Electrospray-IonizationTime-of-FlightMassS pectrometry: pH-DependenceofPhosphomolybdateS pecies

C.ELLIOTTEASTERLY,DAVIDM.HERCULES, * andMARWANHOUALLA Departmentof Chemist ry, VanderbiltUniversit y, Box1822 ,StationB,Nashville,Tenness ee 37235(C .E.E.,D.M.H.); andUniversi te´deCa en,L aboratoire Catalyse et Spectrochimie,6 Boulevarddu M are´chalJu in,1405 0Caen,France (M .H.)

Electrosprayionizat ion(ESI) time-of-ig htm assspectrom etry has Solutionswere preparedat a concentrationo f2mg/mL been usedto investig atethe pH-dependenceofphosph omolybdates phosphomolybdatein w ater.ThepHwasadjustedby insolution from pH 5 1.7to 10.2 .Massspectra are reported for dropwiseadd itionofdilu team moniumhydroxide.Sam - 22 32 32 the Žrst timeforH 2Na2P2Mo5O23 , PMo9O31 , PMo10O 34 , and 32 pleswe re allowedtoeq uilibratefo r18handthepHwas PMo11O37 .The pHdependence ofphosp homolybdatespecies as measured byelectrosprayis comparedwith m easurements reported measuredju stp riortom assspe ctrometrican alysis.The forcom binedpotentio metry—31PNMR.The pHrangesofmostspe- pHwasdeterminedwitha FisherAccumetpHMeter9 10 cies are similar( 6 1pH unit)when m easured bythese twom ethods, usingaCorningG eneralPu rposeco mbinationprobe(cat- althoughsom edifferences are observed.Proton ationand solvati on alog#4 67569)calibratedusingFisher b uffer solutionspH ofphosph omolybdate species measured byE SIdiffer fromthose 5 2.00,5.00,7.00,and10.00.Thephosphomolybdicacid observed by 31PNMR.However, the core phosphomolybdatespecies solutionwastitrate dwithd ilutea mmoniumhydroxide are measured correctly byESI. andsevenaliqu otswe re takenfo ranalysisat pH va lues IndexH eadings: Electrospray;Phosphomolybdates; Speciation; of1.76,2.57,3.59,4.63,5.30,6.77,and10.22.Thein- MassSpectro metry. strumentationandm ethodfor masscalibr ationusedin thisstud ywere thesameasin an an alagousstudyo f phosphotungstates. 9 INTRODUCTION 1 Firststu diedb yBerzeliusin18 26, phosphomolybda- RESULTSANDDISCUSSION tesare u sedincataly sis 2 aswellas otherareas, 3 and may taketh eform oftheK egginan ion. 4 ThedeŽn itivew ork LowpH Region. Themasssp ectrumo fapH 5 1.76 onthespeciationofphosphomolybdatesin so lutionisa phosphomolybdateso lutionis sh owninFig. 1 .Clusters combinedE mf- 31PNMRstudyby Pette rsson,Andersson, correspondingtop hosphomolybdates,w hichc ontain12 ¨ 5 and Ohman. TheŽrst electrosprayio nization(ESI)phos- molybdenumatom s,pre dominatein th ism asssp ectrum phomolybdatem asssp ectra(M S)were reportedb yLe andare theonlyc lusterslab eledin Fig. 1 .Theo ther QuanTuoiandMullerusing a triplequ adrupoleinstru - clusters inthe m ass spectrumare either phosphomolyb- 32 22 ment;spec ieso bservedwere PMo 12O 40 , HPMo12O 40 , datesthatwillbe identiŽed insu bsequentsectionsormo- 2 and H 2PMo12O 40 ,witha mass spectralreso lutionof lybdatesp eciesiden tiŽed b yWalandaetal., 10 summa- 6 2200. Deery,Ho warth,andJenningsused a tandem rizedin Ta bleI. Eachspe ciesin Tab leI wasp reviously quadrupolem asssp ectrometerand p ublishedm assspec - identiŽed o rwasseen in the prese ntw orkb utisre lated trao btainedfrom apH9aqueousphosphomolybdateso- toa previouslyid entiŽed fra gmento rpolymer asin di- lution,whichsho wedth eprotonatedm olybdatean ion, cated.Then umber ‘‘1792’’ontheupperrig hth andcor- andspectrao fapH3aqueousphosphomolybdateso lu- nerofthemass spectruminFig .1isthetotaln umberof tion,whichsho wedPM o O 32 and HPMo O 22.7 12 40 12 40 countsreco rdedfo ritsba sep eakat ;607 Da from Severalph osphomolybdatesp ecieswere observedin 32 thecu rrentworku singESI-MSthatwe re notreported HPMo12O 40 .Theotherclustersin Fig.1thatco mefrom previously,includingH N a P M o O 2 2 , PM o O 3 2 , phosphomolybdatescontaining12molybdenumatoms 2 2 2 5 23 9 31 22 2 32 32 are H2PMo12O40 at ;m/z 913 and H 3PMo12O 40 at ;m/z PMo10O 34 , and PMo11O 37 .Time-of-ig ht(T OF)ESI- MSwasusedto p robethe aq ueoussolutionspeciationof 1825.Thefractionoftheto talin tensityd ueto 12 -Mo phosphomolybdateso lutionsovera pHrangefro m2to phosphomolybdatesis6 3%.Itis po ssibleto d etermine 10.A lthoughdifferencesin p rotonation,cationization, thech argeonclusters thatco ntainm olybdenumbymea- andhydrationwereo bserved,thec orestru ctureso fmost suringthespa cingbetweenthe in dividualisoto pepeaks. phosphomolybdatesremainedintact .Thepresentwork Theno rmalsp acingis 1 Dalton(Da) betweenadjace nt andstudieson iso polyoxometalates 8 andphosphotung- isotopepeaksfo rasinglyc hargedc luster,an dthespac ing states9 havede monstratedth atES I-TOF-MSisa viable willb e1/nDaformultiplycha rgedspecies.Th einsets techniquefo rstudyingequilibriain p olyelectrolyteso lu- inFig. 1 provideadetailedv iewoftheclustercentered tions. at60 7Dawhere a isthe o bservedm asssp ectrumand b isthecalculatedmasssp ectrum.There existsclea ragree- EXPERIMENTAL mentbetweenth ecalculatedando bservedmass spectra; Aldrichph osphomolybdicacid hy drate(22 ,185-6)w as theiso topedistributionrelativ eintensitiesare within dissolvedinHPLCgradewaterfrom FisherScientiŽc Co. 65%an dthem assag reementofeachin dividualisotope peakiswithin 60.05Da.Thedoublycharged 22 Received 16Septemb er 2000;accepted 2August20 01. H 2PMo12O40 cluster at ;m/z 913inFig. 1 agrees to *Authorto whom correspo ndence shouldbe sen t. within 60.05D aofitscalcul atedm asssp ectrumwith

