Laser-Ablation Inductively Coupled Plasma Mass Spectrometry …tetide.geo.uniroma1.it/riviste/permin/testi/V77.3/2008PM0019.pdf · Laser ablation has been applied to the determinations

Per. Mineral. (2008), 77, 3, 65-74 doi:10.2451/2008PM0019 http://go.to/permin

PERIODICO di MINERALOGIAestablished in 1930

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYandappliedtopicsonEnvironment,ArchaeometryandCultural Heritage

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AbstrAct. — The concentration of thirty ninegeochemically relevant trace elements, from 7Lito238U,wasdeterminedinstandardsilicateglasses(NIST610,NIST612,BCR-2)usingtheLaserAblationInductivelyCoupledPlasmaMassSpectrometry(LA-ICP-MS)instrumentationattheDepartmentofEarthSciencesof theUniversityofCagliari (Italy).ThedeviceisaQuadrupoleICP-MS(PerkinElmerElanDRC-e)coupledwitha213nmNd:YAGlaserprobe(NewWave Research).This configuration allowsrapid,highquality, in-situ traceelementsanalysisin glasses and minerals.The calibration strategy,achieved using synthetic multi-element glasses(NIST612),with44Caasinternalstandard,givesananalyticalaccuracywithin5%errorlevel,providingaprecisionbetween1%and9%,at40µmofcratersizeforallelements.At a laser spot size of 40 µm,Atalaserspotsizeof40µm,thelowerlimitofdetection(LLD)rangesbetween0.001and1ppmforalltheelements;itincreasesbyaboutoneorderofmagnitude,withoutanysignificantfractionationamongthedifferentelements,foralaserspotsizeof15µm.Quality control of LA-ICP-MSQualitycontrolofLA-ICP-MSanalysesisroutinelyperformedanalysinganaturalstandardglass, theBCR-2,certifiedbytheUSGS,consideredasunknownsample.Resultsindicatethattheinstrumentationcapabilitiesaresuitableforthegeochemical characterisation of various materials

of mineralogical, petrological, geological andenvironmentalinterest.

riAssunto.—Leconcentrazionidi39elementiintracce,dal7Lial 238U,sonostatedeterminateinvetrisilicaticistandard(NIST610,NIST612,BCR-2)usandolastrumentazioneLaser Ablation Inductively Coupled Plasma Mass Spectrometry(LA-ICP-MS)installata presso il Dipartimento di Scienze dellaTerradell’UniversitàdiCagliari(Sardegna,Italia).LostrumentoècompostodaunICP-MSdeltipoPerkinElmerElanDRC-eaccoppiatoconunsistemalaserNd:YAG,sviluppatodallaNewWaveResearch,chelavoraadunalunghezzad’ondadi213nm.Questoinsiemeconsentedideterminarerapidamente,econun’altaqualità,leconcentrazionidituttiglielementiintraccainvetrieminerali.Lacalibrazioneèstataottenutausandovetrisinteticicertificatidall’USGScomematerialediriferimento(NIST612)eilCacomestandardinterno,analizzatomedianteSEM-EDS.Lastrategiadi calibrazioneutilizzatahapermessodiottenererisultaticonunaaccuratezzaintornoal5%,econunaprecisioneanaliticachevariatra1%e9%,utilizzandoundiametrodelfasciolaserdi40µm.Perdimensionideldiametrodel fascio laserdi40µm,ilminimolimitedirilevabilitàvariapertuttiglielementitra0,001and1ppm,edaumentadicircaunordinedigrandezza,senzachesiverifichinoprocessidifrazionamentotraidiversielementi,perdiametridi 15µm. Inogni sessione analitica si effettuano

Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS): setting operating conditions and instrumental performance

clAudiA d’oriAno 1, 2, *, stefAniA dA Pelo 1, frAncescA PoddA 1 and rAffAello cioni 1, 2

1Dip.todiScienzedellaTerra,UniversitàdegliStudidiCagliari,ViaTrentino,51,09127,Cagliari,Italy2IstitutoNazionalediGeofisicaeVulcanologia,SezionediPisa,ViadellaFaggiola,32,56126,Pisa,Italy

