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IntroductiontoElementalAnalysis AduststormchargesacrosstheSaharadesert,whippingupsandand blowingitwest,towardstheocean.Meanwhile,inthemiddleoftheAtlanticOcean,a teamofscientistusesfilterstocollectdustparticlesintheair,inhopesofidentifying whichelementsarebeingtransportedacrosstheAtlantic,fromadesertthousands ofmilesaway.ThesescientistswilluseICPtechnologytostudytheelemental compositionofthedust. Everyobjectonearthismadeofelements,fromthesandintheSaharatothe snowintheAntarctic.Infact,allordinarymatterintheuniverseismadeupof elements.Someelementswillsoundveryfamiliar,suchashelium,whichisusedto fillballoons,orsodiumandchloride,whichyouaddtoyourfood.Othersmaybea bitmoreobscure,likepraseodymium(usedtocreateaircraftengines)orruthenium (usedtomakejewelry).Theword“element”isatermusedtorefertoagroupof atomsthatsharethesamedefiningcharacteristic,thenumberofprotonsinthe nucleus.Iftwoatomshavethesamenumberofprotons,butdifferentnumberof neutrons,theseatomsarereferredtoasisotopesofanelement.Inotherwords,all atomsoftheelementgoldcontainexactly79protonsinthenucleus,howeverthere are19differentisotopesofgold,withthenumberofneutronsrangingfrom90to 126. AtomicEmissionSpectroscopyOverview AtomicEmissionSpectroscopyisalsoknownasOESandisatechnique whereinelementsinamixtureareidentifiedandquantifiedbyobservingthe interactionsofatomswithelectromagneticradiation(e.g.light).InOESasampleis ionized,atwhichtimeelectronsareexcited.Uponrelaxationoftheexcitedspecies, energyisemittedintheformoflight.Theemissionofthislightismeasuredbya detector. OESisbasedontwoimportantprinciplesguidingthebehaviorofatoms.The firstprincipleisthatatomscanonlyexistindiscretestates,alsoknownaslevels, whicharecharacterizedbydistinctamountsofenergy(thisisreferredtoas quantizedenergy).Onewaytothinkaboutthisisbycomparingittolivinginatwostorybuilding.Yourhomemaybeonthegroundfloororitmaybeonthesecond floor,andyoucanliveoneitherfloor,butyoucannotliveinthespacebetween.This isthesameprincipleforatoms,andwhentheatomchangesfromoneleveltothe next,itcaneitherabsorboremitaphoton(def.:aphotonisasmallpacketofenergy thatcancarryelectromagneticradiation)withenergythatisequivalenttothe differenceinenergybetweenthetwolevels.Thesecondprincipleisthatthe frequency,ν,orwavelength,λ,oftheradiationemittedorabsorbedastheatom transitionsfromoneenergyleveltothenextisdefinedbytheequation: E f − Ei = hc , λ whereErepresentstheenergyoftheatomatitsinitial(i)andfinal(f)state,handc areconstants,andλrepresentsthewavelengthofthephotonreleased.Inother words,thegreatertheamountenergyreleased,theshorterthewavelengthofthe electromagneticradiation. Tyingthesetwoprinciplestogether,wecanseethatatlowtemperaturesthe atomsofasampleofmatterareessentiallyattheirmostrelaxedlevels,knownas groundstate,buttheseatomscanbeexcitedtoahigherenergylevelwithheatfrom aflame,aplasma,oranelectricarcorspark.Whentheatomrelaxesfromthe excitedenergyleveltoalowerenergylevel,energyisreleasedintheformof electromagneticradiationandthewavelengthofthatlightisinverselyrelatedtothe differenceinenergy. InanOES,atomsareexcitedintheinductivelycoupleplasmaandupon relaxation,photonsareemitted.Theemittedelectromagneticradiationthenpasses throughanopticalspectrometerwhereeachwavelengthisseparatedinspace(this isthesameprocessbywhichaprismseparateswhitelightintothevarious,distinct