1 - Sample Prep Basics Paul ICP Conf 2016 · with a pneumatic nebulization (concentric or...

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SamplePreparationBasics

• DefinitionsandObjectivesofaSamplePreparationforICPmeasurement• ChemicalCompatibility• SelectingaSamplePreparationApproach• ContainerMaterialProperties• ClassicSamplePreparationChemistry— ashing,aciddigestion,fusion

Discussiontopicswillinclude:


DefinitionsSamplePreparation(forICPapplicationsforthepurposesofthispresentationisdefinedas)theconversionofasolidorliquidintoaliquidthatissuitableforICPMeasurementswithapneumatic nebulization(concentricorcross-flow)/spraychamber(ScottorCyclonic)introductionsystem.

SampleSolution– Aliquidformofthesample.Thepreferenceishavingtheanalytesinanionicsolvatedformorobservingthattheanalyteswillpassthrua0.3micronfilter.Thismayormaynotinvolveachemicalattack.

StandardSolution– Aliquidformofthecalibrationandqualitycontrolstandards

MatrixMatching– TheabilitytomatchtheSampleSolutiontotheStandardSolutionsuchthatallnebulizerandplasmarelatedeffectsareeliminatedorcanbecorrected.


Objectives•Conversionofsampletoasolution— samplewillpassthrua0.3micronfilter•Eliminatesamplelosses– volatilization,adsorption,precipitation,mechanicalloss•Eliminatecontamination– fromreagents,apparatus,environment,analyst,container

SamplePreparation(forICPapplicationsforthepurposesofthispresentationisdefinedas)theconversionofasolidorliquidintoaliquidthatissuitableforICPMeasurementswithapneumatic nebulization(concentricorcross-flow)/spraychamber(ScottorCyclonic)introductionsystem.

SamplePreparationBasics— Objectives

ObjectivesContinued

• Eliminateoraccountfornebulizerrelatederrors

• Eliminateoraccountforplasmarelatederrors

• Theoverallobjectiveiseliminationorminimizationofsystematicerrorswhileconvertingthesampletoaliquidformthatiscompatible withcommonICPintroductionsystems,otherelements,apparatus,etc.

SamplePreparationBasics— Objectives

SamplePreparation(forICPapplicationsforthepurposesofthispresentationisdefinedas)theconversionofasolidorliquidintoaliquidthatissuitableforICPMeasurementswithapneumatic nebulization(concentricorcross-flow)/spraychamber(ScottorCyclonic)introductionsystem.

• Compatibilitybetweenelementsandthematrixwhenmakingblendsofelementsi.e.eliminatingprecipitation,adsorption,volatility

• Compatibilitywiththeintroductionsystem

• Compatibilitywithlaboratoryapparatususedinpreparationandholdingofsampleandfinalsamplesolution

• Compatibilitywiththeanalyst— safety,contamination

ElementandMatrixCompatibility

ElementandMatrixCompatibility— Grouping

1. Alkalis:Group1— Li,Na,K,Rb,andCs;likeHCl,HF,HNO3, H2SO4 andwater

2. AlkalineandRareEarth:Groups2and3— Be,Mg,Ca,Sr,Ba,Sc,Y,La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu,ThandU;LikeHCl andHNO3 butavoidHF andH2SO4

3. HF Elements:Ti,Zr,Hf,Nb,Ta,Mo,W,Si,Ge,Sn,Sb,Te

4. HCl Elements:Ru,Os,Rh,Ir,Pd,Pt,Au

5. HNO3 Elements:V,Cr,Mn,Re,Fe,Co,Ni,Cu,Ag,Zn,Cd,Hg,B,Al,Ga,InTl,Pb,As,Bi,Se

6. Non-Metals:C,P,S,F,Cl,Br,I;chemicalformcriticaltocompatibility

• MicrowavedigestionmethodologystronglyinfluencesMEBmatriceswhereHNO3 ,HCl,andHFaremostcommon

• The#1matrixacidisHNO3withHCl being2nd

• HF istypicallyallowedandisalwaysmixedwithHNO3 orHCl atlow%totrace(<0.1%)levels

• H2SO4 isstillusedbutislesscommon

• Analystsrequireone workingsolution— multipleconcentratesareallowedonlyiftheycanbemixedatworkinglevels

• Solutionstabilityisamust

FactorsThatInfluenceMEBPreparation

F denotesthattheelementcanbedilutedinHNO3 ifcomplexedwithF-.Cl denotesthattheelementcanbedilutedinHNO3ifcomplexedwithCl-.HF denotesthattheelementshouldhaveexcessHFpresentwhendilutedwithHNO3.T denotesthatthetartaricacidcomplexcanbedilutedinHNO3.1. OsshouldneverbemixedwithHNO3 duetotheformationoftheveryvolatileOsO4.2. ClisoxidizedtomolecularCl2 whichisvolatileandadsorbsonplastic.3. BrandIareoxidizedtomolecularBr2 andI2 whichadsorbontoplastic4. DilutionsofHgandAuinHNO3 below100ppmshouldbestoredinborosilicateglassduetoHg+2 adsorptiononplastic.5. Notsolubleaboveconcentrationsof1000µg/mL.6. TracelevelsofHClorCl- willformAgC,whichwillphotoreducetoAg0.