Submitted, September 2008 - Accepted, November 2008

*Correspondingauthor,E-mail:[email protected]

66 c. d’oriAno, s. dA Pelo, f. PoddAandr. cioni

almenodueanalisidelBCR-2,unvetro silicaticonaturale certificato USGS, considerato in questocaso come campione sconosciuto, per monitorarela qualità delle analisi realizzate durante tutta lasessione.IrisultatidelpresentelavoroindicanochelastrumentazioneLA-ICP-MSinstallataaCagliarihabuonepotenzialitàperquantoriguardalostudiogeochimico di materiali di diversa composizione,e quindi può essere utilizzata a supporto di varietipologiediricercheasfondosiaminero-petrograficochegeologicoogeologico-ambientali.

Key words: laser ablation ICP-MS; trace elements; USGS glass reference materials; operating conditions; precision and accuracy; LLD.

introduction

In the last 20 years, Earth scientists havedevelopedtheLaserAblationInductively-CoupledPlasmaMassSpectrometry(LA-ICP-MS)asananalyticalmethodaimedatinvestigatingthe in-situchemicalandisotopicvariationsingeologicmaterials(Gray,1985;Arrowsmith,1987;Hager,1989;Denoyer,1991;Denoyeret al.,1991). In1985,aLaser-AblationdevicewasassociatedforthefirsttimewithanICP-MSforthedeterminationoftraceelementabundancesinsolids;sincethen,LA-ICP-MShasbeen increasing its importanceasananalyticaltoolfortheEarthSciences.Thepotentialoflaserradiationtocauseablationandvaporization,whenitinteractswithsolidmaterials,isexploitedasamethodofsampleintroductionformassspectrometry.Thedifferentspeciesproduced,includingparticulates,ground-stateatoms,excitedatoms,andions,areutilisedforelementalanalysisviamassspectrometry(MS).Theinteractionoflaserlightwiththesampledependsuponboththecharacteristicsofthelaserbeamandthephysicalproperties of the solid. The early commerciallaserablationsystems,specificallydevelopedforICP-MS, utilised a Nd:YAG design, operatingat the primary wavelength of 1064 nm in theinfraredregion.However,intheanalysesofpalecolouredminerals(i.e.thosewithlowtransitionmetalabundances)orthosemineralswithalowabsorptionintheinfrared,muchofthelaserlightpassesthroughthesampleandisabsorbedbythevolatilebearingmountingmaterial.Thiscanleadtocatastrophicfailureofthemineralaschunksare

blownfromthesurfacebytheexpandingbubblesofgasunderneath(Jacksonet al.,1992).ToovercomesomeoftheseproblemsanumberofresearchershavemodifiedtheNd:YAGlaserbymultiplyingthe laser frequency, reducing itswavelength to532, 266 and 213 nm (Longerich and Diegor,2001). Nowadays, laser ablation is consideredaveryreliablesamplingtechniqueforICP-MS,capable of producing very high quality datadirectlyonsolidsamplesandpowders.Itsupportsmanygeologicalstudiessuchasthoseinvolvingmineralexploration,isotopicagedeterminations,geochemicalinvestigationsofmeltingandmasstransport in natural and experimental systems,andenvironmentalstudies.ManyotherusesinthefieldofEarthSciencesarecontinuouslyproposedthankstotheincreasingpresenceofLA-ICP-MSdevicesinEarthScienceslabs.

Withrespecttootheranalyticaltechniquesfortrace elements, some of the many advantagesofferedbythistechniqueinclude:minimalsamplepreparation,lowblanks,highspatialresolutionatthesub-mmscale(to20µm),elementalmappingacross the surfaceofa sample,high sensitivityanddetection limitsbelowtheppmlevel, rapidanalysistimes(typically2minperpointanalysis)andminimummatrixeffects forawidevarietyof target materials, allowing straightforwardcalibrationoftheanalyses.