wavelengthsandproducesrainbows).Thewavelengthsemittedarecharacteristicof theatomspresent,muchlikefingerprints.Inthisway,OESisusedqualitativelyto identifytheatomspresentinasample.Theintensityoftheradiationcanalsobe quantifiedtoinfertheconcentrationofeachelement. ICP-OES–Theory Plasma AmajorcomponentoftheICP-OESistheinductivelycoupledplasma. OneofthefirststepsinICP-OESistheintroductionofsampleintotheplasma whereitisatomized.Plasmaisconsideredafourthstateofmatter,distinctfrom gas,solidandliquidphases.Tobetterunderstandthepropertiesofplasma,itis usefultocompareittotheotherstatesofmatter. Allmatterexistsinvariousstates.Inasolid,atomsaretightlyand orderlyarranged.Whenenergyisaddedtoanelementinthesolidstate,the solidchangesphaseandbecomesaliquid.Inaliquid,theatomsadheretoa definitevolume,butdonotpossessafixedshape.Asmoreenergyisappliedto thesystem,theliquidwilltransforminagas.Gassesdonotpossessafixed volumeorshape.Theatomsinthegaseousphasemoveaboutfreely.When sufficientenergyisappliedtoagas,theatomsgainenoughenergytocausean electrontodetachfromthenuclei.Thisproducesagaseouscloudconsistingof positivelychargedions(cations)andelectrons.Thisfinalphaseisidentifiedas plasma. AnargonplasmaisutilizedICP-OES.Here,theargonionsmaintain temperaturesasgreatas10,000K.Theenergyoftheplasmaiscapableof atomization(theconversionofasampleintoatoms)andexcitationofthe atoms.Uponexcitation,anelectronfromeachatomtransitionsfromtheground statetoastateofhigherenergy.Anatomintheexcitedstateisnotstableand willdecaybacktoalessexcitedstate.Astheatomdecaysbacktoalessexcited state,energyislostbytheemissionofaphoton.Themagnitudeofthephoton’s energyisequivalenttotheamountofenergyinitiallyrequiredtoexcitethe atom. ObtainingInformation Becauseeveryelementhasauniqueandcharacteristicsetofenergy levels,thewavelengthsproducedbytheatomscanbeusedforidentification purposes.InOES,asampleisintroducedintotheplasmawherethesampleis atomizedandtheatomsareexcited.Theemittedwavelengthsarethenanalyzed withadetectorforqualitativedeterminationandtheintensityofthe wavelengthiscomparedtothatofstandardsofknownconcentrationto determinetheconcentrationofelementsinthesample. Caveats:InstrumentalInterferences SeveraltypesofinterferenceoccurinICP-OES,andthesecanbebroadly dividedintonon-spectralandspectralinterferences.Amajornon-spectral interferenceinICP-OESisduetoeasilyionizableelements,particularlygroupI andgroupIIelements.InICP-OESthegoalsistoprovidesufficientenergyto exciteanelectron.Ifmoreenergyissupplied,however,theelectronmaybe completelydislodged,resultinginacation.Asthereisnoemissionof electromagneticradiationduringthisprocess,eachatomthatisionizedislost fromdetection.Thenetresultisadecreaseintheintensityofemissionlinesfor thatelement,resultinginanobservedconcentrationlowerthanactual.Another majortypeofnon-spectralinterferenceoccurswhenthereisashiftinthe equilibriumofthesystem.Thisequilibriumexistsbetweenthegroundstate atoms,excitedatoms,andions.Theoverabundanceofanelementmayshiftthis equilibrium,affectingtheintensityofemission.Forexample,anoverabundance ofpotassium(K)maycauseanincreaseintheapparentconcentrationof sodium(Na).ThelargenumberofKatomsmaycollidewiththeNaatoms, resultinginanincreasedsignalwhencomparedtoastandardsolutionwitha considerablylowerconcentrationofKatoms. ThetwotypesofspectralinterferencesthatoccurinICP-OESare