ElementandMatrixCompatibility— NitricAcid

F denotesthattheelementismorestabletohydrolysisifcomplexedwithF.

1. Concentrated(35%)HClwillkeepupto100µg/mLofAg+ insolutionastheAg(Cl)X-(X-1) complex.Formoredilutesolutions,theHClcanbeloweredsuchthat10%HClwillkeepupto10µg/mLAginsolution.

NOTE: TheAg(Cl)X-(X-1) complexisphotosensitive andwillreducetoAg0 whenexposedtolight.HNO3 solutionsofAg+ arenotphotosensitive.

2. Parts-per-billion(ppb)dilutionsofHg+2 inHClaremorestabletoadsorptiononthecontainerwallsthanaredilutionsinHNO3.

ElementandMatrixCompatibility— HCl

Thefollowingchecklistshouldbeconsideredpriortoselectingamethod:

ü Theidentityoftheanalytesandpotentialchemicalforms.

ü Theconcentrationrange(s)oftheanalyte(s)andthedetectionlimitrequirement(s).

ü Thechemicalandphysicalcompositionofthesamplematrix.

ü Theavailabilityofapparatusandequipment.

ü Thesamplesizethatisavailableorrequired.

ü Thepotentialforcontaminationduringsomepartofthesamplepreparationprocess.

SelectingaSamplePreparationApproachTheChecklist

• Theanalystisnowinapositiontoselectthepreparationtechnique.

• Thisinvolveschoosingthemodeofattack(aciddigestion,ashing,fusion).

• Thespecificchemicalreagentsandthecontainer(s)materialsneededtocarryoutthepreparation.

• Theanalystmustkeepinmindthatcontaminationissues,plusanydifficultieswithinthefinalsamplesolutionmatrix,willhaveanimpactupontheICP-OESand/orICP-MSmeasurementtechniques

SelectingaSamplePreparationApproachFinalApproach

SelectingaSamplePreparationApproachHNO3-HF / HCl / HF HNO3 / HCl HNO3-HF / HCl HNO3 - HCl HNO3-HF / HCl

All mineral acids OK mDL avoid HF & SO4= mDL HF suggested mDL no SO4 mDL be Alert to reduction mDL

Li Th Te Ba RhNa, cntmnt rsk Ge Sb Pb cntmnt rsk PdK Mg cntmnt rsk Mo Hg PtRb Ca cntmnt rsk Ti HNO3-HF IrCs Sr Zr no HCl mDL AuB cntmnt rsk Sc Sn Ag RuP YAs La HNO3-HF HClS Ce HF a must mDL avoid HNO3 mDLSe Pr Si cntmnt rsk Os loss as OsO4V Nd Ge Cr Sm HfMn Eu NbFe cntmnt rsk Gd TaCo Tb WNi DyCu HoZn cntmnt rsk ErGa TmIn YbRe LuUAl cntmnt rskCd cntmnt rskTlUBi

mDL = minimum Detection Limit

White lettering is volatility risk

SelectingaSamplePreparationApproachOrganic Inorganic %Ash

Fnctlgrps grams Matrix/Description

PharmacueticalTransportationEnvironmentalMetallurgicalConstructionAgriculturalAutomotivePetroleumCosmeticChemicalMedicalEnergyMiningTextileFood

A.AshAshingcanbecombinedwith

aciddigestionorfusion.

GrindingD.AcidDig.F.Fusion

SamplingVolatility

Contamination

Chemical HF HNO3 HCl SO4 CRO4 MiscRestrictions

Laboratoriesusemanydifferentcontainermaterialsforhandlingsamplesduringsamplepreparation.Somematerialsaremoreadvantageoustousethanothers.

Herewe'lllookatthepropertiesof:

• BorosilicateGlass

• Porcelain

• Quartz

• Platinum,5%Au/Pt

• Graphite,GlassyCarbon

• PlasticsincludingPFAandPTFETeflon

ContainerMaterialProperties— Materials

• BorosilicateglassisusedextensivelyforaciddigestionsinvolvingHNO3,HCl,H2SO4.

• Shouldnot beusedforashing orfusions andattemperatures>500ºC.

• Itisresistanttomostacids,butshouldnot beusedwithHF orboilingH3PO4.

• Asageneralrulealkalinesolutionsshouldnotbeheatedorstoredinborosilicateglass.

• Borosilicateglasscancontributeavarietyofcontaminants.[Ca(1760),Si,(170),Na(130),Fe(3),K(30),B(60),Mg(53),Al(50),Mn(1.2),Zn(22),Sr(1.3),Sn(0.8),Sb(0.4),Ba(17)–valuesare(ppb)obtainedbyleachingnewborosilicateglasswith1%HNO3 for7daysat60˚C.

• Itshouldnotbeheatedovertemperaturesachievableusingahotplate(500°C).Forexample,ifyouneedtoashasampleusingamufflefurnace,donotuseborosilicateglass.

ContainerMaterialProperties— BorosilicateGlass

• Porcelainisapopularmaterialused forashing purposes.

• Affordable

• PorcelaincontainsNa,K,Al,andSiinincreasingconcentration.