0003-7028 /01/5512-1671$2.00/0 Volume 55, Number 12, 2001 q 2001 Society for Applied Spectroscopy APPLIEDSPECTROSCOPY 1671 TABLEI.Molybdatespecies observed inESI-M SatpH 5 4.5 and 6.0. PreviouslyidentiŽ ed Species m/z in Ref. 10

22 [Mo2O7] 151 yes 2 [HMoO4] 163 yes 22 [Mo3O10] 224 yes 22 [Mo4O13] 297 yes 2 [HMo2O7] 303 yes 2 1 4[HMoO4] 1 H2O 1 2H 336 22 [Mo5O16] 368 yes 22 [Mo4O16Na6] 1 2H2O 408 2 [HMo3O10] 449 yes 22 [Mo7O22] 510 yes 22 1 [Mo4O13] 1 H 593 22 2 1 [Mo8O25] 1 [HMoO4] 1 H2O 1 Na 694 42 1 [Mo15O47] 1 H2O 1 H 737 42 1 [Mo15O47] 1 6H2O 1 H 767 42 1 1 [Mo11O35] 1 2H2O 1 H 1 Na 839

monwhenu singelectronmultiplierdetectors. Theeffect couldalso re sultfrom differencesin p rotonefŽcien cies inth egasphase.Thesensitivityfa ctorforMo:Pislarg er thanth atfo rW:P,whichis 2 .5. 9 Intermediate pH Region. Figure 3shows theESI masssp ectrumo fanaq ueouspH 5 3.59phosphomolyb- dateso lutionwhichis repres entativeofspectrain th e intermediatep Hregion,pH ; 3 to pH ; 6. The base 32 peakinF ig.3at ;560Daisdu etoPM o 11O37 , i.e.,