In this paper we describe the technicalcharacteristics of the LA-ICP-MS microprobeinstalledattheEarthScienceDepartmentoftheUniversity of Cagliari (Italy), and we presentsomepreliminaryresultsobtainedonthereferencestandardmaterialsinordertoevaluatesensitivity,detection limits, precision and accuracy of theinstrument.Thecalibrationoftheinstrumentandtheoperatingconditionshavebeenderivedusingstandard silicate glasses (NIST610, NIST612,BCR-2).Theconcentrationof39traceelements,from7Lito238UweredeterminedinNIST612andBCR-2sample,consideredasunknown,inordertotesttheinstrumentonmaterialswithdifferentcompositionandconcentrationoftraceelements.

APPlicAtions of the lA-icP-Ms

Laser ablation has been applied to thedeterminationsofthetraceelementconcentrations

Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS).... 67

in a wide range of materials in many differentstudies, including volcanology, petrology,mineralogy,oredeposits,materialsandforensicstudies, cultural heritage and environmentalsciences.Intheenvironmentalscience,LA-ICP-MSanalysesisappliedtoavarietyofbiologicalstructures, such as tree rings, mollusc shells,otoliths, finrayandfishscales(Veinott,2006).Thisisaconsequenceoftwoimportantfactors:analysesof solids by laser requires little or nosample preparation; the determination of traceelementscontentorisotopiccompositioninalmostalltypesofmaterialsisachievedathighspatialresolution.

instruMentAtion And PeculiArity of lA-icP-Ms

Experimentswere carriedout using aPerkinElmerElanDRC-eQuadrupole-ICP-MS,coupledwithaNewWaveResearchUP213serieslaserablationdevice.

TheICP-MSstandardinstrumentconfigurationconsistsofaplasma(luminousvolumeofpartiallyionisedgas)asionsource(ICP),generatedfromradio frequency magnetic fields induced by acoppercoil,woundedaroundthetopofapartlydemountableglasstorch,withremovablealuminainjector.Nickelcones(samplerandskimmer),invacuumenvironmentrepresent the interfaceforextractingionsfromtheplasmaandtransferringthemintheregionofthemassspectrometer(MS).ThePerkinElmerElanDRC-eoffersthepossibilityofworkingwiththeAutolenssystem,i.e.usingasingleionlens,thatdoesnotinteractwithotherlenses, providing optimal transmission of ionsateverymassinamultielementalanalysis.ThevoltageoftheELANsingleionlenscanbereadilycontrolledinstepwiththequadrupoletoprovidetheoptimumion transmission foreachmass.Aquadrupole,whichprogressivelyselectionswithspecific mass to charge ratio, is used as massanalyser.IonsaredetectedbyaChannelElectronMultiplier(CEM),attheendofwhichthereisacomputerfordatacollectionandprocessing.

The laser ablation system consists of anYttriumAluminiumGarnet(Y3Al5O15)roddopedwith approximately 3 wt% Nd2O3, working atwavelength of 213 nm.This wavelength gives

significantlyimprovedablationcharacteristics,inparticularreducingthedamageoftheareaaroundthecrater(Fig.1)andyieldingconstantablation(measureofmassofablatedmaterialforamineralrelativetoablatedmassfromexternalstandard),forwidelydifferentmatrices,suggestingthattheabsorptionof213nm radiation is substantiallymatrix-independent(Jackson,2001).

The Q-switched laser is characterised by asingle large output pulse which releases theenergy stored in the cavity as shorter pulsewithbothhigher average andpeakpower.TheoutputofaQ-switchedNd:YAGlaser isa train(typically1-20Hz)ofhighintensitypulses.Thelaserbeamishighlydirectional,hasaveryhighwavelength purity (λ = 1064 nm) and strong spatialandtemporalcoherence,thatresultsinanenhancedablationefficiencyandarepresentativecomposition of the material ablated.The pulserepetitionratecontrolsthesampleremovalrateforagivenpulseenergyand,consequently,controlspeak/backgroundratiosandablationtimeavailableforagivenpitsize.Themaximumrepetitionrateofthelasercangenerallybeused(typically10-20Hz),withlowerpulsefrequenciesbeingusedtoablateverythinsamples,toreducecountrates

