Per. Mineral. (2008), 77, 3, 65-74 doi:10.2451/2008PM0019 http://go.to/permin PERIODICO di MINERALOGIA established in 1930 An International Journal of MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage l ABSTRACT. — The concentration of thirty nine geochemically relevant trace elements, from 7 Li to 238 U, was determined in standard silicate glasses (NIST610, NIST612, BCR-2) using the LaserAblation Inductively Coupled Plasma Mass Spectrometry (LA- ICP-MS) instrumentation at the Department of Earth Sciences of the University of Cagliari (Italy). The device is a Quadrupole ICP-MS (Perkin Elmer Elan DRC-e) coupled with a 213 nm Nd:YAG laser probe (New Wave Research). This configuration allows rapid, high quality, in-situ trace elements analysis in glasses and minerals. The calibration strategy, achieved using synthetic multi-element glasses (NIST612), with 44 Ca as internal standard, gives an analytical accuracy within 5% error level, providing a precision between 1% and 9%, at 40 µm of crater size for all elements. At a laser spot size of 40 µm, At a laser spot size of 40 µm, the lower limit of detection (LLD) ranges between 0.001 and 1 ppm for all the elements; it increases by about one order of magnitude, without any significant fractionation among the different elements, for a laser spot size of 15 µm. Quality control of LA-ICP-MS Quality control of LA-ICP-MS analyses is routinely performed analysing a natural standard glass, the BCR-2, certified by the USGS, considered as unknown sample. Results indicate that the instrumentation capabilities are suitable for the geochemical characterisation of various materials of mineralogical, petrological, geological and environmental interest. RIASSUNTO. — Le concentrazioni di 39 elementi in tracce, dal 7 Li al 238 U, sono state determinate in vetri silicatici standard (NIST610, NIST612, BCR-2) usando la strumentazione Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) installata presso il Dipartimento di Scienze della Terra dell’Università di Cagliari (Sardegna, Italia). Lo strumento è composto da un ICP-MS del tipo Perkin Elmer Elan DRC-e accoppiato con un sistema laser Nd:YAG, sviluppato dalla New Wave Research, che lavora ad una lunghezza d’onda di 213 nm. Questo insieme consente di determinare rapidamente, e con un’alta qualità, le concentrazioni di tutti gli elementi in tracca in vetri e minerali. La calibrazione è stata ottenuta usando vetri sintetici certificati dall’USGS come materiale di riferimento (NIST612) e il Ca come standard interno, analizzato mediante SEM-EDS. La strategia di calibrazione utilizzata ha permesso di ottenere risultati con una accuratezza intorno al 5%, e con una precisione analitica che varia tra 1% e 9 %, utilizzando un diametro del fascio laser di 40 µm. Per dimensioni del diametro del fascio laser di 40 µm, il minimo limite di rilevabilità varia per tutti gli elementi tra 0,001 and 1 ppm, ed aumenta di circa un ordine di grandezza, senza che si verifichino processi di frazionamento tra i diversi elementi, per diametri di 15 µm. In ogni 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 1 Dip.to di Scienze della Terra, Università degli Studi di Cagliari, Via Trentino, 51, 09127, Cagliari, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via della Faggiola, 32, 56126, Pisa, Italy Submitted, September 2008 - Accepted, November 2008 * Corresponding author, E-mail: [email protected]
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An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYandappliedtopicsonEnvironment,ArchaeometryandCultural Heritage
l
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
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.
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.
68 c. d’oriAno, s. dA Pelo, f. PoddAandr. cioni
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.
Quantitative calibration of the LA-ICP-MSsystemrequirestheknowledgeoftheconcentrationof one element (internal standard), analysedindependentlyinthesampleandinthereferencematerial(Perkinset al.,1997;Pearceatal.,1999).Weuse 44Ca,measuredduringeachanalysis asaninternalstandard,andthedataarenormalisedtotheCaOcontentofthesampleasdetermined
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
Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS).... 71
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.
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
72 c. d’oriAno, s. dA Pelo, f. PoddAandr. cioni
Fig.3 – Low Limit Detection (LLD) measured on the BCR-2 sample at spot size of 40 μm and 15 μm.
Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS).... 73
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.
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
74 c. d’oriAno, s. dA Pelo, f. PoddAandr. cioni
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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