostlyprovidedbysteelcylinders.Theacetyleneinthesteelcylinderisdissolvedunderpressureintheporousadsorptionmaterialofacetone.Itneedstobeusedafterpressurereducingandstabilizing.Themaximumpressureinthecylinderis15×105Pa(15atm).Thepressurecanbeadjustedbythepressurestabilizingregulatortothedesiredvalue.Forthespecificpressureandflowrateofacetyleneofaninstrumentshouldbesettotherequiredvaluewithreferencetotheinstrumentspecification.13Ⅰ.ThestructureoftheatomicAbsorptionSpectrophotometer-Atomizationsystem③Nitrousoxideisagasthatirritatesthesenseofsmellandexcitespeople,soitisknownaslaughinggas.Itismostlysuppliedbysteelcylinders.Thepressureinsidethecylinderisabout7×106Pa.Itneedstobeusedafterpressurereducingandstabilizingbythepressurestabilizingregulator.14Ⅰ.ThestructureoftheatomicAbsorptionSpectrophotometer-AtomizationsystemInatomicabsorptionanalysis,attentionshouldbepaidtotheuseoffuelgasandthefollowingspecialsafetymeasuresshouldbetakentoavoidpersonalorequipmentaccidents.(a)Fuelgas,combustion-supportinggassteelcylindersandacetylenegascylindersshallbeabsolutelyfarfromthesourceoffire.(b)Checkandensurethatthegassystemisproperlysealedbeforeignitionoperation.Beforeignition,thegassourceshouldbeturnedon,andthegaspressureandflowrateshouldbesetandconfirmedastheparametersspecifiedintheinstrumentspecification.(c)Whentheflameisshutdown,thefuelgasvalveshallbeshutdownfirst,andaftertheflameisextinguished,thefuelgasvalveshallbeshutdownincaseoftempering.Foranewtypeofinstrumentcontrolledbyamicrocomputer,whenthegassourceisclosed,thetotalpowerswitchoftheinstrumentshouldbeshutdownaftertheflameisextinguished.15Ⅰ.ThestructureoftheatomicAbsorptionSpectrophotometer-AtomizationsystemInatomicabsorptionanalysis,attentionshouldbepaidtotheuseoffuelgasandthefollowingspecialsafetymeasuresshouldbetakentoavoidpersonalorequipmentaccidents.(d)Expansionchambersorburnersshallbefittedwithasafetyventplugandshallalwaysbekeptingoodworkingordertoensuresafety.(e)Temperingshallbeprevented,especiallyfornitrousoxide-acetyleneflame.Ignitionandflameoutmustbestrictlythroughtheair-acetyleneflametransition,thatis,toensurethatthenitrousoxide-acetyleneflameisalwaysarichflame(redflame).(f)Thecompletionoftheworkshallensurethatthesourceoffuelgasiscutoffandthatanypossibilityofleakageisasafetyhazard,especiallytheneedlevalveoffuelgasflowmeteriseasytobesealedbadly,onceitleaksintotheinstrument,itiseasytoexplode.(g)Theupperpartoftheinstrumentshallbefittedwithanexhaustdeviceandmaintainedingoodorderandeffectively.16Ⅰ.ThestructureoftheatomicAbsorptionSpectrophotometer-Atomizationsystem(2)Non-flameatomizationdeviceNon-flameatomizationisageneraltermforotheratomizationmethodsotherthanflameatomization,includingmanyspecificmethods.Atpresent,theelectricallyheatedhigh-temperaturegraphitefurnaceatomizerismorewidelyused,whicharecalledthemethodof"atomizationingraphitefurnace"(GFA).StructureofatomizationingraphitefurnaceAssemblyofgraphitetube17InsulatorGraphitetubeLightbeamSpecimenLightbeamGraphitetubeCoolingwaterCasingpipeInertgasQuartzwindowSampleinletpowersourceⅠ.ThestructureoftheatomicAbsorptionSpectrophotometer-AtomizationsystemTheatomizationingraphitefurnacetakesthewayofprogrammedheating,andisdividedintofoursteps:drying,ashing,atomizing,andpurifying(or"residualremoving"),whichiscontrolledautomaticallybymicrocomputer.TheheatingmethodsoftheatomizationingraphitefurnacehavethesteptypeandtheslopetypeasshowninFigure3-10.Thelattercanmakethesamplemoreeffectivelyashing,reducethebackgroundinterference,andcontrolthechemicalreactionspeedwithincreasingtemperature,whichismorebeneficialtothedeterminationofthedifficultvolatileelements.Figure4.4.5Heatingprocessofatomizationingraphitefurnace18absorbanceTimeAtomicabsorptionrodMolecularabsorptionorscatteringCurrentAshingTimeDryingatomizerⅠ.ThestructureoftheatomicAbsorptionSpectrophotometer-AtomizationsystemThebestadvantageofgraphitefurnaceatomizationisthatthesampleinjectedintothegraphitetubecanbeatomizedalmostcompletely(theatomizationefficiencycanreachabout90%),becausethecavityspaceismuchsmallerthantheflame,sotheatomicvaporconcentrationisgreatlyincreased,sothesensitivityofthedeterminationisalsogreatlyimproved.Thisatomizationmethodisappropriatewhenthesamplecontentisverylow,oronlyaveryfewamountofsamplesareprovided.19Table3-2ComparisonofFlameMethodandGraphiteFurnaceMethodElectricalenergyAbout3000℃(temperatureofgraphitetube,lowergastemperatureintube)Flatpeak(1)Subsidizedthermalenergysupply(2)Maximumtemperature(3)Atomizationefficiency(4)Samplevolume(5)Signal(Peakshape)(6)SensitivityDetectionlimit(7)Accuracy(8)MatrixeffectSpikeAbove90%Flame29551℃(toacetylene-nitrousoxideflame)LargeSmallAbout10%About1mlProjectflameatomizationElectrothermalatomizationlowhighDeformationcoefficientDeformationcoefficientⅠ.ThestructureoftheatomicAbsorptionSpectrophotometer-Atomizationsystem(3)Otheratomizationmethods1)Hydrideatomizationdevices.Themethodofhydrideatomizationisalsocalledhydridegenerationmethod.Thismethodisoneofthe"lowtemperature"atomizationmethods.Itismainlyusedtodetermineelementssuchas,As，Sb，Bi，Sn，Ge，Se，andPb,whichcanformhydridebychemicalreactionatnormaltemperature.Whentheaboveelementsreactwiththestrongreducingagentofsodiumborohydride(potassium)atalowertemperatureinanacidicmedium,agaseoushydrideisgenerated.2)Cold-vapouratomizationdevice.Itisalsocalled"coldatomicabsorptionmethod".Inthismethod,mercuryionsinthesolutionarefirstreducedtosimplemercurybySnCl2orhydroxylaminehydrochloride,andthenthemercuryvapor(usingitslowboilingpointcharacteristic)isbroughtintothegasabsorptiontubewithquartzwindowbyairflowforatomicabsorptionspectrometry.20I.StructureofAtomicAbsorptionSpectrophotometer-OpticalSystemTheopticalsystemofatomicabsorptionspectrometeriscomposedoftwosystems:concentratedlight(externalopticalpath)andspectroscopic(monochromator).ItsgeneralstructureprincipleisshowninFigure4.3.6.(1)ExternalopticalsystemItsfunctionisthatthefirstlensfocusestheresonancelineemittedbythelightsourcecorrectlyinthecenteroftheatomicvapor(flame)ofthesamplebeingtested,andthenthesecondlensfocusesthespectrallinepassingthroughtheatomicvaporontheincidentslitofthemonochromator.Figure4.3.6Single-beamexternalopticalsystem21LightsourceMonochromatorIncidentslitFlameAlensAlensI.StructureofAtomicAbsorptionSpectrophotometer-OpticalSystem(1)SpectroscopicSystem(Monochromator)TheportionofthedottedlineinFigure4.3.7.Mainlycomposedbythedispersionelement(gratingorprism),collimationmirror,slitandsoon.Figure4.3.7SpectrumSystemS1-incidentslitM-collimationmirrorG-gratingS2-exitslitPM-detector22Ⅰ.Structure-DetectionSystemofAtomicAbsorptionSpectrophotometerThedetectionsystemofatomicabsorptionspectrometermainlyincludesdetector,amplifier,andlogarithmicconverteranddisplaydevice.(1)DETECTORThefunctionoftheatomicabsorptionspectrometerdetectoristoconverttheopticalsignalfrommonochromatorintoelectricalsignal.Atpresent,thephotomultiplierismainlyusedasphotoelectricconverter(detector).InordertoselecttheneededgainandtoadjustthetransmissivityT%easilyto100%(thatis,theabsorbanceAequalszero),thepowersupplyvoltageofthephotomultiplier(alsoknownasgainnegativehighvoltage)isadjustable.PhotomultiplierdiagramK-photosensitivecathode1~4-dynodesA-anodeR,R1~R5-resistance23NegativevoltageLightⅠ.Structure-DetectionSystemofAtomicAbsorptionSpectrophotometer(2)AmplifierItsfunctionistoamplifytheoutputelectricsignalofthephotomultiplier.Thelightemittedbythelights