FIG.1.Neg ative-ionelectrosp raymass spectrum ofphosphomolybdate at pH 5 1.76.N opeakswere observedabove m/z 2000nor was any- thingsigniŽ can tobservedbelow m/z 50.The insets sh owa detailed 32 view of HPMo12O40 ; (a)observed,( b)theoretical. relativeintensitiesw ithin 68%an dappears muchlik e previouslypu blishedsp ectra. 6 HighpH Region. Figure 2shows themassspec trum obtainedfro mapH 5 10.22phosphomolybdateso lution. Thebasep eakco mesfrom theprotonatedm olybdate 2 2 monomer,HMoO 4 at ;163 Da; H 2PO 4 at97 .0D arep- resentstheo ther majorpe ak.T hein setin F ig.2shows 2 thedetailedmass spectrumofHMoO 4 .Theobserved isotopedistributionisb aselineresolved,ha smassag ree- menttowithin 60.05Daofthetheoreticalvalue,andthe isotopedistributionag reestow ithin 62%re lativein ten- sity.T hespe ctrumresemblesth atp ublishedb yDeeryet al.,7 showingth attheprotonatedm onomerisobservedin ESI-MSanalysisw ithTO Fdetectionasw ellas w ith quadrupoled etection.Becausep rotonationofthemolyb- 22 4 datem onomer (MoO 4 ) with a pKa of 3.89 at basic pH isn otfeasiblein so lution,theESIprocessm ustbecaus- ingtheprotonadd ition;th isw illb ediscussedla ter. Therelativ esensitivityfa ctorfor molybdatean dphos- phatecan b ecalculatedbysummingtheintegratedare as 2 ofth epeaksinthe iso topicd istributionsofH MoO 4 2 (6843) and H 2PO 4 (93).Dividingbythem olera tios(12: 1Mo:Pasphosphomolybdicacid),the relati vesensitivity 2 2 ofM o(as HMoO 4 ) to P (as H 2PO 4 )is57 0:93;Mois FIG.2.Neg ative-ionelectrosp raymass spectrum ofphosphomolybdate 6.13tim esmore sensitivelyde tectedth anP .Thislev elof at pH 5 10.22.The on lysign iŽcant peakobser vedbelow m/z 50 was 2 differenceinsen sitivities,be tweensp eciescon taining OH 1 H2O at ;35D a, andnothin gwas observedabove m/z 2000. metalsan dthoseha vingonlyno nmetals,is no tuncom- (a)observed,( b)theoretical.

1672 Volume 55, Number 12, 2001 FIG.3.Negative -ionelectrosp raymass spectrum ofphosphomolybdate at pH 5 3.59.The only signiŽ cant peak observedbelow m/z 50 was 2 OH 1H2O at ;35Da, andnothingwas observedabove m/z 2000. The 32 insetssho wadetailedview of PMo 11O37 ; (a)observed,( b)theoretical.