Fig. 1 –ComparingspotareausingArgonandHeliumascarriergas.In thefirstcase theareaaroundthecrater isdamagedandinterestedbythedepositionofalittlequantityof ablated material, while using He these problems areavoided.Theblackbarcorrespondto50µm.


for very concentrated samples or to produce aprolongedstablesignalwhenrequirede.g.aswhenoptimisingtheoperatingconditionsoftheICP-MS(Güntheret al.,1999).

Helium gas was used as carrier inside theablationcellandmixedwithArgon,themakeupgas,beforeenteringtheICP.Inthisway,reductionoftheobservedfractionationmagnitude,bothattheablationsiteandduringthetransporttowardtheICP-MS,isobtained,enhancingthetransportefficiency of ablated material (Günther andHeinrich,1999;Jackson,2001;HornandGünther,2003)(Fig.1).

Ablation spot diameter ranges between 10and100µm.Theselectionofthespotsizeusedduringablationisacriticalparameter thatmustbeadjustedaccordingtoapplication.Analyses ofAnalysesofsmallsamples,forexample,requireareductionin the sizeof ablationcrater,without changingsensitivity. In last-generation instruments, laserenergycanbedecreasedandlaserspotsizereduced(consequently reducing the volume of ablatedmaterial)without changing theLowerLimitofDetection (LLD), thusoffering thepotential togenerate data from smaller samples. The LAsystemincorporatesavariableorinterchangeableapertureinthelaserbeampaththatisimagedontothesamplesurfacebythelaserobjectivelens.Theinstrumentischaracterisedalsobya“Resonator-flat”beamthatyieldsuniformcraterswithenergydensitiesthatremainconstantwhenchangingspotsizes.Moreover,theLAsystemisequippedwithanadditionalcontrolofthespotsizethatisachievedusingabeamexpander.

The pulse energy ranges from 0.2 and 2 mJand is controlledbyanoptical attenuator,withtheadvantageofalinearpolarisationofthelaserbeam.The optical attenuator is placed directlyaftertheIRhead,beforeanyharmonicgenerationcrystals, because IR polarizer does not requireexpensiveUV transmittingmaterials.This alsoprotectssubsequentcomponentsfromhighenergyphoton(UV).

Thelaserablationdeviceisequippedwithavideocamera for thehighresolutionsampleviewing,ensuringanaccuratefocussingofthesampleandaneasieridentificationoftheareastobeanalysed.ThesystemisremotelycontrolledusingtheElanVersion3.0PerkinElmerSciexpackage.

oPerAting conditions

To achieve maximum intensity across thefull range of masses, the ICP-MS parameters,suchastheRFpower,ionlensvoltageandmasscalibration, were optimised by continuouslyaspiratingasolutionof10µg/lofMg,In,Ce,BaandUin0.5%HNO3.ThenebulisedgasflowwasadjustedsothatCeO/Ceresults<3%anddoublychargedionproductionasmeasuredbyBa2+/Ba+was<3%.

Todetermine traceelementconcentrations intheablatedmaterial,datawerecollectedbypeakhopping.Mass resolution, measured as peak width,Massresolution,measuredaspeakwidth,wasset to0.7AtomicMassUnit(amu)andthepeakintensityforeachelementwasobtainedbymeasuringthesignalintensityatthecentrepointinthemassrangedefinedforeachelement.Thedwelltimes(measurementtimesoneachisotopeduringonequadrupolesweep)was10ms per amu,peramu,,onesweepofthemassrangeperreplicateand300replicatesperanalysis,including80replicatesonthedrygastoestablishthebackgroundprior toablation.Totalanalysistimewas2minperspot,including1minofbackgroundsacquisition(Fig.2).

