膜技术4Word文档下载推荐.docx
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Received11September2002;
receivedinrevisedform22April2003;
accepted22June2003
Abstract
Ana-spectrometricmethodfortherapiddeterminationofradiumisotopes(223Ra,224Raand226Ra)inenvironmentalsamplesispresented.UsingEmporeTMRadiumRadDiskscompleteseparationofthetargetradionuclidesisachieved.ThehighselectivityoftheseRadDisksallowsthestraightforwarduseof225Raasyieldtracer.Chemicalyieldisupto9279%.Thechemicalprocedurecanbeaccomplishedwithin5h.a-Sourcesshowenergyresolutionintherangeoftypically26–40keV(FWHM).Despiteminimalthicknessofthesourcesnosignificantradon(Rn)lossescanbeobserved.
r2003ElsevierLtd.Allrightsreserved.
Keywords:
Radiumisotopes;
Membranetechnology;
Alphaspectrometry;
Electrodeposition;
Radondiffusion
1.Introduction
Theuseofion-selectivemembranetechnology(Gokenetal.,1999)combinestheeasyhandlingofcolumnchromatographywiththehighselectivityandrapidextractionkineticsofsolvent-extractionchromatogra-phy.Previousstudies(Smithetal.,1997;
SeelyandOsterheim,1998)usedEmporeTMRadiumRadDiskswithhighenrichmentfactorsforRa.However,noneofthesemethodsuseinternalstandardstocheckthe
chemicalyieldofthewholechemicalprocedurepriortoradiometricormass-spectrometric(JoannonandPin,
2001)measurement.
Furthermore,ifinadditionto226Ra(uranium–
radium-series)also228Ra(thorium-series)hastobedeterminedtheappliedmethodsrequiretime-consumingprocedureslikeprecipitation,centrifugation,filtration
andredissolutioninordertoobtaintherequestedpurity
*Correspondingauthor.Tel./fax:
+49-431-6001-407.
E-mailaddress:
spurkl@geomar.de,stefan.purkl@gmx.de
(S.Purkl).
forb-spectrometry.Improvedbackgroundcharacteris-ticsofliquidscintillationcounting(LSC)(Scho.nhoferandWallner,2001)allowdeterminationof228RaeveninmBqquantities.Themethodmentionedisveryrapidandisintendedforsurveillanceofdrinkingwater.Nevertheless,poorenergyresolutionofLSCresultsinextensivepeakoverlappinginthea-andb-windowsand
thusmakingidentificationofinterferingradionuclides(224Raand210Pb)remaininginthepreparedsourcedifficult.
Incontrast,high-resolutiona-spectrometryallowssimultaneousdeterminationofalla-emittingRaiso-topes,includingshort-livedisotopes223Ra(actinium-series)and224Ra(thorium-series)onasinglea-source.(Note:
thedeterminationof228Raisalsopossibleonthisa-sourcewaitingseveralmonthsfortheingrowthofthea-emittingdaughter228Th.)Theselectivityofthenewanalyticalmethodpresentedhereallowsstraightforwarduseof225Ra(neptunium-series)asaninternalstandardandfurthermorecombineslowdetectionlimit(ontheorderofmBq)ofhigh-resolutiona-spectrometrywitheasyhandling,highselectivityandtheadvantageof
0969-8043/$-seefrontmatterr2003ElsevierLtd.Allrightsreserved.doi:
10.1016/S0969-8043(03)00172-6
246
S.Purkl,A.Eisenhauer/AppliedRadiationandIsotopes59(2003)245–254
extractionkineticsofsolid-phaseextractiondiscs,makingtheprocedurerapidandsensitive.
2.Chemicalprocedures
2.1.Flowdiagramforsamplepreparation
Fig.1depictsinaflowdiagramthechronologicalsequenceoftheindividualstepsinvolvedinthepreparationofa-countingsamples.
2.2.Procedureforion-selectiveextractionofRaviamembranetechnology
2.2.1.Preparationofaqueoussamples
Ifnecessary,thecollectedaqueoussamplesareprefilteredthrougha0.45mmfilter.Subsequenttoexactmeasurementofthesamplevolume,whichshouldcontainananalyteactivityofabout2–500mBq,concentratedHNO3isaddedtoraisetheHNO3
concentrationto2M.Theamountof229Th/225Ra(in
equilibrium)traceractivitiesaddedshouldcorrespondtotheanalyteactivitylevelsexpectedinthesamplevolumes.Thesamplepreparedisstirredvigorouslyforseveraltimestoassureawell-equilibrateddistributionoftraceractivities.
SelectiveExtractionRadiumretainedonRadiumRadDisksThoriumpassesthroughthemembrane
Column-Chromatography
Purifyingsolutionusinglow-pressurecolumns
α-SourcePreparation
Electrodepositioninethanolicsolution
α-Counting
Fig.1.Flowchartillustratingthesample-preparationproce-dureforthesimultaneousdeterminationofnaturallyoccurringRaisotopesandtheinternalyieldtracer225Raonasinglea-source.