• Itistypicallycoatedwithaglazewhichisabout70%SiO2,withroughlyequalamountsoftheoxidesofAlandCa,andlesseramountsofNaandK.

• Attackwilloccurifthesamplecontainsevenminoramountsofthealkalimetals.Thisismadeevidentbyadullinginthenormallyshinysurface.

• Ifalkalisarepresent,thenthesampleistypicallytreatedwithconc.H2SO4priortoashing.

• Thefollowingshouldnot beheatedinporcelain:HF;boilingH3PO4;andtheoxides,hydroxides,orcarbonatesofthealkalioralkalineearthelements.

• Themajoradvantageorporcelainoverglassisthatitcanbeheatedupto1100°C

ContainerMaterialProperties— Porcelain

• Therearetwotypesofquartz— opaqueandtransparent

• Opaquequartzhasthehighesttraceelementconcentration

• Transparentquartzcomesinfourdifferentvarieties

ContainerMaterialProperties— Quartz

• TypesI&IIaremadefromnaturallyoccurringquartzcrystalsorsands.TypeIiscreatedbyelectricmeltingandtypeIIbyflamemelting.TypeIIhasslightlylessimpuritiesthantypeI(someimpuritiesarevolatilizedbytheflame).

• TypeIIIquartzismadesyntheticallybyvaporphasehydrolysisofpuresiliconcompoundssuchasSiCl4.Thistypeofquartzismorepurethanthenaturalquartz,withtheexceptionofCl.

• TypeIVquartzissyntheticallymadefromSiCl4 usingaprocessinvolvingelectricalfusionoftheoxidizedstaringmaterial.ItisaspureastypeIII,withrespecttotracemetalcontent,andcontainsfarmoreCl- whichis~50ppm.

• Usethetransparentquartzwheneverpossible.

• Quartzistypically99.8+%SiO2.

• ItisattackedbyHF,boilingH3PO4,andthealkaliandalkalineearthoxides,hydroxides,andcarbonates.

• Itcanbeheatedto1100°C.

• ItsmainadvantageoverthatofporcelainisthatmajorcontaminationoccursfromonlySi— however,thiscontaminationcanbesignificant.

ContainerMaterialProperties— Quartz(cont.)

• Itisresistanttoattackbymostacidsandreagents.AvoidconcentratedH3PO4 athightemperatures,HCl+HNO3 mixturesandfusions usingLi2CO3,Na2O2,orthealkalihydroxides.

• Itheatsupandcoolsdownrapidly,makingitexcellentfor%ashdeterminations wherethe%ashisatlowlevels.

• Fusions usingNa2CO3 arecommoninadditiontofusionsusingthealkaliborates,fluorides,nitrates,andbisulfates.Avoid heatingatprolongedtemperaturesinexcessof1100°C(M.P.=1772°C).

• Platinumcanbedestroyedbyheatingwithmetalswithwhichitcanalloy.Avoidhightemperatureheatingwithsamplescontainingsignificantlevelsofanymetalthatmaybeinorreducedtothemetallicstateduringtheheatingprocess.(Allpreciousmetals,Cu,Hg,Sn.)

• Platinumisknowntocontaintraceamountsoftheotherpreciousmetalsandshouldnotbeusedfortheirpreparation.

• AvoidashingsamplescontainingPinanyform,includingthephosphates.

ContainerMaterialProperties— Platinum

• Graphiteisveryinexpensiveandrelativelyclean,butverymessytoworkwith.

• ItisaninexpensivewaytoperformLi2CO3 fusionswherethecrucibleslowlyoxidizesawayoverthecourseof7–10fusions.

• Itispopularbecauseitdoesnotwetbysomemeltswhichcanbepouredoutquantitatively.

• Lossesduetotheporosityofgraphiteshouldexcludeitsuseforashingsamplescontainingtracemetals.

• Graphite'smainadvantagetothetraceanalystisbeingamaterialthatcanwithstandfusionsthatmightdestroyplatinum.

• OurchemistsusegraphiteforperformingLi2CO3fusions.

ContainerMaterialProperties— Graphite

PhysicalPropertiesofCommonPlastics:

FEP (FLUORINATEDETHYLENEPROPYLENE)PFA (PERFLUOROALKOXY)FLEP (FLUORINATEDHIGH-DENSITYPOLYETHLYENE)PMP (POLYMETHYLPENTENE)

ContainerMaterialProperties— Plastics

PP (POLYPROPYLENE)HDPE (HIGH-DENSITYPOLYETHYLENE)LDPE (LOW-DENSITYPOLYETHYLENE)

ContainerMaterialProperties— Plastics(cont.)

