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SamplePreparationBasics
SamplePreparationBasics
Discussiontopicswillinclude:
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DefinitionsandObjectivesofaSamplePreparationforICPmeasurement
ChemicalCompatibility
SelectingaSamplePreparationApproach
ContainerMaterialProperties
ClassicSamplePreparationChemistry— ashing,aciddigestion,fusion
SamplePreparationBasics
Definitions
SamplePreparation(forICPapplicationsforthepurposesofthispresentationisdefinedas)
theconversionofasolidorliquidintoaliquidthatissuitableforICPMeasurements
withapneumatic nebulization(concentricorcross-flow)/spraychamber
(ScottorCyclonic)introductionsystem.
SampleSolution– Aliquidformofthesample.Thepreferenceishavingtheanalytes
inanionicsolvatedformorobservingthattheanalyteswillpassthrua0.3micron
filter.Thismayormaynotinvolveachemicalattack.
StandardSolution– Aliquidformofthecalibrationandqualitycontrolstandards
MatrixMatching– TheabilitytomatchtheSampleSolutiontotheStandardSolution
suchthatallnebulizerandplasmarelatedeffectsareeliminatedorcanbecorrected.
SamplePreparationBasics— Objectives
SamplePreparation(forICPapplicationsforthepurposesofthispresentationisdefinedas)
theconversionofasolidorliquidintoaliquidthatissuitableforICPMeasurements
withapneumatic nebulization(concentricorcross-flow)/spraychamber
(ScottorCyclonic)introductionsystem.
Objectives
•Conversionofsampletoasolution— samplewillpassthrua0.3micronfilter
•Eliminatesamplelosses– volatilization,adsorption,precipitation,mechanicalloss
•Eliminatecontamination– fromreagents,apparatus,environment,analyst,
container
SamplePreparationBasics— Objectives
SamplePreparation(forICPapplicationsforthepurposesofthispresentationisdefinedas)
theconversionofasolidorliquidintoaliquidthatissuitableforICPMeasurements
withapneumatic nebulization(concentricorcross-flow)/spraychamber
(ScottorCyclonic)introductionsystem.
ObjectivesContinued
• Eliminateoraccountfornebulizerrelatederrors
• Eliminateoraccountforplasmarelatederrors
• Theoverallobjectiveiseliminationorminimizationofsystematicerrorswhile
convertingthesampletoaliquidformthatiscompatible withcommonICP
introductionsystems,otherelements,apparatus,etc.
ElementandMatrixCompatibility
• Compatibilitybetweenelementsandthematrixwhenmakingblendsof
elementsi.e.eliminatingprecipitation,adsorption,volatility
• Compatibilitywiththeintroductionsystem
• Compatibilitywithlaboratoryapparatususedinpreparationandholding
ofsampleandfinalsamplesolution
• Compatibilitywiththeanalyst— safety,contamination
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 butavoid
HF 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
FactorsThatInfluenceMEBPreparation
• MicrowavedigestionmethodologystronglyinfluencesMEBmatriceswhere
HNO3 ,HCl,andHFaremostcommon
• The#1matrixacidisHNO3 withHCl being2nd
• HF istypicallyallowedandisalwaysmixedwithHNO3 orHCl at
low%totrace(<0.1%)levels
• H2SO4 isstillusedbutislesscommon
• Analystsrequireone workingsolution— multipleconcentratesare
allowedonlyiftheycanbemixedatworkinglevels
• Solutionstabilityisamust
ElementandMatrixCompatibility— NitricAcid
F denotesthattheelementcanbedilutedinHNO3 ifcomplexedwithF-.
Cl denotesthattheelementcanbedilutedinHNO3ifcomplexedwithCl-.
HF denotesthattheelementshouldhaveexcessHFpresentwhendilutedwithHNO3.
T denotesthatthetartaricacidcomplexcanbedilutedinHNO3.
1.
2.
3.
4.
5.
6.
OsshouldneverbemixedwithHNO3 duetotheformationoftheveryvolatileOsO4.
ClisoxidizedtomolecularCl2 whichisvolatileandadsorbsonplastic.
BrandIareoxidizedtomolecularBr2 andI2 whichadsorbontoplastic
DilutionsofHgandAuinHNO3 below100ppmshouldbestoredinborosilicateglassduetoHg+2 adsorptiononplastic.
Notsolubleaboveconcentrationsof1000µg/mL.
TracelevelsofHClorCl- willformAgC,whichwillphotoreducetoAg0.
ElementandMatrixCompatibility— HCl
F denotesthattheelementismorestabletohydrolysisifcomplexedwithF.
1.
Concentrated(35%)HClwillkeepupto100µg/mLofAg+ insolutionastheAg(Cl)X-(X-1) complex.Formoredilutesolutions,theHClcanbeloweredsuchthat
10%HClwillkeepupto10µg/mLAginsolution.
