ICP-MS

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Lead isotope determinations by inductively-coupled plasma mass spectrometry (ICP-MS): potential of sector field instruments BELlN DA FLEM, ANDREASGRIMSTVEDT & NIGEL COOK

Flem, B., Grim st vedt, A. & Cook, N.J. 2000: Lead isot ope dete rm inat ion s by inductively-coup led p lasma mass spectro metry (ICP-MS): potenti al of secto r field instr uments. Nor ges geo log iske undersekelse Bulletin 436, 203-207 . Lead isoto pe measurements have been carried out using a high -resolut ion induc tively-coup led p lasma mass spect romet er (Finnigan MAT ELEMENT, config ured wi th CD-1 opt io n). The metho d can be applied to all types of geological samples brought into solution fo rm. Three com mon lead standa rds are used for document at ion of t he met hod. Detection limit s fo r 204 p b, 206p b, 207Pb and 208Pb lie bet ween 0.124 and 2.86 pg mr 1 . Optimi sation of scanning condit ions at a count rate of approximat ely 200,000 cps for 206Pb and 207pb, led to a relative standard deviatio n of O.OS% for t he mean 207Pb/ 2o6Pb isot ope ratio, based on 15 consecut ive measurement s. The meth od can be rout inely app lied to gain geo chro nolog ical and petrol ogical data fo r genet ic st udiesof mineral deposit s, includin g deposits of indust rial minerals. Because of space charge effects in t he skimmer cone region and mass-dep endent sensitivi ty of th e mass spectro mete r, t he measured int ensity rati o of samples must generally be calib rated using external or int ernal standards of known isoto pe ratios to correct for mass biases and mass fractionat ion. Belinda Flem, Andreas Grimst vedt & Nigel Cook, Geol ogi cal Survey of No rw ay, N-749 7 Tron dheim , Norway .

Introduction Geochronol ogical, isotopic and petr ological data are important eleme nt s in st udies of minera l deposi t s. This is t rue for all typesof min eral deposit s, bo t h metalliferou s oresand deposit s of indu stri al min erals. Alt ernatively, geoch ron ological and isotope data are useful to olsfor pro specting, asthe y can help esta blish geolog ical correlati on s o n the basis of age and/ or isot opi c signat ure, and aid ident ificatio n of form ati on s offe ring econo mic pote nt ial. This paper describes development w ork carried out at th e Geolog ical Survey of Norway t o use induct ively-cou pled plasma mass spectrometry (lCP-MS) as a met hod for lead isoto pe measurement s. The possibilit y t o use th e techniqu e in inv estigation s of min eral deposits on a routine basis represents an important addit ional tool for unr avelling th e geo log ical framewor k hosting economic deposit s of indust rial min erals such as limeston e, dolo mit e, talc and ot her mineral produ cts. Lead isotopes can be measured in min erals w hose U/Pb and Th/Pb ratios are suffici ently low such t hat th e isot opic composit ion wi ll not chang e appreciably w it h t im e. Examples include th e commonest lead min eral, galena (PbS). PbPb isochron s for such mi nerals, and for w ho le rocks with similarly low rati os, can give an age of form ation . Such meth od s have been in w ide usage for several decades and have been app lied to meta lliferous ore s (e.g., Bjerlykke et al. 1993, Billstr orn & Vivallo 1994); to a broad range of igneou s rocks, f rom granit e t o basalt; and also to various types of sedimentary rocks. Reliabl e ages can be ob tained for rocks w it h a sim ple crystallisation histor y. How ever, because of t he sig nificant difference in geoc hemical behaviour and mobility of U and Pb, maj or pro blems can arise in int erpret at ion of sit uat ions in