FEP (FLUORINATEDETHYLENEPROPYLENE)PFA (PERFLUOROALKOXY)FLEP (FLUORINATEDHIGH-DENSITYPOLYETHLYENE)PMP (POLYMETHYLPENTENE)

PP (POLYPROPYLENE)HDPE (HIGH-DENSITYPOLYETHYLENE)LDPE (LOW-DENSITYPOLYETHYLENE)

• BorosilicateGlass– aciddigestions(noHF)

• Porcelain– ashing (wetandsulfated),fusion(acidic)

• Quartz– ashing (wetandsulfated),aciddigestions(noHF),fusions(acidic)

• Platinum– dryashing (notPcontaining)ashing (wetandsulfated),fusion(sodiumcarbonatebutnotLithiumcarbonate,alkalihydroxidesandornitrates),fusion(lithiumtetraborate/carbonate)fusion(acidic)

• Graphite– lithiumcarbonatefusion

• Plastics– Teflon(PTFE,PFAforaciddigestion),containingdigestates/samplesolutions(LDPEisbyfarthebest)

ContainerMaterialProperties— CommonUses

SeetheTraceMetalsAnalysisguideinthe‘TechCenter’ontheInorganicVentureswebsitefor:

• EnvironmentalContamination(Chapter8)

• ContaminationfromReagents(Chapter9)

• ContaminationfromtheAnalystandApparatus(Chapter10)

Contamination

• TheabilitytodecomposelargesamplesizesachievinglowDLs.

• Lowreagentblanks- theneedforlittleornoreagents.

• Thetechniqueisrelativelysafe.

• Elapsedtimesarehoursbutactualanalystattentionrequiredisminutes.

• Theabilitytopreparesamplescontainingvolatilecombustionelementssuchassulfur,fluorineandchlorine(theSchönigeroxygenflaskcombustiontechniqueisverypopularinthiscaseinadditiontolowtemperatureplasmaashing).

• Usingashingaidsveryfewelementsarelost(S,Se,andHgareproblematic).

• Thetechniquelendsitselftomassproduction.

SamplePreparationBasicsAshing– TechnicalAdvantages

• DryAshingisusuallyperformedbyplacingthesampleinanopeninertvesselanddestroyingthecombustible(organic)portionofthesamplebythermaldecompositionusingamufflefurnace.

• Typicalashingtemperaturesare450to550°C.• Charringonahotplateortheuseofaprogrammablefurnaceisadvantageous.• Magnesiumnitrateiscommonlyusedasanashingaid.• Forlargersamplesizescharringthesamplepriortomufflingispreferred.• Charringcanbeaccomplishedusinganopenflame.• Ignitionandburningofthesamplecanbeveryhelpful.Thisiswidelyusedinthe

polymerandpetroleumindustries.• Porcelainandplatinumaremostpopularcruciblematerialsfollowedbyfused

silicaandquartz.

SamplePreparationBasicsAshing— DryAshing

• TheadditionofsmallquantitiesofH2SO4 or‘sulfatedashing’involvestreatmentofthesampleaftercharringwithsulfuricacid.

• Thechariswettedusingtheminimumamountofsulfuricacidandthenbroughttodryness.

• Sulfationcanpreventvolatilizationlossofvolatilechlorides,preventattackofsilicaandaluminabasedcrucibles,preventformationofalightfriableashandpreventformationofsomerefractoryoxidesmakingthefinalasheasiertodissolve.

• Beforeplacinginamufflefurnace,thecharshouldbeheatedto>400˚C.

• Attempttokeeptheashingtemperaturebetween450and500˚C.

• Sulfatedashesgenerallydiscouragetheformationofrefractoryoxidesduringmufflingmakingdissolutionseasier.

• DonotsulfatesamplescontainingBaorPb.


Ashing– DryAshingwithH2SO4 asanAshingAid

• TheadditionofmLquantitiesofH2SO4 or‘wetashing’involvestreatmentofthesamplebeforecharringwithsulfuricacid.

• Charring/digestionisperformedusingeitheranopenflameorhotplate.

• Liquidsamplestendtofoam.Constantattentionfromtheanalystisadvisedandthetreatmentprocessistediousandslow.Consequentlythisapproachisnotpopular.

• Aftertheexcesssulfuricacidisdrivenoff,thesampleismuffledasabove.

• Wetashingischosenoversulfatedashingtoavoidcrucibleattackwith‘caustic’samplesoranalytelossthroughreactionwithcruciblecontainer.

• NotsuggestedforsamplescontainingBaorPb.


Ashing– ‘Wet’AshingwithH2SO4 asanAshingAid

• Low-temperatureAshinginvolvestreatmentofthesampleat~120°Cusingactivated(singletstate)oxygen.Lifetimeofexcitedoxygenis~1second.(Oxygenispassedthroughahigh-frequencyelectricfieldof13.5MHz.)

• Used fortraceanalysisoforganicsamplessuchascoal.Lookforpossiblelossesofhalogens,SandHg.

• Oftenusedonsamplestoavoidanalytelossesofvolatileanalytes(As,Cd,Sb)andvirtuallyeliminatesrxn.betweenresidueandashingcontainerreducingcontamination.

• Usedforthetraceanalysisofcoals.Samplesize isgreater thanforaciddigestion.

• Sampleisspreadoutoverashingcontainertospeedupprocess— is stillslowrequiring1–3daysdependinguponsamplesizeandorganicstructure.

SamplePreparationBasicsAshing– LowTemperatureAshing

• ClosedSystemAshing involvesthermaldecompositioninoxygeninaclosedsystemsuchasaSchöniger flask.

• SampleiswrappedinpaperandheldinaPtbasket.Asolutionatthebottomoftheflaskisusedtoabsorbthecombustionproducts.