NOTE: TheAg(Cl)X-(X-1) complexisphotosensitive andwillreducetoAg0 whenexposedtolight.HNO3 solutionsofAg+ arenotphotosensitive.
2.
Parts-per-billion(ppb)dilutionsofHg+2 inHClaremorestabletoadsorptiononthecontainerwallsthanaredilutionsinHNO3.
SelectingaSamplePreparationApproach
TheChecklist
Thefollowingchecklistshouldbeconsideredpriortoselectingamethod:
ü Theidentityoftheanalytesandpotentialchemicalforms.
ü Theconcentrationrange(s)oftheanalyte(s)andthedetection
limitrequirement(s).
ü Thechemicalandphysicalcompositionofthesamplematrix.
ü Theavailabilityofapparatusandequipment.
ü Thesamplesizethatisavailableorrequired.
ü Thepotentialforcontaminationduringsomepartofthesample
preparationprocess.
SelectingaSamplePreparationApproach
FinalApproach
• Theanalystisnowinapositiontoselectthepreparationtechnique.
• Thisinvolveschoosingthemodeofattack(aciddigestion,ashing,fusion).
• Thespecificchemicalreagentsandthecontainer(s)materialsneededto
carryoutthepreparation.
• Theanalystmustkeepinmindthatcontaminationissues,plusany
difficultieswithinthefinalsamplesolutionmatrix,willhaveanimpact
upontheICP-OESand/orICP-MSmeasurementtechniques
SelectingaSamplePreparationApproach
HNO3-HF / HCl / HF
All mineral acids OK
Li
Na,
cntmnt rsk
K
Rb
Cs
B
cntmnt rsk
P
As
S
Se
V
Cr
Mn
Fe
cntmnt rsk
Co
Ni
Cu
Zn
cntmnt rsk
Ga
In
Re
U
Al
cntmnt rsk
Cd
cntmnt rsk
Tl
U
Bi
HNO3 / HCl
mDL
avoid HF & SO4=
Th
Ge
Mg cntmnt rsk
Ca cntmnt rsk
Sr
Sc
Y
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
HNO3-HF / HCl
mDL
HF suggested
Te
Sb
Mo
Ti
Zr
Sn
HNO3-HF
HF a must
Si cntmnt rsk
Ge
Hf
Nb
Ta
W
White lettering is volatility risk
mDL = minimum Detection Limit
HNO3 - HCl
mDL
mDL
no SO4
Ba
Pb cntmnt rsk
Hg
HNO3-HF
no HCl
Ag
HNO3-HF / HCl
mDL
mDL
be Alert to reduction
Rh
Pd
Pt
Ir
Au
Ru
HCl
avoid HNO3
Os loss as OsO4
mDL
mDL
SelectingaSamplePreparationApproach
Organic
Inorganic
%Ash
Fnctl
grps
grams
Matrix/Description
Pharmacuetical
Transportation
Environmental
Metallurgical
Construction
Agricultural
Automotive
Petroleum
Cosmetic
Chemical
Medical
Energy
Mining
Textile
Food
A.Ash
D.AcidDig.
F.Fusion
Grinding
Ashingcanbecombinedwith
aciddigestionorfusion.
Sampling
Volatility
Contamination
Chemical
Restrictions
HF
HNO3
HCl
SO4
CRO4
Misc
ContainerMaterialProperties— Materials
Laboratoriesusemanydifferentcontainermaterialsforhandlingsamplesduring
samplepreparation.Somematerialsaremoreadvantageoustousethanothers.
Herewe'lllookatthepropertiesof:
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BorosilicateGlass
Porcelain
Quartz
Platinum,5%Au/Pt
Graphite,GlassyCarbon
PlasticsincludingPFAandPTFETeflon
ContainerMaterialProperties— BorosilicateGlass
• BorosilicateglassisusedextensivelyforaciddigestionsinvolvingHNO3,HCl,
H2SO4.
• Shouldnot beusedforashing orfusions andattemperatures>500ºC.
• Itisresistanttomostacids,butshouldnot beusedwithHF orboilingH3PO4.
• Asageneralrulealkalinesolutionsshouldnotbeheatedorstoredin
borosilicateglass.
• 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,donotuse
borosilicateglass.
ContainerMaterialProperties— Porcelain
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Porcelainisapopularmaterialused forashing purposes.
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Affordable
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PorcelaincontainsNa,K,Al,andSiinincreasingconcentration.
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Itistypicallycoatedwithaglazewhichisabout70%SiO2,withroughlyequal
amountsoftheoxidesofAlandCa,andlesseramountsofNaandK.
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Attackwilloccurifthesamplecontainsevenminoramountsofthealkalimetals.
Thisismadeevidentbyadullinginthenormallyshinysurface.
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Ifalkalisarepresent,thenthesampleistypicallytreatedwithconc.H2SO4priortoashing.