2.2.2.RapidandselectiveRaenrichmentusingmembranetechnology
PriortothesampleprocessingtheRadiumRadDiskisconditionedusing20mlof2MHNO3underaslightunderpressure.Thesampleisextractedina47mmdiametervacuumfilterapparatus(Roth,Karlsruhe)adjustingtheflowrateto50ml/min.Afterwardsthemembraneiswashedthreetimeswith20mlof2MHNO3toremoveanyremnantoftheprocessedsolution.Likethetargetradioisotopestheaddedinternalyieldtracer225Raisretainedonthemembrane,whereas229Thpassesthrough.Timet0oftheRaextractionisnoted.Forsamplesizesexceeding1l,bothstarttBandendpointtEarenoted.Thediscisnotallowedtogodry.
2.2.3.QuantitativeRaelution
ElutionoftheRa-loadedmembraneisaccomplishedusing3mlof0.25MEDTA/1.7Mammoniumacetatein
6MNH4OHwhichisdirectlyaddedontotheextractivedisc,followedbyawashof16ml0.01MEDTApreviouslyadjustedtoaboutpH10usingNH4OHaswell.TheEDTAwashingsolutionisaddedcontinuouslyusinggravityfloworifnecessarybyapplyingaslightunderpressuretoensureaflowrateof1ml/min.Theobtainedeluatesarecombinedandsemi-concentratedHNO3isaddedtoachieveapHof4.5.Thesamplevolumenowcorrespondstoabout22ml.
2.3.Low-pressurecolumn-chromatography
2.3.1.Conditioning
Alow-pressurecation-exchangecolumn(Bio-RadAG50W-X12,200–400mesh,7mmindiameter)ispreparedandconvertedintheammoniumformbywashingwith15ml1.5Mammoniumacetate,followedby15ml0.25MammoniumacetatesolutionpreviouslyadjustedtopH=4.5(HancockandMartin,1991).Resultingcolumnheightis75mm.Forthepurposesofarapidseparationgravityflowisnotrecommended.Applyingaslightpressuretherequiredstableflowrateof1.0–1.2ml/mincouldbeensuredduringthewholeseparationprocedure.
2.3.2.EDTAremovalandingrowthofactinium(Ac)
TheeluateobtainedfromtheRaextraction(2.2.3)ispassedthroughthecolumnandRaisboundstronglyontothetopofthecolumn.Bywashingwith50ml1.5Mammoniumacetatein0.1MHNO3,EDTAiseluted,avoidingformationofprecipitatesandfilteringpro-cessesafterwards.Residualactinidesarecomplexedandlikewiseeluted.Afterammoniumacetatewash225Acstartstobuild-up.Thetimet1oftheendofEDTAremovalhastobenoted.NotenowthesampleshavetobeprocessedwithoutdelaytoavoiderraticresultsassociatedwiththedifferentbehaviourofRaandAcduringtheplatingprocedure.
S.Purkl,A.Eisenhauer/AppliedRadiationandIsotopes59(2003)245–254247
2.3.3.SeparationofBaandelutionofRa
About54mlof2.5MHClisusedtoremovebariumfromthecolumn.Thenbyaddinganother25mlof6MHNO3theRafractionretainediselutedtogetherwiththesmallfractionofAcalreadybuiltup.TheseparationofBafromRahastobeverifiedbecauseitiscrucialforlaterelectroplating.ThesolutionisevaporatedinaPFA(PerfluoralkoxyPTFE)beakertoremoveHNO3althoughcompleteevaporationtodrynesshastobeavoided.
2.4.Electrodepositionfromethanolicsolution
Aftertheseparationfromcomplexingagentsandotherinterferingelementstheanalyteiselectrodepositedontoapolishedstainless-steeldiscfora-spectrometricassay.Priortoelectrodeposition,planchetteshavetobecleanedwithacetonetoremoveanyoilysubstancetoensurethattheanalytesolutionisingoodcontactwithitalloveritssurface.
Theevaporatedresidueisdissolvedin0.5ml0.1MHNO3andthebeakerissuccessivelywashedwith1ml
0.05MHCl(HancockandMartin,1991).Combined
solutionsaretransferredwith9mlethanolintoanelectrodepositioncell.Distancebetweentheelectrodesisadjustedto7mminordertoattainahomogeneouselectricalfield.Theshapeandmaterialoftheanodearecrucialandconsistofaplanesurfacewithlaser-weldedplatinum(Pt)-rings.TheuseofPtassuresthatitdoesnotappreciablycorrodeduringtheelectrodeposition.However,asmallamountofPtdissolves(FerreroCalabuigetal.,1998)andservesasacarrier(Weber
etal.,1999).Electroplatingisperformedatacurrentof
120mAandavoltageofabout90–100Vfor1h.About
1minbeforetheendofelectrodeposition1mlNH4OHisaddedtothesolutiontopreventredissolutionofRaoncetheelectricalcurrentisturnedoff.
Afterdepositionthedisciscautiouslyrinsedwitheitherethanoloracetone.Thebrownishfilmappearingaftervolatilis
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