Signaldetectionwasperformedindualmodes,ioncountingandanalog,operatingsimultaneously.Datawerecollectedintime-resolvedgraphicsmodetomonitoreventualcompositionalheterogeneitiesthatmightbepresentinthesampleatthescaleofthelasersamplingandtomonitortheinter-elementfractionation.TheinstrumentwasoperatedwiththeAutolenson;datawere transmitted toaPCand processed by the GLITTER program (VanAchterberghet al.,2001).OperatingconditionsusedinthisworkaresummarisedinTable1,bothfortheICP-MSandLAdevices.

reference MAteriAls

In this work, the NIST 612 and NIST 610reference materials, partially certified by theNational InstituteofStandardsandTechnology(NIST),wereadopted.TheyrepresentthereferencematerialusedinalmostalltheLA-ICP-MSlabsforthecalibrationofmicrobeamanalyses.

Botharesilicateglassesspikedwitharangeoftraceelementsatanominalconcentrationof50and500µgg-1respectively.Pearceet al.(1997)


compiledadatabaseofpublisheddataforthesereferencematerialsmorethannewanalysesfromfouradditionallaboratories,toprovideaninterimworkingvalueforawiderangeofelements(Table2).Forthequalitycontroloftheanalyses,BCR-2glass,astandardpreparedfromColumbiaRiverbasaltbytheUSGeologicalSurvey,wereroutinelyutilisedinaccordancewiththeotherLA-ICP-MSlabs(Table2)(Gaoet al.,2002).

DisksofglassesNIST610,NIST612andBCR-2wereincludedtogetherinasinglemountofepoxyresin,andpolishedwithSiCdisksuntil1µmwithalumina.

cAlibrAtion strAtegy

Quantitative calibration of the LA-ICP-MSsystemrequirestheknowledgeoftheconcentrationof one element (internal standard), analysedindependentlyinthesampleandinthereferencematerial(Perkinset al.,1997;Pearceatal.,1999).Weuse 44Ca,measuredduringeachanalysis asaninternalstandard,andthedataarenormalisedtotheCaOcontentofthesampleasdetermined

ELANDRC-eICP-MS

Nebulisergasflow 0.86lmin-1

Auxiliarygasflow 1.40lmin-1

Plasmagasflow 15.5lmin-1

Lensvoltage 7.50VICPRFpower 1200WAutolens on

NewWaveResearchUPseries213LaserAblation

HecarriergasFlow 0.67lmin-1

Energy(10Hz) 0.028-0.28mJSpotsize 20-50µmLaserfrequency 10Hz

tAble 1Instrumental parameters for the ICP-MS and Laser

Ablation instruments

Fig.2 –Realtimewindowofthesignalintensityduringtimeforeachelement.Duringthefirst60softheanalysis,thelaserisswitchoffforthebackgroundacquisition.


independentlybyenergy-dispersive �-ray (SEM-energy-dispersive�-ray(SEM-EDS) technique, at the Department of EarthSciencesofPisa.The use of an internal standardTheuseofaninternalstandardaccountsforanyvariationintheamountofablatedmaterialreachingtheplasma(the‘ablationyield’)betweensamplesandreferencematerial,andanydifferencesinconcentrationoftheinternalstandardarecorrectedforthecalculationofconcentrations(Normanet al.,1998).Thisalsomeansthat thereferencematerialusedtoproducethecalibrationshould not have necessarily the same matrixcompositionas theunknown,althoughastrongdifferenceinmatrixcompositionmayintroducefurthercomplications(Pearceet al.,2007). EachEachanalytical run consists of a maximum of 20analysesinwhichthefirstandthelastarestandardmaterials.Thenumberof20analysesrepresentthe

bestcompromisetoensureinstrumentalstabilityintherelatedtimeinterval.The concentration of anyTheconcentrationofanyunknownelementiscalculatedfromthefollowingequation,assuming that thecalibrationcurve islinearandpassesthroughtheorigin(PerkinsandPearce,1995):

rm

unkunk

rm

rmunk

isis

isel

iselel

elCC

ACPSACPS

ACPSACPSCC

where C = concentration; el = analyte element; is = element selected as internal standard; rm = reference material; unk = in the unknown sample; ACPS = area counts per second (i.e. the integrated countsnormalisedtothedwelltimeperpeak).