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Thefollowingshouldnot beheatedinporcelain:HF;boilingH3PO4;andtheoxides,
hydroxides,orcarbonatesofthealkalioralkalineearthelements.
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Themajoradvantageorporcelainoverglassisthatitcanbeheatedupto1100°C
ContainerMaterialProperties— Quartz
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Therearetwotypesofquartz— opaqueandtransparent
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Opaquequartzhasthehighesttraceelementconcentration
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Transparentquartzcomesinfourdifferentvarieties
• TypesI&IIaremadefromnaturallyoccurringquartzcrystalsorsands.TypeIis
createdbyelectricmeltingandtypeIIbyflamemelting.TypeIIhasslightlyless
impuritiesthantypeI(someimpuritiesarevolatilizedbytheflame).
• TypeIIIquartzismadesyntheticallybyvaporphasehydrolysisofpuresilicon
compoundssuchasSiCl4.Thistypeofquartzismorepurethanthenaturalquartz,
withtheexceptionofCl.
• TypeIVquartzissyntheticallymadefromSiCl4 usingaprocessinvolvingelectrical
fusionoftheoxidizedstaringmaterial.ItisaspureastypeIII,withrespecttotrace
metalcontent,andcontainsfarmoreCl- whichis~50ppm.
ContainerMaterialProperties— Quartz(cont.)
• Usethetransparentquartzwheneverpossible.
• Quartzistypically99.8+%SiO2.
• ItisattackedbyHF,boilingH3PO4,andthealkaliandalkalineearthoxides,
hydroxides,andcarbonates.
• Itcanbeheatedto1100°C.
• Itsmainadvantageoverthatofporcelainisthatmajorcontaminationoccurs
fromonlySi — however,thiscontaminationcanbesignificant.
ContainerMaterialProperties— Platinum
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Itisresistanttoattackbymostacidsandreagents.AvoidconcentratedH3PO4 athigh
temperatures,HCl+HNO3 mixturesandfusions usingLi2CO3,Na2O2,orthealkalihydroxides.
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Itheatsupandcoolsdownrapidly,makingitexcellentfor%ashdeterminations wherethe
%ashisatlowlevels.
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Fusions usingNa2CO3 arecommoninadditiontofusionsusingthealkaliborates,fluorides,
nitrates,andbisulfates.Avoid heatingatprolongedtemperaturesinexcessof1100°C
(M.P.=1772°C).
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Platinumcanbedestroyedbyheatingwithmetalswithwhichitcanalloy.Avoidhigh
temperatureheatingwithsamplescontainingsignificantlevelsofanymetalthatmaybeinor
reducedtothemetallicstateduringtheheatingprocess.(Allpreciousmetals,Cu,Hg,Sn.)
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Platinumisknowntocontaintraceamountsoftheotherpreciousmetalsandshouldnotbe
usedfortheirpreparation.
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AvoidashingsamplescontainingPinanyform,includingthephosphates.
ContainerMaterialProperties— Graphite
• Graphiteisveryinexpensiveandrelativelyclean,butverymessytoworkwith.
• ItisaninexpensivewaytoperformLi2CO3 fusionswherethecrucibleslowly
oxidizesawayoverthecourseof7–10fusions.
• Itispopularbecauseitdoesnotwetbysomemeltswhichcanbepoured
outquantitatively.
• Lossesduetotheporosityofgraphiteshouldexcludeitsuseforashing
samplescontainingtracemetals.
• Graphite'smainadvantagetothetraceanalystisbeingamaterialthatcan
withstandfusionsthatmightdestroyplatinum.
• OurchemistsusegraphiteforperformingLi2CO3fusions.
ContainerMaterialProperties— Plastics
PhysicalPropertiesofCommonPlastics:
FEP (FLUORINATEDETHYLENEPROPYLENE)
PFA (PERFLUOROALKOXY)
FLEP (FLUORINATEDHIGH-DENSITYPOLYETHLYENE)
PMP (POLYMETHYLPENTENE)
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)
ContainerMaterialProperties— CommonUses
• 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/sample
solutions(LDPEisbyfarthebest)
Contamination
SeetheTraceMetalsAnalysisguideinthe‘TechCenter’onthe
InorganicVentureswebsitefor:
• EnvironmentalContamination(Chapter8)
• ContaminationfromReagents(Chapter9)
• ContaminationfromtheAnalystandApparatus(Chapter10)
SamplePreparationBasics
Ashing– TechnicalAdvantages
• TheabilitytodecomposelargesamplesizesachievinglowDLs.
• Lowreagentblanks- theneedforlittleornoreagents.
• Thetechniqueisrelativelysafe.
• Elapsedtimesarehoursbutactualanalystattentionrequiredisminutes.
• Theabilitytopreparesamplescontainingvolatilecombustionelements
suchassulfur,fluorineandchlorine(theSchönigeroxygenflaskcombustion
techniqueisverypopularinthiscaseinadditiontolowtemperatureplasma
ashing).
