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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.