• ThistechniqueismostcommonlyusedforhalogensPandSincombustibleorganicmatrices.Convenientforanalyzingcompoundsseparatedbypaperchromatography.Usefulintheanalysisofradio-isotopesinbiologicalmaterials(3H,14C,and35S).

• Flasksaremadeofborosilicateglassorpolypropyleneforfluorineanalysis.

• A1literSchöniger flaskholdsenoughoxygen tocombustupto150mgofsample.

• Thistechniqueiswidelyusedinmicroelemental analysis.

SamplePreparationBasicsAshing — ClosedContainerAshing

• Possiblelossduetoretentiontotheashing container.

• Possiblelossduetovolatilization.

• Contaminationfromtheashing container.

• Contaminationfromthemufflefurnace.

• Physicallossof'lowdensity'asheswhenthemuffledoorisopened(aircurrents).

• Difficultyindissolvingcertainmetaloxides.

• Formationoftoxicgasesinpoorlyventilatedareas.(Notethatallcharringshouldtakeplaceinahoodandthemufflefurnacemusthaveahoodcanopyforproperventilation).


Ashing– PotentialProblems

• Ifthesampletypeisunknown(withrespecttothematrix)thenanEDXRFscan,IRscan,andC,H,andNanalysiswillprovidesufficientinformationinmostcasestomakeinformeddecisions.

• ProtectyourPt0 warebylookingforP(highlevelswillattackandattachtothePt0)andelementsthatalloywithPt0 whichincludethepreciousmetals,Cu,andHg.

• Whenusing'silica'containingcrucibles(porcelain,Vycor,quartz,glass,andfusedsilica)lookforelementsthatformbasicoxidessuchasthealkaliearthelements.Naiscommonlyfoundandit'soxidewillform(unlessthecharissulfated)andattackthesilica.


Ashing— AvoidingProblems

• Lookforvolatileelements(Cd,B,Hg,Pb,Se,Zn,As,Sn,Sb,S,andhalogens)especiallyifmoderatetolargeamountsofForClarepresent.

• SiisacommonelementthatistypicallydeterminedbydissolutionofanashperformedinPt0.Methylsiliconesarewidelyusedandverycommon.IfSiispresentasasiliconeoilthenitwillbepartiallylostasthehexamethycyclotrisiloxaneandthehexamethydisiloxane.

• Retentionandphysicallossofanalyte(s).TheuseofhighpurityMg(NO3)2 asanashing aidwillhelppreventlossesof'lowdensity'ashes,andwillhelpinpreventingretentionlosses.


Ashing– AvoidingProblems(cont.)

• Fordifficulttodissolveoxidesuseaslowanashingtemperatureaspossible(400to550°Cmaximum)forsamplestobeanalyzedforTi,Zr,Nb,Hf,Ta,W,Ni,Co,Fe,Cr,Sb,andMo.Pt0 isnotattackedbyHFwhichwilldissolveseveraloftheaboveoxides.

• Lossduetoreductiontothemetalcanoccur.LookforeasilyreducedelementssuchasCuandthepreciousmetals.Usetheappropriatecruciblematerialtoallowforthenecessarydissolutionreagentsforthemetal.Ptcruciblesshouldnotbeused.

• WhenusingPtrememberthatcertainelements‘alloy’withitincludingHg,Cuandmanyofthepreciousmetals.

• WhenusingPtrememberthatithasarathercomplexspectrumandmayinterferewithICP-OESmeasurement(somePtisalwayslostduringeachpreparation).

• Rememberthatatleasttraceamountsofyourcruciblematerialwillendupinyoursamplesolution.


Ashing— AvoidingProblems(cont.)

• Aciddigestionshavetheadvantageofretaining'volatile'analytes(refluxcondenserisneededforsometraceelements).

• Aciddigestionshavethedisadvantageofbeingtediouswhenlargesamplesizesarerequired.

• Aciddigestionsareidealifthesamplesizeis<1gram.

• Nitricacid(HNO3)isusedinpracticallyeveryaciddigestionprocedureandiscommonlyusedincombinationwithotheracids.

• Nitricacidispopularbecauseofitschemicalcompatibility,oxidizingability,availability,purity,andlowcost.


AcidDigestions

• Nitricacid* isusedprimarilyinthepreparationofinorganicsampletypes.++Itisaveryusefulcomponentinthedestructionoforganicsbutcannotbyitselfcompletelydecomposeorganicmatrices.

*AllreferencetoHNO3 willmean69%'concentrated'nitricacidunlessspecifiedotherwise.++Theconventionalmeaningofinorganicisintendedalongwiththepresenceoflowmolecularweightwatersolubleorganiccmpds.andorganometalliccmpds.containingrelativelysmallmolecularweightorganiccomponents.


AcidDigestions— InorganicSamples

• Dilute10–15%aqueousdilution— AlkaliandAlkalineearthoxides,lanthanideoxides,actinideoxides,Sc2O3,Y2O3,La2O3.

• 1:1HNO3 /H2O— V2O5,Mn oxides,CuO,CdO,HgoxidesTloxides,Pb oxides,Bioxides,Cu0,Zn0,Cd0,Hg0,Pb0.