• Usingashingaidsveryfewelementsarelost(S,Se,andHgareproblematic).
• Thetechniquelendsitselftomassproduction.
SamplePreparationBasics
Ashing— DryAshing
• DryAshingisusuallyperformedbyplacingthesampleinanopeninertvesseland
destroyingthecombustible(organic)portionofthesamplebythermal
decompositionusingamufflefurnace.
• Typicalashingtemperaturesare450to550°C.
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Charringonahotplateortheuseofaprogrammablefurnaceisadvantageous.
Magnesiumnitrateiscommonlyusedasanashingaid.
Forlargersamplesizescharringthesamplepriortomufflingispreferred.
Charringcanbeaccomplishedusinganopenflame.
Ignitionandburningofthesamplecanbeveryhelpful.Thisiswidelyusedinthe
polymerandpetroleumindustries.
• Porcelainandplatinumaremostpopularcruciblematerialsfollowedbyfused
silicaandquartz.
SamplePreparationBasics
Ashing– DryAshingwithH2SO4 asanAshingAid
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TheadditionofsmallquantitiesofH2SO4 or‘sulfatedashing’involvestreatmentofthe
sampleaftercharringwithsulfuricacid.
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Thechariswettedusingtheminimumamountofsulfuricacidandthenbroughttodryness.
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Beforeplacinginamufflefurnace,thecharshouldbeheatedto>400˚C.
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DonotsulfatesamplescontainingBaorPb.
Sulfationcanpreventvolatilizationlossofvolatilechlorides,preventattackofsilicaand
aluminabasedcrucibles,preventformationofalightfriableashandpreventformationof
somerefractoryoxidesmakingthefinalasheasiertodissolve.
Attempttokeeptheashingtemperaturebetween450and500˚C.
Sulfatedashesgenerallydiscouragetheformationofrefractoryoxidesduringmuffling
makingdissolutionseasier.
SamplePreparationBasics
Ashing– ‘Wet’AshingwithH2SO4 asanAshingAid
• TheadditionofmLquantitiesofH2SO4 or‘wetashing’involvestreatmentofthe
samplebeforecharringwithsulfuricacid.
• Charring/digestionisperformedusingeitheranopenflameorhotplate.
• Liquidsamplestendtofoam.Constantattentionfromtheanalystisadvisedand
thetreatmentprocessistediousandslow.Consequentlythisapproachisnot
popular.
• Aftertheexcesssulfuricacidisdrivenoff,thesampleismuffledasabove.
• Wetashingischosenoversulfatedashingtoavoidcrucibleattackwith‘caustic’
samplesoranalytelossthroughreactionwithcruciblecontainer.
• NotsuggestedforsamplescontainingBaorPb.
SamplePreparationBasics
Ashing– LowTemperatureAshing
• Low-temperatureAshinginvolvestreatmentofthesampleat~120°Cusing
activated(singletstate)oxygen.Lifetimeofexcitedoxygenis~1second.
(Oxygenispassedthroughahigh-frequencyelectricfieldof13.5MHz.)
• Used fortraceanalysisoforganicsamplessuchascoal.Lookforpossiblelosses
ofhalogens,SandHg.
• Oftenusedonsamplestoavoidanalytelossesofvolatileanalytes(As,Cd,Sb)
andvirtuallyeliminatesrxn.betweenresidueandashingcontainerreducing
contamination.
• Usedforthetraceanalysisofcoals.Samplesize isgreater thanforaciddigestion.
• Sampleisspreadoutoverashingcontainertospeedupprocess— is stillslow
requiring1–3daysdependinguponsamplesizeandorganicstructure.
SamplePreparationBasics
Ashing — ClosedContainerAshing
• ClosedSystemAshing involvesthermaldecompositioninoxygeninaclosed
systemsuchasaSchöniger flask.
• SampleiswrappedinpaperandheldinaPtbasket.Asolutionatthebottomof
theflaskisusedtoabsorbthecombustionproducts.
• ThistechniqueismostcommonlyusedforhalogensPandSincombustible
organicmatrices.Convenientforanalyzingcompoundsseparatedbypaper
chromatography.Usefulintheanalysisofradio-isotopesinbiologicalmaterials
(3H,14C,and35S).
• Flasksaremadeofborosilicateglassorpolypropyleneforfluorineanalysis.
• A1literSchöniger flaskholdsenoughoxygen tocombustupto150mgof
sample.
• Thistechniqueiswidelyusedinmicroelemental analysis.
SamplePreparationBasics
Ashing– PotentialProblems
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Possiblelossduetoretentiontotheashing container.
Possiblelossduetovolatilization.
Contaminationfromtheashing container.
Contaminationfromthemufflefurnace.
Physicallossof'lowdensity'asheswhenthemuffledoorisopened(aircurrents).