Calibration requires that the backgroundcorrected signal fromall orpartof anablationspectrumofanunknownsamplebereferencedto

Elements NIST610 NIST612 BCR-2Li7 484.6 41.54 9B11 356.4 34.73Mg25 465.3 77.44 21650Sc45 441.1 41.05 33Ti49 434 48.11 13500V51 441.7 39.22 416Cr53 405.2 39.88 18Co59 405 35.26 37Ni60 443.9 38.44Zn66 456.3 37.92 19Rb85 431.1 31.63 48Sr88 497.4 76.15 346Y89 449.9 38.25 37Zr90 439.9 35.99 188Nb93 419.4 38.06Cs133 360.9 41.64 1.1Ba137 424.1 37.74 683La139 457.4 35.77 25

tAble 2Trace elements concentrations (µg g-1) of the reference materials. NIST610 and NIST612 are from Pearce at

al. (1997), and BCR-2 from Gao et al. (2002)

Elements NIST610 NIST612 BCR-2Ce140 447.8 38.35 53Pr141 429.8 37.16 6.8Nd146 430.8 35.24 28Sm149 450.5 36.72 6.7Eu151 461.1 34.44 2Gd157 419.9 36.95 6.8Tb159 442.8 35.92 1.07Dy163 426.5 35.97Ho165 449.4 37.87 1.33Er167 426 37.43Tm169 420.1 37.55 0.54Yb173 461.5 39.95 3.5Lu175 434.7 37.71 0.51Hf177 417.7 34.77 4.8Ta181 376.6 39.77Pb208 413.3 38.96 11Th232 450.6 37.23 6.2U238 457.1 37.15 1.69


thatofastandardmaterial.Backgroundsignalsareusuallyobtainedmeasuring the‘gasblank’, i.e.withthelaserofffor60secondsbeforeablation(Fig.2)(Güntheret al.,1999).Thesameoperatingconditionshavebeenmaintainedfortheanalysisofboththestandardandtheunknowntoovercomeeffects of differential element fractionation(PerkinsandPearce,1995).

results

Sensitivity

The instrument sensitivity (cps/ppm) is an animportantparameterthatneedstobeevaluatedtoensureoptimuminstrumentconditions,especiallyat the minimum level of response or limit ofdetection.Sensitivitydepends on several factorsdependsonseveralfactorsincludingablationefficiency,ionisationefficiencyandiontransmission, essentially controlled by spotessentiallycontrolledbyspotsizeandlaserpoweraswellasisotopicabundance(Halter et al., 2002). The response curve was The response curve wasThe response curve wasobtainedontheNIST612glassreferencematerialusinga40µmspotsize,alaseroutputenergyof0.28mJandarepetitionrateof10Hz.Thesensitivityvariesasafunctionoftheatomicnumberoftheelements,andfivedifferentregionsofmass-rangewere recognized in the intervals between 7-29amu,43-66amu,85-93amu,133-177amuand181-238amu.Forthefirstgroup(fromLitoSi)sensitivityisthelowest,rangingbetween22and165 cps/ppm, with the lower value for the Li. Fromcps/ppm,withthelowervaluefortheLi.FromFromthemassrangebetween43-66sensitivityabruptlyincreasesincorrespondenceoftheCa,withvalueof300 cps/ppm, regularly decreasing until 166 cps/cps/ppm,regularly decreasing until 166 cps/regularlydecreasinguntil166cps/cps/ppmfortheZn.ThesametrendisshownfromRbtoNb,withasensitivity3timeshigher.For amuoramuhigherthan95,sensitivityisquiteconstantforalltheelements,rangingbetween500-700cps/ppm,cps/ppm,andfinally,atvery high mass-range, a decrease isveryhighmass-range,adecreaseisobserved.