• Concentrated(69%)HNO3 —Mn0,Fe0 (hot),Co0,Ag0,Ni0,Pd0(hot),Se0,As0,Bi0,Re0.

• 1:3HNO3 /HCl — Pt0,Au0,steel,Fe/Nialloys,Cualloys,Cr/Nisteel.

• 1:1:1HNO3 /HF/H2O— ThemetalandoxidesofTi,Zr,Hf,Nb,W,Sn,Al,Si,Ge,Sb,Te,As,Se,Moandnumerousalloysandoxidemixturescontainingoneormoreoftheseelements.


AcidDigestions— InorganicSamples

• Thismethodisapplicabletothemicrowaveassistedaciddigestionofsoils,ash,sediments,sludgesandsiliceouswastes.

• TypicalAcidmixis9mLnitric+3mLHF.Sometimes2–4mLofHClisadded.

• Ithasbeenclaimedtobea‘UniversalMethod.’

• Typicallynomorethan0.5gramsofsampleisdigested.

• Themethodallowstheanalystthefreedomtovarytheratiosofacidsbutlimitsaredefinedbythemethod.

• Withsmallamountsoforganicstheuseof30%hydrogenperoxideisallowed(0.1to2mL).


AcidDigestions— InorganicSamples— EPAMethod3052

• Theadditionofwater(0to5mL)isallowedandmayimprovethesolubilityofmineralsandpreventtemperaturespikesduetoexothermicreactions.Mayalsoalterthereductionpotentialofthenitricacid.

• Thismethodisdesignedtoachievetotaldecompositioninsealedinertpolymericmicrowavedigestionvesselswherereactionsreachingtemperaturesof180˚Cin5minutesareallowedtoreactatthistemperaturefor9.5minutes.

• Themethodrequiresamicrowavesystemthatwillsenseandmaintainthetemperaturebyadjustingthemicrowavefieldoutput.Temperaturefeedbackcontrolprovidestheprimarycontrolperformancemechanismforthemethod.


AcidDigestions— InorganicSamples— EPAMethod3052

• Intheexampleslistedabovenitricacidisactingasastrongacidwhereinorganicoxidesarebroughtintosolution…

• Andasanoxidizingagent/acidcombowherezerovalenceinorganicmetalsandnonmetalsareoxidizedandbroughtintosolution…

• Inaddition,nitricaciddoesnotformanyinsolublecompounds.Thesamecannotbesaidforsulfuric,hydrochloric,hydrofluoric,phosphoric,orperchloricacids.


AcidDigestions— InorganicSamples— NitricAcid(cont.)

1. CaO+2H3O+ =Ca+2 +3H2O

2. Fe0 +3H3O+ +3HNO3 (conc.)Fe+3 +3NO2(brown)+6H2O3. 3Cu0 +6H3O+ +2HNO3 (dilute)2NO(clear)+3Cu+2 +10H2O

Nitricacidundergoesbothoneandthreeelectronchanges.Theoneelectronchangeisobservedwhenconcentrated.Incomparison,the3electronchangeisobservedwhendiluteinreaction(3).Thepresenceofbrownfumesisindicativeofreactionsgoingby1electron.



4. H3O+ + HNO3 + e-1 = NO2 (brown) + 2H2O : Concentrated

5. 3H3O+ + HNO3 + 3e-1 = NO (clear) + 5H2O : Dilute

• Nitrateisgenerallyconsideredtobea'poorligand'inthatitscoordinationabilityisnotenoughtokeephydrolysisfromoccurring.Thisstatementmaybecontradictedincertaininorganictextbooks.

• Themostcommon‘goodligands’usedincombinationwithnitricacidareHCl,HF,H3PO4 andtartaricacid(forSb).Ifnitricacidwasabetterligand,theseadditionalacidswouldnotbeneeded.

• ConcentrationsofHNO3 between65%and69%areknownas‘concentrated;’concentrationsgreaterthan69.2%areknownas‘fumingnitricacid.’

• 100%nitricacidislightandheatsensitiveandboilsat84°C.‘Concentrated’nitricacidboilsasanazeotrope(withwaterat69.2%HNO3)atatemperatureof121.8°C.ThedistilledHNO3 (tracemetalsgrade)shouldbeatthe69.2%concentrationlevel.

SamplePreparationBasicsAcidDigestions— InorganicSamples— NitricAcid(cont.)

• Checkwithyourmanufacturerofdoublydistillednitricacidtodetermineifthecontainerinwhichitispackagedisnitricacidleachedpriortouse.InthecaseofTefloncontainers,thecontainermaterialisgenerallyassumedtobepure.

• PTFEandPFATefloncanbeheatedwithconcentratednitricacid,evenathighpressuresorwithcombinationssuchasnitric+HCl,nitric+HF,andnitric+H2O2.

• NitricacidisnotastrongenoughoxidizingagentbyitselftoconvertorganicmoleculestoCO2 andH2O(completelyoxidize).



• Thelackofnegativesideeffectsisunfortunatelylimitedtotheinorganicsideofthetable.Theabilityofnitricacidtoreactwithalcoholsandaromaticringsformingexplosivecompounds(nitroglycerinandTNT,tonametwo)callsforcautionwhenusingnitricacidaloneorincombinationwithotherreagentsinthedecompositionoforganicmatrices.