Difficultyindissolvingcertainmetaloxides.
Formationoftoxicgasesinpoorlyventilatedareas.(Notethatallcharring
shouldtakeplaceinahoodandthemufflefurnacemusthaveahoodcanopy
forproperventilation).
SamplePreparationBasics
Ashing— AvoidingProblems
• Ifthesampletypeisunknown(withrespecttothematrix)thenanEDXRFscan,
IRscan,andC,H,andNanalysiswillprovidesufficientinformationinmostcases
tomakeinformeddecisions.
• ProtectyourPt0 warebylookingforP(highlevelswillattackandattachtothePt0)
andelementsthatalloywithPt0 whichincludethepreciousmetals,Cu,andHg.
• Whenusing'silica'containingcrucibles(porcelain,Vycor,quartz,glass,andfused
silica)lookforelementsthatformbasicoxidessuchasthealkaliearthelements.
Naiscommonlyfoundandit'soxidewillform(unlessthecharissulfated)and
attackthesilica.
SamplePreparationBasics
Ashing– AvoidingProblems(cont.)
• Lookforvolatileelements(Cd,B,Hg,Pb,Se,Zn,As,Sn,Sb,S,andhalogens)
especiallyifmoderatetolargeamountsofForClarepresent.
• Siisacommonelementthatistypicallydeterminedbydissolutionofanash
performedinPt0.Methylsiliconesarewidelyusedandverycommon.IfSiis
presentasasiliconeoilthenitwillbepartiallylostasthehexamethycyclotrisiloxane
andthehexamethydisiloxane.
• Retentionandphysicallossofanalyte(s).TheuseofhighpurityMg(NO3)2 as
anashing aidwillhelppreventlossesof'lowdensity'ashes,andwillhelpin
preventingretentionlosses.
SamplePreparationBasics
Ashing— AvoidingProblems(cont.)
• Fordifficulttodissolveoxidesuseaslowanashingtemperatureaspossible
(400to550°Cmaximum)forsamplestobeanalyzedforTi,Zr,Nb,Hf,Ta,W,Ni,Co,Fe,
Cr,Sb,andMo.Pt0 isnotattackedbyHFwhichwilldissolveseveraloftheaboveoxides.
• Lossduetoreductiontothemetalcanoccur.Lookforeasilyreducedelementssuchas
Cuandthepreciousmetals.Usetheappropriatecruciblematerialtoallowforthe
necessarydissolutionreagentsforthemetal.Ptcruciblesshouldnotbeused.
• WhenusingPtrememberthatcertainelements‘alloy’withitincludingHg,Cuand
manyofthepreciousmetals.
• WhenusingPtrememberthatithasarathercomplexspectrumandmayinterferewith
ICP-OESmeasurement(somePtisalwayslostduringeachpreparation).
• Rememberthatatleasttraceamountsofyourcruciblematerialwillendupinyour
samplesolution.
SamplePreparationBasics
AcidDigestions
• Aciddigestionshavetheadvantageofretaining'volatile'analytes
(refluxcondenserisneededforsometraceelements).
• Aciddigestionshavethedisadvantageofbeingtediouswhenlargesample
sizesarerequired.
• Aciddigestionsareidealifthesamplesizeis<1gram.
• Nitricacid(HNO3)isusedinpracticallyeveryaciddigestionprocedureandis
commonlyusedincombinationwithotheracids.
• Nitricacidispopularbecauseofitschemicalcompatibility,oxidizingability,
availability,purity,andlowcost.
SamplePreparationBasics
AcidDigestions— InorganicSamples
• Nitricacid* isusedprimarilyinthepreparationofinorganicsampletypes.++
Itisaveryusefulcomponentinthedestructionoforganicsbutcannotby
itselfcompletelydecomposeorganicmatrices.
*AllreferencetoHNO3 willmean69%'concentrated'nitricacidunlessspecifiedotherwise.
++Theconventionalmeaningofinorganicisintendedalongwiththepresenceoflowmolecularweightwatersoluble
organiccmpds.andorganometalliccmpds.containingrelativelysmallmolecularweightorganiccomponents.
SamplePreparationBasics
AcidDigestions— InorganicSamples
• Dilute10–15%aqueousdilution— AlkaliandAlkalineearthoxides,lanthanide
oxides,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,Moandnumerousalloysandoxidemixturescontainingone
ormoreoftheseelements.
SamplePreparationBasics
AcidDigestions— InorganicSamples— EPAMethod3052
• Thismethodisapplicabletothemicrowaveassistedaciddigestionofsoils,ash,
sediments,sludgesandsiliceouswastes.
• TypicalAcidmixis9mLnitric+3mLHF.Sometimes2–4mLofHClisadded.
• Ithasbeenclaimedtobea‘UniversalMethod.’
• Typicallynomorethan0.5gramsofsampleisdigested.