backgroundemissioninterferencesandtheoverlapoflinesemittedfromother elements.Backgroundemissionresultsfromtheemissionofexcitedmolecules intheplasma.Moleculesthatareformedduringtherearrangementofatomsin theplasmacanabsorbenergyandemitlight.However,unlikethediscrete bandsemittedfromatoms,broadbandsareproducedwhenmoleculesemit light.Thebroadbandsmaythenoverlapwithlinesofinterest.Thebestmethod fordealingwiththisinterferenceistoanalyzeareagentblankandsubtractthe signalofinterferencesfromthesample. Theothermajorsourceofspectralinterferenceoccurswhenanother elementinthesamplematrixemitslightatasimilarwavelengthastheelement ofinterest.Inthiscase,identificationandquantificationarenotpossible.One work-aroundistoselectadifferentemissionwavelength,onethatlacks interference. ICP-MS-Overview ICP-MSisananalyticaltechniquethatenablesmulti-elementanalysisoftrace elements.Thismethodisoptimalforsub-part-per-millionanalysis,withideal elementalconcentrationsranginginthehundredspart-per-billion(ppb)topartsper-trillionrange.ThemajorcomponentsofanICP-MSare:thesampleintroduction systemandionizationsource,aninterfacefortransferringanalytesfromahightemperatureandatmosphericpressuresystemtoalow-temperaturevacuum,a massfilter(calledamassanalyzer),andultimatelyadetector. ThebasicprincipleofICP-MSinvolvestheintroductionofasampleinto inductivelycoupledplasma—wheretemperaturesrangefrom6,000to10,000K (that’sashotasthesurfaceofoursun!).Whenthesampleisintroducedintothe plasma,moleculesarebrokendownintogaseousatomsthatarethenionizedinto, predominately,cations(ionswithapositivecharge).Theionsarethentransferred intoaninterfaceregiondesignedtoputtheionsundervacuum.Theinterfaceregion inanICP-MSconsistsoftwo(ormore)metalcone-shapeddiskswithasingleholein thecenterofeach.Thesmalldiameteroftheholeallowsforsamplingofions concentratedatthecenteroftheplasma—hereinliesoneofthelimitationsofICPMSasthesmalldiameteroftheconesinherentlyrequiressamplestobefreeof particulates,whichmaynotbecompletelyatomizedandfurtherionized,thatcan clogthecones.Oncetheionshavetraversedtheinterface,electrostaticlensesthen focustheionsfromtheplasmasourceintoasmallbeam.Becausemostionsformed intheplasmahaveapositivecharge,andthereforewillberepelledbyan electrostaticfieldofthesamepolarity(i.e.positivecharge),theelectrostaticlenses (small,positivelychargedtubesthroughwhichtheionstraverse)collimatetheions intoanarrow,focusedbeam.Theionbeamisfocusedintotheentranceofthemass spectrometerwheretheionsarethenseparatedbytheirmass-to-chargeratio (m/z).Themajorityofionsproducedintheplasmahaveasingle-charge(i.e.+1),so them/ziseffectivelyequivalenttothemassofeachion.Inotherwords,acopper-63 (63Cu)atomwillbeionizedto63Cu+;themass(63)tocharge(1)ratioisthen63. Ionswitham/zof63willbeseparatedfromotherionswithadifferingmassinthe massanalyzer.Onceseparated,theionswithadefinedmassaredirectedtoa detectorwheretheyarecounted. ICP-MS-Theory QuadrupoleMassAnalyzer Thereareseveraldifferentmassanalyzertechniques,eachwitha distinctmethodforseparatingions.OneofthemostpopulartechniquesinICPMStechnologyisthequadrupolemassspectrometer.Thequadrupolemass analyzerworksbyfilteringouteverythingexceptionswithaspecificm/z.This separationisachievedbyemployingelectrostaticforcesofattractionand repulsionaswellasinertia.Theelectrostaticforcesatplayinthequadrupole