• Ifyoursamplecontains-OHfunctionalityitisbesttopre-treatthesamplewithconcentratedsulfuricacid.Whenconcentrated,thesulfuricwillactasadehydratingagent.— R-CH2-CH(OH)-R'+H2SO4 R-CH=CH-R'+H2O

• Idonotrecommendtheuseofnitricacidforthedigestionofhighlyaromaticsamples.

• NitricaciddoesnotbreakdownorganiccomponentstoCO2andH2O.


AcidDigestions— OrganicSamples

• Nitricacidisrarelyusedalone.

• Itisbestusedincombinationwithsulfuricand/orperchloric acidsfororganicsampledigestion.

• Forsamplesthatarenothighlyaromaticand/orcontainahigh-OHfunctionality,Iprefertousenitricacidfollowedbyperchloric acid.

• Theonlyelementthatmay belostfromanitric/perchloric digestionisHg.

• Careshouldbeexercisedandtheliteratureconsultedbeforeattemptingtousenitricacidincombinationwithotheracidsfororganicsampledigestions


AcidDigestions– OrganicSamples(cont.)

• Organicmatricesshouldalwaysbepre-treatedwithnitricacid(seeexceptionsabove).

• Perchloric acidshouldneverbeusedalone.

• Perchloric aciddigestionsshouldneverbeallowedtogotodryness.

• Hotperchloric acidshouldneverbeaddedtoanorganicmatrix.

• Samplesizesshouldneverexceed1gram(dryweightforbiologicals).

• Perchloric acidfumesshouldbenotbeallowedto‘gofree’unlessaperchloric acidhoodisused.

• Unknownorganicmatricesshouldbeanalyzedbymolecularspectroscopytodetermineprimarystructurebeforeattemptingtheuseofeithernitricorperchloric acid.

SamplePreparationBasicsAcidDigestions— OrganicSamples— Nitric+PerchloricAcidfordetailedproceduregotoInorganicventures.comandthengothechapter12oftheTraceMetalsGuide locatedthe‘TechCenter’

• Dilutesulfurichasnooxidizingpowerbuthotandconcentratedis.

• SulfuricacidisusedtooxidizeSbandalloysofAs,Sb,andSn.

• Pb presentinalloysisoxidizedandprecipitatedasthesulfate.

• Sulfuricacidwithcatalysts(CuSO4,orSeO2 orHgSO4)isbasisfortheKjeldahl digestionmethod(determinationofNitrogen).

• Thedropwiseadditionof30%H2O2 tohotfumingsulfuricacidisusefulfororganicmatrices.HaveusedforBeingrease.

• TheadditionofnitricaciddissolvesalloysofMo,Zr,Sn,steels,carbides,oresofmolybdenum.Sulfides,andZnores.

• Theadditionofnitric+HCl willoxidizesteels.

• Theadditionofnitric+perchloric acidisverypowerfulforhardtooxidizeorganicsamples.

SamplePreparationBasicsAcidDigestions– 98%H2SO4

• Theyareexpensiveandoftennotavailable(highpurityfluxes).

• TheyyieldhighsolidssolutionsthatcansaltoutinthenebulizerandcausesensitivitylosswithICP-MS.

• Largedilutionsofthesamplearesometimesrequired.

• Contaminationofthesamplewiththecrucibleconstructionelementandimpuritiesmustbeconsidered.

• However— fusionisanessentialcapabilityforalaboratoryreceivingawidevarietyofsampletypes.

• Itisthebestapproachformanyinorganicrefractories.


Fusion— Fusionsareconsideredtobemoreofa‘lastresort’bytraceanalysts:

• Flux— K2S2O7 orKHSO4 whichconvertstothepyrosulfatewhenheated

• Crucible— Pt0orfusedsilica/quartz(lessthan.3mgsilicalost/fusion)

• Flux:Sampleratio— 20:1

• Temperature— 500˚C

• Time~20minutes

• MostPopularApplications— TiO2,ZrO2,Nb2O5,Ta2O5

• Easy toperform.Mosteasilyperformedoveraflame.

• Iffluxbeginstosolidifyduringheatingadd98%sulfuricaciddropwisetoregenerate


Fusion— Pyrosulfate(K2S2O7)orBisulfate(KHSO4)

KHSO4 (MP197˚C)heatedformsK2S2O7(MP325˚C)whenheatedformsK2SO4 (MP1069˚C)

• Ifaflameisnotavailablethenuseamuffleat500˚C.

• Swirlingmeltwhileheatingispreferred.

• Swirlmeltontowallsafterremovingheat.Meltwillcoolandcrackintosmallpiecesthatareeasiertodissolve.

• Dissolvemeltinaqueoussolutionscontainingappropriateacidorstabilizer.


Fusion— Pyrosulfate(K2S2O7)orBisulfate(KHSO4)(cont.)

Heatuntilmeltisclear

• Duringfusionlosses ofsulfur asH2S,fluoride asHF,carbonate asCO2,borate asH3BO3,arsenite asH3AsO3,arsenate asH3AsO4,selenite asH2SeO3,andchlorideasHClwilloccur.