• Themethodallowstheanalystthefreedomtovarytheratiosofacidsbutlimits
aredefinedbythemethod.
• Withsmallamountsoforganicstheuseof30%hydrogenperoxideisallowed
(0.1to2mL).
SamplePreparationBasics
AcidDigestions— InorganicSamples— EPAMethod3052
• Theadditionofwater(0to5mL)isallowedandmayimprovethesolubility
ofmineralsandpreventtemperaturespikesduetoexothermicreactions.
Mayalsoalterthereductionpotentialofthenitricacid.
• Thismethodisdesignedtoachievetotaldecompositioninsealedinert
polymericmicrowavedigestionvesselswherereactionsreachingtemperatures
of180˚Cin5minutesareallowedtoreactatthistemperaturefor9.5minutes.
• Themethodrequiresamicrowavesystemthatwillsenseandmaintainthe
temperaturebyadjustingthemicrowavefieldoutput.Temperaturefeedback
controlprovidestheprimarycontrolperformancemechanismforthemethod.
SamplePreparationBasics
AcidDigestions— InorganicSamples— NitricAcid(cont.)
• Intheexampleslistedabovenitricacidisactingasastrongacidwhere
inorganicoxidesarebroughtintosolution…
1. CaO+2H3O+ =Ca+2 +3H2O
• Andasanoxidizingagent/acidcombowherezerovalenceinorganicmetals
andnonmetalsareoxidizedandbroughtintosolution…
2. Fe0 +3H3O+ +3HNO3 (conc.)Fe+3 +3NO2(brown)+6H2O
3. 3Cu0 +6H3O+ +2HNO3 (dilute)2NO(clear)+3Cu+2 +10H2O
• Inaddition,nitricaciddoesnotformanyinsolublecompounds.
Thesamecannotbesaidforsulfuric,hydrochloric,hydrofluoric,phosphoric,
orperchloricacids.
SamplePreparationBasics
AcidDigestions— InorganicSamples— NitricAcid(cont.)
Nitricacidundergoesbothoneandthreeelectronchanges.Theoneelectronchange
isobservedwhenconcentrated.Incomparison,the3electronchangeisobserved
whendiluteinreaction(3).Thepresenceofbrownfumesisindicativeofreactions
goingby1electron.
4. H3O+ + HNO3 + e-1 = NO2 (brown) + 2H2O : Concentrated
5. 3H3O+ + HNO3 + 3e-1 = NO (clear) + 5H2O : Dilute
SamplePreparationBasics
AcidDigestions— InorganicSamples— NitricAcid(cont.)
• Nitrateisgenerallyconsideredtobea'poorligand'inthatitscoordination
abilityisnotenoughtokeephydrolysisfromoccurring.Thisstatementmaybe
contradictedincertaininorganictextbooks.
• Themostcommon‘goodligands’usedincombinationwithnitricacidare
HCl,HF,H3PO4 andtartaricacid(forSb).Ifnitricacidwasabetterligand,
theseadditionalacidswouldnotbeneeded.
• ConcentrationsofHNO3 between65%and69%areknownas‘concentrated;’
concentrationsgreaterthan69.2%areknownas‘fumingnitricacid.’
• 100%nitricacidislightandheatsensitiveandboilsat84°C.‘Concentrated’
nitricacidboilsasanazeotrope(withwaterat69.2%HNO3)atatemperature
of121.8°C.ThedistilledHNO3 (tracemetalsgrade)shouldbeatthe69.2%
concentrationlevel.
SamplePreparationBasics
AcidDigestions— InorganicSamples— NitricAcid(cont.)
• Checkwithyourmanufacturerofdoublydistillednitricacidtodetermineifthe
containerinwhichitispackagedisnitricacidleachedpriortouse.Inthecaseof
Tefloncontainers,thecontainermaterialisgenerallyassumedtobepure.
• PTFEandPFATefloncanbeheatedwithconcentratednitricacid,evenat
highpressuresorwithcombinationssuchasnitric+HCl,nitric+HF,and
nitric+H2O2.
• Nitricacidisnotastrongenoughoxidizingagentbyitselftoconvertorganic
moleculestoCO2 andH2O(completelyoxidize).
SamplePreparationBasics
AcidDigestions— OrganicSamples
• Thelackofnegativesideeffectsisunfortunatelylimitedtotheinorganicsideofthe
table.Theabilityofnitricacidtoreactwithalcoholsandaromaticringsforming
explosivecompounds(nitroglycerinandTNT,tonametwo)callsforcautionwhen
usingnitricacidaloneorincombinationwithotherreagentsinthedecomposition
oforganicmatrices.
• Ifyoursamplecontains-OHfunctionalityitisbesttopre-treatthesamplewith
concentratedsulfuricacid.Whenconcentrated,thesulfuricwillactasadehydrating
agent.— R-CH2-CH(OH)-R'+H2SO4
R-CH=CH-R'+H2O
• Idonotrecommendtheuseofnitricacidforthedigestionofhighly
aromaticsamples.