H PMo O 32 less2 waterm olecules.A detailedcom- 4 11 39 FIG.4.Pho sphorus-containingspecies as afunctionof pH .( a) Mea- 32 parisonofth eclustercorrespondingtoPM o 11O 37 with suredby ESI-TO F-MS.( b)Frompoten tiometry- 31PNMRmeasure- thatcalcu latedfor thesa mecompositionissh owninin- ments basedon R ef. 5,co nstructedsim ilarlyto Fig. 4a. sets a and b ofFig.3 .Them ass agreementforthepeaks 32 from PMo11O 37 is within 60.1D aanditsiso topedis- tributionisw ithin 67%ofth erelativeintensityo fthe actionw asob servedb yWalandaeta l.w ithiso polymo- calculatedd istribution.Thefractionoftotalin tensitycor- lybdates.10 Theclusterat m/z 607.6in F ig.3hasbee n 32 32 respondingto1 1-Mophosphomolybdates(PM o 11O 37 ) identiŽed as H PMo 12O 40 (see Fig.1)andisa minor inFig .3is21 %.Thefra ctionofphosphomolybdatein - speciesat th ispH . tensity(notincludingisopolymolybdatein tensities)re- Speciation. Figure 4ash owsasemi-quantitativedia- 32 sultingfrom the11-Mocontainingspecies,PM o 11O 37 , gram depictingtherelativ eabundanceof e achp hospho- is 43%. ruscontainingspeciesas afunctionofpHcalculatedfro m There are fourotherphosphomolybdateclu stersin Fig. ESI-TOF-MSresults,co nstructedin an analogousfashion 3 at ;m/z 464,4 80,511,and608,andtheyare alliso- toFig .6inth erelatedphosphotungstatewo rk. 9 Figure 4 polymolybdatesp ecies,as liste dinTa bleI. Thecluste r compares thespeciationob servedb yTOF-ESIwithth at 2 2 5 at ;480DaisduetoH 2Na 2P 2 M o 5O 23 , i.e., expectedfro mPetterssonetal. Itw asassu medth atall 42 1 H 2P2Mo5O23 1 2Na ,andshowsmassagreementwithin phosphomolybdatespec iesh aveequald etectorresponse 60.05Da ofthe ory,a ndth epeaksinth eisotoped istri- factors. Phosphomolybdateswe re groupedaccordingto butionag ree within 69%relativ eintensity.Tw ospecies thenu mber ofmolybdenumatoms,a ndeac hseriesw as withn inemolybdenumatomcoresare observedin F ig. plottedv s.p Has‘ ‘relativea bundance,’’theratioo fa 32 3 at ;m/z 464an d511,co rrespondingtoPM o 9O 31 and singleserie stoth esumofall se ries. 32 PMo10O 34 ;eachh asan isoto picd istributionsimilarin It isd ifŽcult to discer ntheabundanceo findividual 32 shape to PMo11O 37 (inset a inFig .3).Th emassag ree- speciesby ex aminingindividualm asssp ectra.Th isis 32 32 ment of PMo9O 31 and PMo10O 34 is within 60.07 Da, whya graphical(F ig.4a) representationishelp ful,es- andtheiso topedistributionis w ithin 612%relati vein - peciallyforcomparisonw ithab undanced ataca lculated 32 tensityofthepredictedv alue.PMo 10O34 appears tore- usingadifferenttechnique(Fig.4b).Thegeneralsh apes sultfro magasp hasep olymerizationreactionth ateffec- ofthecur vesforthehomologousseriesa re similarb e- 32 tivelya ddedM oO 3 to PMo9O31 .Thisd ehydrationre- tweenF igs.4a an d4b.Forexample,th egeneralsh apes

APPLIEDSPECTROSCOPY 1673 ofthecurvesfor the12 -Mo,11-Mo,and9-Moseriesare Theth irdd ifferenceb etweenthe tw oresultsis tha t 2 verysimilarb etweenthetwodiagrams.Thestrikingdif- predominanceof H 2PO4 occurs at pH . 5.5 for ESI. ferenceis th atthe P 2Mo5 seriesd isappears aroundpH 5 Theform ofphosphateo bservedb yESI-MSwassin gly 2 9 5.5 and H2PO 4 predominatesat p H . 5.5ac cordingto charged,as for thep hosphotungstates. Phosphateis ex - (62 n)2 ESI-MS.Petterssonetal.p redictedth atH nP2Mo5O 23 pectedb yNMRtob efoundinbo thsing lya nddo ubly 5 2 1 predominancew ouldo ccurfrompH 5 6to7 andfree chargedanions, with a pKa of 7.2 for H2PO 4 . H 1 5 22 22 phosphatew ouldbe m ostab undantatpH . 7. Even so, HPO4 .Itis un derstandableth atH PO 4 isp rotonated theESI-MSandN MR/potentiometricresu ltssh owcon- inthe g asp hase,bu tthepredominanceo fphosphateis (62 n)2 siderableag reement,w hichis d iscussedb elowingre ater likelyd uetothefragmentationo fH nP2Mo5O23 in the detail. range of pH 5 5.5to 7 asd iscussedab ove. Theresu ltsfro mNMR/potentiometryandESI-MS Comparisono fESI-MSAnalysiso fPhosphomolyb- phosphomolybdatea nalysisd iffer ina tleastthre esignif- dates andPhosphotungstates. Oneofthedifferences icantways.First,m anyisopolymolybdatespec iesare ob- betweenE SI-MSandN MRresultscommontob othtun g- servedbyESI-MSthata re notpredictedby N MR.Sec- statesan dmolybdatesis th eexistenceo ftheprotonated (62 n)2 2 2 ond, the H nP 2Mo5O 23 series,predictedb yNMR/po- monomers (HWO 4 , HMoO4 )athigh(basic)pH.The tentiometrytop redominatebe tweenp H ; 6 and pH ; protonatedm onomer isn otex pectedto e xistat h ighpH