• IffluorideispresentsomeSiwillbelostandifCl,BrorIarepresentsomeSb,Sn,Ge,andVwillbelost.

• Thisfusionisusedfordealingwithrefractoryoxidestypicallyformedwhenignition/ashingtemperaturesaretoohigh.TheoxidesofBeFe,Cr,Mo,Te,Ti,Zr,Nb,andTa.

• Ihavefoundthismethodveryusefulfordealingwithbrookite (averyrefractoryformofTiO2).


Fusion— Pyrosulfate(K2S2O7)orBisulfate(KHSO4)(cont.)

• Flux— Li2B4O7(tetraborate);LiBO2 (metaborate);H3BO3 +Li2CO3 at1:1(matterofpreference— metaborateusedforchromite(Fe++Cr2O4).

• FusionsallowforthedeterminationofNaandK.

• Crucible— Pt0;Pt0/Au0;occasionallyAu0 orgraphite.


• Temperature— 950to1200˚C

• MostPopularApplications— SiO2,Al2O3,alumino-silicates(variousminerals,zeolites,etc.),chromite.

• Fusionanddissolutionoffuseate(typicallyin5%nitric/water)hasbeenautomatedusingcommerciallyavailable‘fluxers.’


Fusion— LithiumBorate

• Flux— NaOH(MP321˚C);KOH(MP404˚C)advantageoverthecarbonatefusionasamuchlowertemperatureisrequired.

• Crucible— Ag0;Ni0 (alkalihydroxidesattackPt0)–MessywithlotsofNicontamination


• Timeis~30minutes

• Temperature— 500˚C(maxof600˚C/Ni0 and700˚C/Ag0).Morecrucibleattackathighertemperatures.

• MostPopularApplications— Silicates(glass,porcelain,kaolinetc.)andaluminosilicates


Fusion— NaOHorKOH

• Flux— (7:1)KOH:KNO3

• Crucible— Ag0


• Temperature— 650˚C(maxof700˚C/Ag0).Morecrucibleattackathighertemperatures.

• MostPopularApplications— Ruthenium,Chromite,“Organics”(“Organics”referstoorganiccontainingsamples.Boththesodiumandpotassiumhydroxide/nitratemixtureshavebeenusedforawidevarietyofbiologicalmaterials,soils,coalandorganicsamples.)


Fusion— KOH+KNO3

• Flux— Na2CO3;mp=853ºC

• Crucible— Pt0


• Temperatureoffurnace– 1000˚C;Time~20minutes

• 5–9spurityfluxisavailable

• MostPopularApplications— Na2CO3 fusionsareverypopularinthedecompositionofminerals,silicates,refractories,insolublemetalfluorides,etc.forexample,brookiteTiO2 willnotbeattackedbyacids,andAl2O3 isveryresistanttoacidattack.

• Ipersonallylikeitformanyorganicmatrixdecompositionswhereitisaddedtothesamplebeforeashing.


Fusion— Na2CO3

• Flux— Li2CO3

• AdvantageislowertemperaturebutdisadvantageisthatPtcan’tbeused.

• Crucible— Graphite.CAUTION— Li2CO3 willattackPtcrucibles.


• Temperature— 700˚C

• MostPopularApplications— SimilartoNa2CO3


Fusion— Li2CO3

• Lithiumsaltsareverywaterandorganicsolventsoluble.• LithiumhasrelativelyfewspectrallinesfortheICP-OESuser.• Signalquenching(signalsuppressionbymatrix)isrelativelyless(duetolowmass)

thantheB,Na,orKcontainingfluxesfortheICP-MSuser.• Lithiumcanbeobtainedinpureformasthecarbonateandisrelativelyinexpensive.• Li2CO3 meltsatrelativelylowtemperatures(relativetothepopularsodium

carbonatefusion).• Youcanusegraphitecrucibles,whicharebothcleanandinexpensive(Ipreferglassy

carbonwhichismoreexpensive,butverycleanandnotnearlyasmessy).• Li2CO3 isabasicfusionthatwillattackmanyoftherefractorymetaloxidesand

sampletypesattackedbyNa2CO3.


Fusion— Li2CO3(cont.)

• BasicSamplePreparationTechniqueswerediscussednamelyashing,aciddigestionandfusion.

• Therearemanymodificationtotheaboveapproachesreported.

• Microwaveinstrumentationiscommerciallyavailableforashing,aciddigestionandfusion.

• Microwavecapabilitygivestheanalystadecidedadvantageinspeedandconvenienceaswellasanadvantageinavoidingenvironmentalcontaminationsincemanypreparationareperformedinclosedoranenvironmentallyprotectedapparatus.

• Thematerialdiscussedisintendedtogivethetraceanalystandintroductiontosamplepreparationandshouldbefollowedbyliteratureresearchwhendevelopingorestablishingcapabilitiesnewtoyourlaboratory.

• Theonlybadquestionistheonenotasked.

SamplePreparationBasicsSummary—

• Customers can visit our website’s Tech Center, which includes:– Interactive Periodic Table– Sample Preparation Guide– Trace Analysis Guide– ICP Operations Guide– Expert Advice– And much, much more.

Technical Support – Available to EveryoneOnline Resources at inorganicventures.com

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