• NitricaciddoesnotbreakdownorganiccomponentstoCO2andH2O.
SamplePreparationBasics
AcidDigestions– OrganicSamples(cont.)
• Nitricacidisrarelyusedalone.
• Itisbestusedincombinationwithsulfuricand/orperchloric acidsfororganic
sampledigestion.
• Forsamplesthatarenothighlyaromaticand/orcontainahigh-OH
functionality,Iprefertousenitricacidfollowedbyperchloric acid.
• Theonlyelementthatmay belostfromanitric/perchloric digestionisHg.
• Careshouldbeexercisedandtheliteratureconsultedbeforeattemptingtouse
nitricacidincombinationwithotheracidsfororganicsampledigestions
SamplePreparationBasics
AcidDigestions— OrganicSamples— Nitric+PerchloricAcid
fordetailedproceduregotoInorganicventures.comandthengothechapter12ofthe
TraceMetalsGuide locatedthe‘TechCenter’
• Organicmatricesshouldalwaysbepre-treatedwithnitricacid(seeexceptionsabove).
• Perchloric acidshouldneverbeusedalone.
• Perchloric aciddigestionsshouldneverbeallowedtogotodryness.
• Hotperchloric acidshouldneverbeaddedtoanorganicmatrix.
• Samplesizesshouldneverexceed1gram(dryweightforbiologicals).
• Perchloric acidfumesshouldbenotbeallowedto‘gofree’unlessaperchloric acid
hoodisused.
• Unknownorganicmatricesshouldbeanalyzedbymolecularspectroscopytodetermine
primarystructurebeforeattemptingtheuseofeithernitricorperchloric acid.
SamplePreparationBasics
AcidDigestions– 98%H2SO4
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Dilutesulfurichasnooxidizingpowerbuthotandconcentratedis.
SulfuricacidisusedtooxidizeSbandalloysofAs,Sb,andSn.
Pb presentinalloysisoxidizedandprecipitatedasthesulfate.
Sulfuricacidwithcatalysts(CuSO4,orSeO2 orHgSO4)isbasisfortheKjeldahl digestion
method(determinationofNitrogen).
• Thedropwiseadditionof30%H2O2 tohotfumingsulfuricacidisusefulfororganic
matrices.HaveusedforBeingrease.
• TheadditionofnitricaciddissolvesalloysofMo,Zr,Sn,steels,carbides,oresof
molybdenum.Sulfides,andZnores.
• Theadditionofnitric+HCl willoxidizesteels.
• Theadditionofnitric+perchloric acidisverypowerfulforhardtooxidizeorganicsamples.
SamplePreparationBasics
Fusion— Fusionsareconsideredtobemoreofa‘lastresort’
bytraceanalysts:
• Theyareexpensiveandoftennotavailable(highpurityfluxes).
• Theyyieldhighsolidssolutionsthatcansaltoutinthenebulizerandcause
sensitivitylosswithICP-MS.
• Largedilutionsofthesamplearesometimesrequired.
• Contaminationofthesamplewiththecrucibleconstructionelementand
impuritiesmustbeconsidered.
• However— fusionisanessentialcapabilityforalaboratoryreceivingawide
varietyofsampletypes.
• Itisthebestapproachformanyinorganicrefractories.
SamplePreparationBasics
Fusion— Pyrosulfate(K2S2O7)orBisulfate(KHSO4)
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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
KHSO4 (MP197˚C)heatedformsK2S2O7(MP325˚C)whenheatedforms
K2SO4 (MP1069˚C)
SamplePreparationBasics
Fusion— Pyrosulfate(K2S2O7)orBisulfate(KHSO4)(cont.)
Heatuntilmeltisclear
• Ifaflameisnotavailablethenuseamuffleat500˚C.
• Swirlingmeltwhileheatingispreferred.
• Swirlmeltontowallsafterremovingheat.Meltwillcoolandcrackintosmall
piecesthatareeasiertodissolve.
• Dissolvemeltinaqueoussolutionscontainingappropriateacidorstabilizer.
SamplePreparationBasics
Fusion— Pyrosulfate(K2S2O7)orBisulfate(KHSO4)(cont.)
• Duringfusionlosses ofsulfur asH2S,fluoride asHF,carbonate asCO2,borate as
H3BO3,arsenite asH3AsO3,arsenate asH3AsO4,selenite asH2SeO3,andchloride
asHClwilloccur.
• IffluorideispresentsomeSiwillbelostandifCl,BrorIarepresentsome
Sb,Sn,Ge,andVwillbelost.
• Thisfusionisusedfordealingwithrefractoryoxidestypicallyformedwhen
ignition/ashingtemperaturesaretoohigh.TheoxidesofBeFe,Cr,Mo,Te,
Ti,Zr,Nb,andTa.