7,isnotobservedabovepH ; 5.5b ymasssp ectrometry. values;the p Ka ofth etungstatem onomeris3.5 fo r 2 2 1 22 4 7 Third,thepredominanceo fH 2PO 4 occurs at pH . 5.5 HWO 4 . H 1WO 4 . AccordingtoDe eryetal., pro- accordingtoE SI-MS,whereasit isexpectedby NM Rto tonationofthemonomer couldbe ex plainedin th efol- predominateon lyab ovep H 5 7. lowingway.W henth edryingagent(ex.curtaing aso f Thediffere nceinisop olymolybdatespe ciesprobably nitrogen)desolvatesthe an alyte,the vo lumeofeach relatesto co nditionsu sedfor theESI-MSstudy.In o rder dropletdecreases,inc reasinganalytec oncentrations.It is tosp rayaq ueousphosphomolybdateso lutions,thenozzle possibleth ate vaporationofN H 3 inth edesolvationp ro- potentialwas setat 6 200V,whichis slightlyh igherthan cesscou ldlea vepro tonsbehindinth eresultingd roplet, 1 1 thenozzlep otentialnormallyu sedan dtherefore may therebydecreasingthesolutionpH:NH 4 . NH 31H . havecausedso mefragmentation.Allso lventsystem s Dataw ereo btainedu singNaOHinsteado fNH 4OH to triedin pre liminaryexperiments(m ethanol–w ater,ace- adjusta phosphotungstateso lutionto p H ; 4. A phos- tonitrile–w ater,andpure water) showedfragmentation photungstateso lutionad justedto p HwithN aOHhadth e andso lvationanddidn otsprayread ily.In co ntrast,1:3 samepeaksinth emasssp ectrumason eadjustedto p H methanol–w aterw asu sedforphosphotungstateE SI-MS with NH4OH, at ;m/z 670,8 93,959,and1340.Because analysisbec auseit sp rayedread ilyan dproducedlittle themassspec trao fphosphotungstateso lutionswerein - fragmentation.B ecauseth ere wasnosprayingadvantage dependentofthebaseused to a djustp H,protonationo f 22 fromthemixedsolvents,w aterwaschosenas theso lvent WO 4 andother speciesco uldno tbecausedb yevapo- for comparisonofESI-MSwithaq ueousphosphomolyb- rationofammonia. dateN MRstudies.T herefore,sp eciesm aybe prese ntin Figure5 shows anE SImasssp ectrumofapH 5 5.3 phosphomolybdateso lutionsthatw ere notmeasuredb y phosphomolybdateso lution;itsba sep eakis at ;163 Da 2 onemethodortheother,assu pporteda lreadyb yPet- andis du etoH MoO 4 ;itsfra ctionoftotalin tensityis 5 2 terssonetal. 23%.Thepeakdu etoH MoO 4 probablyresu ltsfro m Theseco ndd ifferenceb etweenE SI-MSandNMR/po- dissociationofamolybdate–w atercomplexin the v acu- tentiometryresultsis tha tthediphosphoruspentamolyb- um.Normallyex pectedto b eobservedas a dianion,the dateo bservedbyESI-MSdoesn otpredominateas ex - molybdatem onomer apparentlygo esth rougha‘charge pected in pH 5 6to7 phosphomolybdatesolu tions.T he separation’rea ction, 11 assho wnbelow. form ofdiphosphorusthatwaso bservedby ES I-MSwas 22 2 2 (62 n)2 (62 n)2 (MO 4 · nH 2O) ® HMO 4 1 (n 2 1)H 2O(OH) H nP2Mo5O23 , where H nP2Mo18O 62 (a slow-form- 5 ing PMo9 dimer, log K 5 2.5) ise xpectedb yNMRbe- (where M 5 Mo, W) 5 (62 n)2 low pH 5 3. The lack of H nP2Mo5O 23 between pH 5 6to7 isth emostsign iŽcan tdifferencebetweenES I- Thebasisis th atth emolybdatem onomer isto osmallto MSandNMR/poteniometryresults.O nepossibilityis accommodatetw onegativechargesin th evacuumw ith- thatthe solu tionchemistrybetweenth etwostu diesm ay outsolvation,soch argeseparationisforcedby C oulom- 2 2 differ becauseth eNMR/potentiometricm easurements bicrep ulsion,producingHMoO 4 and an OH –water 2 usedsolu tionsh avingaconcentrationof2 40mM,while cluster.Apeakco rrespondingto (H 2O)OH is observed theESIspectraw ere obtainedfor8mMsolutions.Iso - at ;35Dainthe m asssp ectruminw hichthep rotonated polymolybdatesp olymerizeat pH 6 –7forconcentrations molybdatem onomer iso bservedas sho wnbyinset a in ato rabove0.1M , 12 soth erec ouldbe sim ilarconcentra- Fig.5.T hem ass spectrumofa1:3m ethanol–w ater tioneffectsfor phosphomolybdates.H igher concentra- ‘blank’atp H 5 6.5,as seen in in set b inFig. 5, showed 2 tionso fphosphomolybdatesw ere notan alyzedby E SI- nopeakdueto(H 2O)OH .Thiswouldb eexpected,g iven MSbecauseth eelectrospraya pparatuswouldb ecome that the OH2 concentrationin the so lventisb elow10 28 (62 n)2 clogged.An other possibilityis tha tH nP2Mo5O 23 Matthis pH .Onthebasiso fthepeakat 35 D aata pH 2 doesn otsurviveelectr osprayio nization,resultinginfo r- where nopeakis o bservedin th eblank,the(H 2O)OH 2 mationoffree phosphatean dthem olybdatem onomer. mustb eproducedb yreactionbetweenM oO 4 and H 2O, Thisap pears tob eapossibilitybe causeth esignalfro m supportingStewartan dHorlick’smechanism. 11 The phos- 2 H2PO 4 becomessign iŽcan tatab outp H 5 5.5,nearthe phomolybdatepe akin F ig.4at m/z 511.7co mesfro m (62 n)2 5 32 5 pHreg ionw hereH nP2Mo5O23 isexp ected. PMo10O 34 andhasafractionoftotalin tensityo f13%.

