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).Onewaytothinkaboutthisisbycomparingittolivinginatwo-

storybuilding.Yourhomemaybeonthegroundfloororitmaybeonthesecond

floor,andyoucanliveoneitherfloor,butyoucannotliveinthespacebetween.This

isthesameprincipleforatoms,andwhentheatomchangesfromoneleveltothe

next,itcaneitherabsorboremitaphoton(def.:aphotonisasmallpacketofenergy

thatcancarryelectromagneticradiation)withenergythatisequivalenttothe

differenceinenergybetweenthetwolevels.Thesecondprincipleisthatthe

frequency,ν,orwavelength,λ,oftheradiationemittedorabsorbedastheatom

transitionsfromoneenergyleveltothenextisdefinedbytheequation:

,

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

Ef − Ei =hcλ

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)toparts-

per-trillionrange.ThemajorcomponentsofanICP-MSare:thesampleintroduction

systemandionizationsource,aninterfacefortransferringanalytesfromahigh-

temperatureandatmosphericpressuresystemtoalow-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—hereinliesoneofthelimitationsofICP-

MSasthesmalldiameteroftheconesinherentlyrequiressamplestobefreeof

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.OneofthemostpopulartechniquesinICP-

MStechnologyisthequadrupolemassspectrometer.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

oscillateinacorkscrewpatternastheyareattractedandrepelledbytherod-

pairs.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-directionhasapositiveDCpotentialandtherod-

pairinthey-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.