areexplainedbyCoulomb’slaw,whichtellsusthatparticleswithadifferent chargeattractandparticleswiththesamechargerepel.Thetendenciesofthe particlestoreacttotheattractiveorrepulsiveforces,however,arealso governedbyinertia.Thelawsofinertiadictatethatthemoremassiveand object,thegreateritstendencytoresistchangesinitsstateofmotion.Within thequadrupoleregion,theseforcesareusedincombinationtotransferonly thoseionswiththedesiredm/z. Inthequadrupoleregion,4rodsarearrangedinadiamond-like orientationwithaspaceinthemiddle.Eachrodispairedwiththerodtoits diametricopposite.Onerod-pairhaspositivedirectcurrent(DC)andan alternatingcurrent(AC)potentialappliedtotherods,whiletheotherrod-pair hasanegativeDCaswellasanACpotentialapplied.ThemagnitudeoftheAC andDCvoltagescontrolsthetrajectoryofionsastheytravelthroughthe quadrupoles,allowingonlyionswithaspecificmass/zthroughtothedetector atanygiventime. Tobetterunderstandthetheorybehindthequadrupole,envisionthe rodsalignedalongaCartesiancoordinate,whereonerod-pairisalignedalong thex-axisandtheotherisalignedalongthey-axis.Imaginetherodspairedin thex-directionhaveapositiveDCandACvoltageapplied,whiletherodspaired inthey-directionhaveanegativeDCandanACvoltageapplied. ConsiderforthemomentonlyACvoltageisappliedtotherods.Whenan alternatingcurrentisappliedtotherods,thevoltageswitchespolarityover time.TheACpotentialappliedtobothsetsofrodsisoffsetby180°suchthatthe voltageononepairispositiveandnegativeontheotherpair.Asaresult,ions oscillateinacorkscrewpatternastheyareattractedandrepelledbytherodpairs.Becauselighterionsaremorereadilyaffectedbyelectrostaticvariations, theseionswillreadilyrespondtochangesinalternatingcurrent.Whenthe alternatingcurrentonarod-pairispositive,therodswillrepelthesesmaller cations.WhentheACisnegative,therodswillattractthesmallercations.A balancemustbemaintainedbetweentheattractiveandrepulsiveforcesto promotefurtheroscillationoftheions.Ifeitherforceistoostrong,thecations willmakecontactwiththerods,atwhichtimeitwillbeneutralizedand disposedfromthesystem.Thefactorsgoverningwhethertheparticlewill oscillateorcollidewitharodincludethemagnitudeofthevoltage,theAC frequency,andthemassandchargeoftheions.AtagivenACvoltage,then,all cationswithamasslessthanthemassofinterestwillhaveanunstable trajectory,eventuallycollidingwitharod,andwillnotbedetected.Meanwhile, themoremassivecations,withgreaterinertia,willresisttheforcesresulting fromtheACpotential. Nowconsiderthateachrod-pairhasaDCpotentialofoppositesign. Supposetherod-pairinthex-directionhasapositiveDCpotentialandtherodpairinthey-directionhasanegativeDCpotential.ThepositiveDCpotentialwill focusthecationstowardsthecenteroftherods,butthenegativeDCpotential willattractthemoremassiveions,andanycationwithamassgreaterthanthe massofinterestwillcollidewiththenegativeDCrodpair. Withthesefactors(ACandDCpotentials)combined,thequadrupole selectivelyallowscationswithasinglem/zthroughthemassfilter.The magnitudesoftheACandDCpotentialsareadjustedtoallowionstobe detectedsequentially.Atthispoint,theionsareconvertedtoanelectronic signalatthedetectorandmeasured. MassInterferences Becausequadrupoleshavelowresolutionandfilterionsbasedonm/z, distinctspecieswithsimilarm/zwillnotbedistinguishable.Thisleadsto interferencesduringanalysis.Therearetwomajortypesofinterferencesin