1674 Volume 55, Number 12, 2001 concentrationan dpHdependenceof the sp eciationof isopolytungstatesis limitedto Žvemajorspe cies,whereas nineexistfo risopolymolybdates. 12 ESI-MSanalysiso fbothp hosphooxometalatesystems agrees withN MRresultsto w ithin1 pHu nit,prov iding awayto q uicklyestim atethe m ajorspeciesin solu tion. Mass spectrafro mphosphotungstateso lutionsare more easilyin terpretedthanth osefro mphosphomolybdateso- lutions.Whilem assspe ctrao fphosphotungstatesolu tions showclustersre presentingcore speciesth oughttob ein solutionfromNMRstudies,m ass spectrao fphospho- 2 molybdatesare oftend ominatedby H MoO 4 and have manyisopolymolybdateclu stersn otexpectedfro mNMR studies. Ineac hphosphooxometalatesystem,theprotonated 2 2 monomer (HWO 4 and HMoO 4 )wasobservedrather thanth edianionspredictedb yNMR.Diphosphorusphos- photungstatesw ere notobservedbyeitherESI-MSor 31P NMRinm ethanol–w ater,whereasoneofthediphospho- rusphosphomolybdatesexpectedfrom NMR 5 was ob- servedby E SI-MS.Finally,in phosphotungstatean alysis, dehydrationwa sobserved,whereasbothdehydrationand cationizationwere observedinp hosphomolybdatean al- ysis.