Detection limits

Lowerlimitsofdetection(LLD)wascalculated(at3standarddeviationsofthebackground)fromtheequation:

ICBLLD 2

123

where LLD= lower limit of detection; B= background intensity for analyte; C= concentration of analyte; I= peak intensity for analyte in reference material.LLDisinverselyproportionalto(ablationvolume)1/3;itrepresentsameasureofthesignaltobackgroundratioofananalytic instrument,andgivesameasureoftheinstrumentresponse(i.e.counts per second per unit of concentration)(PerkinsandPearce,1995;Pearceet al.,2007)..Averageminimumdetectionlimitsoftheselectedelements(Fig.3),weredeterminedonthereferenceglass BCR-2 using the acquisition parametersshowninTable1.Inordertoconstrainthevariationof detection limits as a function of spot size,analyseswerecarriedoutat40and15µmspotsizewith a laser power of 0.15 and 0.028 mJ,respectively.Ataspotsizeof40µm,thedetectionlimitrangesbetween0.001and1ppmforalltheelements,exceptforSi,Ca,TiandCr,duetothestringent relationship between sensitivity andbackground.Theelementswithhigh-abundanceisotopesshowLLDbetween0.001and0.01ppm,whilefortheotherelementsLLDsarehigherthan0.01ppm.Bydecreasingthespotsizefrom40to15µm,detectionlimitincreasesaboutoneorderofmagnitudewithoutanysignificant fractionationamongthedifferentelements.

Precision (RSD%) and Accuracy (RD%)

The precision (RSD% = [standard deviation / average value] * 100) of the LA-ICP-MSof the LA-ICP-MSinstrumentdependsontheacquisitionparameters,onthetotalnumberofionsofanalytedetected,ontheanalyteconcentrationandonlaserspotsize.The accuracy (RD% = [(average value - reference value) / referencevalue]*100)corresponds totherelativedeviationoftheaverageconcentrationobtainedinthisstudyfromthereferencevalues(Pearceet al.,1997).Inthiswork,precisionandaccuracyoftheanalyses are calculated from 13arecalculatedfrom13replicateson reference material NIST612 obtainedreferencematerialNIST612obtainedinoneday,usingaspotsizeof40µm,laseroutputenergyof0.28mJandarepetitionrateof10Hz.Averageandreferencevalues,RSD%andRD%arereportedforeachconsideredelementinTable3.RSD%andRD%arealsoshowninFig.4.The


Fig.3 – Low Limit Detection (LLD) measured on the BCR-2 sample at spot size of 40 μm and 15 μm.

Fig.4 –Precision(RSD%)andAccuracy(RD%)oftheLA-ICP-MSinstrument,measuredusingthereferencematerialNIST612on16replicatesinoneday.


precisiongenerallyrangesbetweenthe1.2and6%,increasingtowardthehighermass.Neverthlessthemaximumvalueofabout9%isreachedfor11B.Accuracyvariesbetween-5%and5%.NegativeaccuracyvaluesarerelativetoLi,B,MgandZndue to an underestimation with respect to thecertifiedmaterial.

conclusive reMArKs

Analyses of certified reference materials(NIST610,NIST612,BCR-2)wereperformedinordertochecktheinstrumentalsetupandoperatingconditions.Sensitivity isa functionbothof theatomicmassnumberandoftherelativeabundanceoftheanalysedelement;anincreaseinthesignalintensityisobservedthroughoutthemassrange,withaminimumresponseforlightelementsandamaximumforheavierelements.Lowsensitivityvaluesarerecordedformajorandlightelements,suchasSi.Highsensitivityvaluesaretypicaloftheheavierandgeochemicalimportantelements,such as the REE, Pb and U. Analyses of thepartiallycertifiednaturalglass,BCR-2,performedat spot size of 15 µm and 40 µm respectively,indicatethatthedetectionlimitsoftheinstrumentrangebetween10ppband20ppm,withhighestvaluesobtainedatsmallerspotsize.Anumberof13replicateanalysesofNIST612carriedinonedaysuggeststhatprecisionisincreasingtowardthe higher mass from 1% to 9%, and accuracy1% to 9%, and accuracy and accuracyisbetter than 5% for all investigated elements.better than5% for all investigatedelements.Thesepreliminaryresults,obtainedonsyntheticandnaturalglassstandards,suggest that the LA-thattheLA-ICP-MSinstrumentationinstalledatDepartmentofEarthSciencesoftheUniversityofCagliariissuitableforthein-situanalysisoftraceelementsingeologicalmaterials.Techniqueandmethodswillbesoonapplied tocharacterisenatural samplesforgeological, geochemical and environmentalstudies.