ICP-MS:isobaricandpolyatomic. Isobaricinterferencesoccurwhentwoisotopeshavethesamemass.For instance,indium(In)andtin(Sn)bothhaveisotopesofmass115.115Snhasa massof114.9033and115Inhasamassof114.9038.Discriminationofthesetwo isotopesrequiresamassanalyzerwitharesolvingpowergreaterthan200,000. SincequadrupoleICP-MSgenerallyhasaresolvingpowerof1,000the quadrupoleICP-MSisincapableofdistinguishingthesetwospecies.Insucha case,bothisotopeswouldbemeasuredatthesametimeandtheconcentration ofeitherwouldnotbeascertainableifbothwereappreciablypresentinthe sample. Thesecondmajortypeofinterferenceispolyatomicinterference. Polyatomicinterferencesoccurwhenmoleculesrecombinewithargonorother matrixcomponents.Whenthesepolyatomic(molecular)specieshavethesame massasanisotopeoftheelementofinterest,thetwospeciesmaybe indistinguishable.Forexample,argonandoxygenrecombineintheplasmato formamolecularspecieswithamasssimilartothatofthedominantiron isotope.Inthisexample,40Arand16Orecombinetoform40Ar16O+witham/zof 56,whichissimilarto56Fe+. Thesemassinterferencescanbeovercomethroughplanningand foresight.Techniquessuchastheuseofcoolplasma,reactionorcollisioncells, orchromatographicseparationcanbeemployedtocorrectoreliminatemany oftheinterferences.Additionally,selectinganalternateisotope,whenan option,canhelpavoidtheseinterferences. OtherMassAnalyzers:TheMagneticSectorandMC-ICP-MSTheory Unlikethequadrupole,whereintheionsoscillatethroughthequadrupole andareremovedbycollisionwithoneofthequadrupoles,inthemagneticsector analyzerionsareacceleratedthroughaflighttubeandthepathtraveledis influencedbyamagneticfield.Althoughthegeometryoftheflighttubecanvary betweendifferenttypesofmagneticsectors,allmagneticsectorsdeflecttheionsin anangularpath.Thespecificangletheionsacquireisdependentontwoforces, centripetalandmagneticfield,experiencedbytheions.Theseforcesare proportionaltomass,andasaresult,allionswithaspecificm/zwillhaveaunique pathradiusdeterminedbythemagneticfieldandvoltagedifferenceapplied.In otherwords,thedeflectionofionsthroughtheflighttubeisdependentonkinetic energy. Whenaconstantmagneticfieldandvoltagedifferentialisappliedtothe flighttube,ionstooheavytobedeflectedatthecorrespondingangleoftheflight tubewillcollidewithonesideoftheflighttube.Ontheotherendofthespectrum, lightionsexperienceagreaterdegreeofcurvature.Whenthedegreeofcurvature experiencedbylighterionsisgreaterthantheangleoftheflighttube,theseionswill alsocollideintothewallsoftheflighttube.Withintheflighttube,ionsthathavethe idealcurvature(asdictatedbythemagneticfield),relativetotheflighttube,will travelunobstructedandexperienceuniquetrajectoriesbasedontheirm/z.Eachof thesetrajectories,orionbeams,canthenbefocusedintoaseriesofdetectors.This isthebasisofbothsingleandmultiplecollector-ICP-MS. InMC-ICP-MS,theionbeamsofvariousisotopesarefocusedintooneof numerousdetectors(i.e.multiplecollectors)--oftentimesFaradayCupstyle detectors.Asaresult,withincertainlimitations(e.g.similarmass),variousisotopes canbemeasuredsimultaneously.Thisdiffersfromthequadrupolemassanalyzer andothersingle-collectormassspectrometers,whichonlymeasuredifferent isotopicspeciessequentially.Theinherentlowerprecisionofsequentialanalysis limitstheapplicabilityofsingle-collectors.Thesimultaneousdetectionand measurementofisotopes,ontheotherhand,offersanefficientandprecisemethod formeasuringandcalculatingisotopicratios.