CONCLUSION ESI-MShasb eenused suc cessfullyto ex aminethe speciationofphosphomolybdatesin aq ueoussolution. Theco re ofmostp hosphomolybdatesrem ainedintact eventho ughtherewere differencesinhy dration,proton- ation,andcationization.Themassspectraof FIG.5.Negative -ionelectrosp raymass spectrum ofphosphomolybdate 2 2 3 2 3 2 at pH 5 5.30.Nothin gwasobservedabove m/z 2000.Thein set labeled H 2 Na 2P 2M o 5O 23 , PM o 9O 3 1 , PM o 10 O 34 , and 32 a showsthe low mass regionof p hosphomolybdateat pH 5 5.30 and PMo11O 37 wererep ortedfo rtheŽrst time. inset b is froma1:3methano l–water ‘blank’(pH 5 6.5).N otethat the mass spectrometrysoftware usedd oes notacquire data below m/z 10. ACKNOWLEDGMENT This workwas supportedin part by the Nationa lScience Foundation underGrant C HE-9985864. Larger phosphooxometallatescanm ore readilyac com- modatem ultipleneg ativech arges,red ucingthenee dfor 1.J. J.Berzelius,Po gg.Ann . 6, 369 (1826). chargeseparation.Hence,1 1-and12-Wphosphotung- 2.M .T.P opeand A. Mu¨ller, Polyoxometales: FromPlatonic Solids statesan dphosphomolybdatesare o bservedm ultiply toAnti-Re troviralActivity (Kluwer AcademicPublishers, Dor- charged. drecht,Neth erlands,1994). 3.D .E.Katsoulis,Chem .Rev. 98, 359 (1998). Themostsign iŽcan tdifferenceb etweenE SI-TOF-MS 4.M .T.Pope, HeteropolyandIsopoly Oxometa lates (Springer,Berlin, analysiso fphosphomolybdatesan dphosphotungstatesis Germany,1983). thatp hosphotungstatesio nizew ithlittle frag mentation 5.L .Pettersson,I.Andersson,an dL.O ¨ hman,Ino rg.Chem. 25, 4726 andpolymerization.It is n otclea rwhythisis th ecase; (1986). 6.J. Le QuanTuoi and E .Muller,Rapid C ommun.M ass Spectrom. possiblyit is b ecausem olybdenumhasm ore readilyac - 8, 692 (1994). cessiblein termediateox idationstates( 13, 14, 15) and 7.M .J.Deery,O. W.Howarth ,andK. R.Jennings,J. Chem. Soc., ismoree asilyred ucibleth antungsten. 12 Theowofelec- DaltonTrans. 478 3(1997). tronsinn egative-ionelectrospraywill cau secath odic 8.C .S.Truebenbach, M.Houalla,and D. M.Hercules,J.Mass Spec- electrochemicalreacti onsatthespraytip ,consistentwith trom. 35, 1121(2000). 9.C .E.Easterly,D .M.Hercules,andD. M.Houalla,Appl.Spectros c. thepro ductionofreducedM ospecies.T hebondstrength 55, (2001). forMo–Ois5 60kJ/mol(134kcal/mol)an d672kJ/mol 10.D .K.Walanda,R. C.Burns,G .A.Lawrance, andE. I.vonNagy- (161kcal/mol)fo rW–O, 13 whichsu ggeststh atth eW–O Felsobuki,J. Chem. Soc.,Dalto nTrans.311 (1999 ). bondisstro nger andlesslik elyto be b rokenth anth e 11.I. I.Stewart andG .Horlick,TrendsAnal.Ch em. 15, 80 (1996). 12.C .F.Baes andR. E.Mesmer, TheHydrolys is ofCations (Kreiger Mo–Obond.Duringthee lectrosprayp rocess,th emolyb- PublishingCo., M alabar, FL,1976). datesmayrea rrangein tod ifferentmetastablecon forma- 13.D .R.Lide, Ed., CRCHandbookofChemistry andPhysics (CRC tionshavingdifferentox idationstates.Furthermore,the Press, Boca Raton,FL, 1994),75th ed.

APPLIEDSPECTROSCOPY 1675