AcKnowledgeMents

ManythankstoMassimoTiepolo(IGG-CNRlabofPavia)andDonatellaBarca(EarthSciencesDept.ofCosenza)forthemanytheoreticalandpracticalcluesandforthetechnicalassistanceattheLA–ICP–MSinstruments.ThisworkrepresentsapartofthePhDthesisofC.D’Oriano,supportedbytheFondazione

Elements Mean St.dev. Ref.value RSD% RD%Li 42.82 2.56 41.54 5.95 -3.09Ba 35.16 5.31 34.73 8.61 -1.66Mg 79.60 3.70 77.44 4.48 -3.05Sc 40.88 1.13 41.05 2.41 -0.21Ti 47.37 2.35 48.11 4.83 2.63V 38.73 1.25 39.22 3.02 1.8Cr 39.65 2.22 39.88 5.25 1.86Co 34.91 0.81 35.26 1.62 1.73Ni 38.11 1.46 38.44 3.18 1.87Zn 38.31 1.81 37.92 4.72 -0.66Rb 31.41 0.83 31.63 2.14 1.21Sr 75.71 0.88 76.15 1.21 0.67Y 37.98 1.44 38.25 3.61 0.07Zr 35.68 1.47 35.99 3.91 0.07Nb 37.50 1.09 38.06 2.9 1.91Cs 40.98 1.51 41.64 2.74 2.65Ba 37.14 1.49 37.74 3.8 2.22La 35.17 1.30 35.77 3.68 2.07Ce 37.61 1.41 38.35 3.12 2.95Pr 36.56 1.29 37.16 2.87 2.45Nd 34.69 1.39 35.24 3.54 2.26Sm 36.50 1.61 36.72 3.51 1.34Eu 34.02 1.59 34.44 3.2 2.27Gd 36.37 2.13 36.95 5.06 2.12Tb 35.47 2.18 35.92 4.54 1.62Dy 35.54 2.55 35.97 5.69 1.95Ho 37.49 2.58 37.87 4.97 1.57Er 37.09 2.75 37.43 5.9 0.8Tm 37.16 2.51 37.55 5.1 1.38Yb 39.48 3.04 39.95 5.77 1.65Lu 37.36 2.62 37.71 5.05 1.29Hf 34.35 2.76 34.77 6.1 1.52Ta 39.01 2.54 39.77 4.36 2.66Pb 37.90 1.72 38.96 3.65 3.88Th 36.17 2.17 37.23 5.39 3.28U 36.04 2.02 37.15 4.61 4.55

tAble 3Average and reference values, Precision (RSD%)

and Accuracy (RD%) for each element considered.The values refers to 13 replicates analyses of NIST

612 in one day


BancodiSardegna.Instrumentaltuningwaspossiblethanks to a Ministero Università e Ricerca PRIN2004 grant and a Dipartimento Protezione Civile–Ist.NazionalediGeofisicaeVulcanologia2005-07granttoRaffaelloCioni.TheinstrumentwasacquiredthankstothefinancialsupportofaPON2000-2006(ScientificcoordinatorProf.LucaFanfani).Reviewsby Prof. S. Conticelli and Prof. R. Vannucci areacknowledged.

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Laser-Ablation Inductively Coupled Plasma Mass Spectrometry …tetide.geo.uniroma1.it/riviste/permin/testi/V77.3/2008PM0019.pdf · Laser ablation has been applied to the determinations