• Ihavefoundthismethodveryusefulfordealingwithbrookite (averyrefractory
formofTiO2).
SamplePreparationBasics
Fusion— LithiumBorate
• Flux— Li2B4O7(tetraborate);LiBO2 (metaborate);H3BO3 +Li2CO3 at1:1(matterof
preference— metaborateusedforchromite(Fe++Cr2O4).
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FusionsallowforthedeterminationofNaandK.
Crucible— Pt0;Pt0/Au0;occasionallyAu0 orgraphite.
Flux:Sampleratio— 10:1
Temperature— 950to1200˚C
MostPopularApplications— SiO2,Al2O3,alumino-silicates(variousminerals,
zeolites,etc.),chromite.
• Fusionanddissolutionoffuseate(typicallyin5%nitric/water)hasbeen
automatedusingcommerciallyavailable‘fluxers.’
SamplePreparationBasics
Fusion— NaOHorKOH
• Flux— NaOH(MP321˚C);KOH(MP404˚C)advantageoverthecarbonatefusion
asamuchlowertemperatureisrequired.
• Crucible— Ag0;Ni0 (alkalihydroxidesattackPt0)– MessywithlotsofNi
contamination
• Flux:Sampleratio— 20:1
• Timeis~30minutes
• Temperature— 500˚C(maxof600˚C/Ni0 and700˚C/Ag0).
Morecrucibleattackathighertemperatures.
• MostPopularApplications— Silicates(glass,porcelain,kaolinetc.)and
aluminosilicates
SamplePreparationBasics
Fusion— KOH+KNO3
• Flux— (7:1)KOH:KNO3
• Crucible— Ag0
• Flux:Sampleratio— 7:1
• Temperature— 650˚C(maxof700˚C/Ag0).
Morecrucibleattackathighertemperatures.
• MostPopularApplications— Ruthenium,Chromite,“Organics”
(“Organics”referstoorganiccontainingsamples.Boththesodiumand
potassiumhydroxide/nitratemixtureshavebeenusedforawidevariety
ofbiologicalmaterials,soils,coalandorganicsamples.)
SamplePreparationBasics
Fusion— Na2CO3
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Flux— Na2CO3; mp=853ºC
Crucible— Pt0
Flux:Sampleratio— 20:1
Temperatureoffurnace– 1000˚C;Time~20minutes
5–9spurityfluxisavailable
MostPopularApplications— Na2CO3 fusionsareverypopularinthe
decompositionofminerals,silicates,refractories,insolublemetalfluorides,etc.
forexample,brookiteTiO2 willnotbeattackedbyacids,andAl2O3 isvery
resistanttoacidattack.
• Ipersonallylikeitformanyorganicmatrixdecompositionswhereitis
addedtothesamplebeforeashing.
SamplePreparationBasics
Fusion— Li2CO3
• Flux— Li2CO3
• AdvantageislowertemperaturebutdisadvantageisthatPtcan’tbeused.
• Crucible— Graphite.
CAUTION — Li2CO3 willattackPtcrucibles.
• Flux:Sampleratio— 5:1
• Temperature— 700˚C
• MostPopularApplications— SimilartoNa2CO3
SamplePreparationBasics
Fusion— Li2CO3(cont.)
• Lithiumsaltsareverywaterandorganicsolventsoluble.
• LithiumhasrelativelyfewspectrallinesfortheICP-OESuser.
• Signalquenching(signalsuppressionbymatrix)isrelativelyless(duetolowmass)
thantheB,Na,orKcontainingfluxesfortheICP-MSuser.
• Lithiumcanbeobtainedinpureformasthecarbonateandisrelativelyinexpensive.
• Li2CO3 meltsatrelativelylowtemperatures(relativetothepopularsodium
carbonatefusion).
• Youcanusegraphitecrucibles,whicharebothcleanandinexpensive(Ipreferglassy
carbonwhichismoreexpensive,butverycleanandnotnearlyasmessy).
• Li2CO3 isabasicfusionthatwillattackmanyoftherefractorymetaloxidesand
sampletypesattackedbyNa2CO3.
SamplePreparationBasics
Summary—
• BasicSamplePreparationTechniqueswerediscussednamelyashing,aciddigestionand
fusion.
• Therearemanymodificationtotheaboveapproachesreported.
• Microwaveinstrumentationiscommerciallyavailableforashing,aciddigestionandfusion.
• Microwavecapabilitygivestheanalystadecidedadvantageinspeedandconvenience
aswellasanadvantageinavoidingenvironmentalcontaminationsincemanypreparation
areperformedinclosedoranenvironmentallyprotectedapparatus.
• Thematerialdiscussedisintendedtogivethetraceanalystandintroductiontosample
preparationandshouldbefollowedbyliteratureresearchwhendevelopingor
establishingcapabilitiesnewtoyourlaboratory.
• Theonlybadquestionistheonenotasked.
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