w hich th e Pb isoto pe systemat ics have been reset. The applicat ion of Pb isotopes for geoch ronology has decli ned in pop ularit y in recent years. How ever, stu dies of Pb isot ope syst ematics are an incre asingl y used tool in petrog enetic stud ies in w hich th e goa l is to establish th e lead source. Lead isot op es, and ot her radiogenic isotope rat ios, are used to identify source region s in th e crust as well as mixing and contaminatio n pro cesses. Modern review s of Pb/Pb isot op e method ology (Faure 1986, Roll inson 1993) show how Pb/Pb geoch ronolo gy is applied t o give ages of met amorp hism (Le. t ime of resetting ), and more rarely to give ages of crustal fo rmati on . Determin ati on of lead isot ope ratios in geo log ical samples may be carried out using a variety of analyt ical meth ods, e.g., TIMS (t hermal ionis ation mass spect romet ry). In th e past decade or so, inductively-coup led plasma mass spect romet ry (lCP-MS) has esta blished it self as an excellent method for determi nati on of Pb isoto pe abund ances. Measurem ent of Pb isotope rat ios w ith ICP-M Shas been shown to be possible on all ty pes of envi ronm ental samples. These can rang e from atmospheri c aerosols (e.g., Surges & Barrie 1987). gasoline (e.g., Lord 1994), fresh and saline wate rs (e.g., Halicz et al. 1994) and digested samples (e.g. Wid enb eck et al. 1995). If th e ICP-MS is used in combin ation w it h a laser prob e, t he determ in ation of Pb isotope ratio s can also be carried out on solid samples, perm itting in situ determinati on of Pb isot op e systemati cs in individ ual min eral g rains (e.g., Feng et al. 1993). Application of ICP-MS methods t o Pb isotope determinati ons is adv ant ageous in st udies of min eral resources and metallog eny in t hat rapid, reliabl e analysis of samples is permit te d at relative ly low cost . Furth erm ore, if th e t echn ique

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BELlNDA FLEM , A N D REAS GRIMS7VEDT & NI GEL CO O K

can be used in conju nction w it h a laser probe, analyses cou ld be perform ed in situ on samples prepared for minera logic al or petrograph ical st udy, t hus fur ther reducing prepa rat ion and analysis t ime. The method offers, the refore, sig nif icant prom ise for th e futu re. In th is paper, th e application of indu ctiv ely-cou pl ed plasma mass spectr ometry (ICP-MS) to direct det ermina t ion of Pb isot ope rat ios in aqueous or digested samples is invest igat ed. Three com mon lead standa rds w it h different con centration s of Pb isot opes have been stud ied in order to show th e suitability of t he t echniqu e for rout ine determin at ion of Pb isotopes.

Instrumental details A standard, doubl e focusing, sector fie ld, ICP-MS (Finnigan MAT, ELEMENT) instrument is used (Feldmann et al. 1994, Giessmann & Greb 1994). The configuration included t he col option from Finnigan, con sist ing of a to rch insert modu le w hich replaces th e standard Elem ent ICP tor ch holder. A grounded Pt electrode (Guard Elect rodeTM) is inserte d bet ween the quartz ICP tor ch and t he RF load coil to prevent Ta ble 1. Ope rating cond itions of the ICP-M5. Plasma con d itions Plasma Power

900Watt

capaciti ve coupli ng from t he load coil into t he ICP. The ICP is t hen sustained by 'pure' inductive coup lin g; secondary discharge between t he ICP and mass spectrome te r is eliminated. The CD-1 option is used fo r t he wi dest range of samp le matr ices and instr ument al operat ing cond it ions, includin g cold plasma ICP-MS. The most impo rt ant ope rat ing parame ters are giv en in Table 1. The Finnigan MAT ELEMENT is a sector field, high -resolution ICPMS, but all measurements were carried out at a mass resolut ion of 300 (mass of isotope M / mass differen ce of isotopes, LlM). The high resolut io n (mass resolut ion 8000-9000) was only used for checking of isob aric int erference. The inst rument ope rat ing parameters wer e t uned on a 1 ng ml! solut ion of NBS-98l to give a peak height of approximately 700,000 cps for 208 Pb. Three common lead isotop e standa rds have been used (NBS-98l , NBS·982 and NBS-983). A specifi c amou nt of each standard was d igested in an auto clave at 200 kPafor 30 minutes wi t h 7 N HN0 3 . Aft er coo ling to room temperat ure, t he solut ion was d iluted to the concen trati ons given in Table 2. Nit ric acid was add ed to a concentr at ion of 0.24 M fo r solution conservation. A samp le blank was prepared in the same way to check and correct fo r conta mination during digestion and dilut ion. The blank solut ion was also used to estima te the limits of detect ion (LOO). The lim it s of detection (LOO) for all Pb isoto pes were est abli shed from t he sensit ivit y dat a given in Table 2 and 1S successive measurem ents on t he blank sample. LOO determinat ion is here based on 3 tim es t he stand ard deviation mult iplied by 2°.5, divid ed by th e sensitiv ity, S; equ at ion [1].

Dat a collect ion Mass resolution (m/c.M)

LOD = 30- -J2 5

300

Sca n type

Escan

No of scans

600

Counting time on each isotope per scan

0.05s

Total counting time on each isotope

30s

Detection mode

Counting

Result s and discussion Mass discrim inat ion effects have been ob served in measuremen ts of isotopic ratios using ICP-MS. Several processes con tribute to t his phen om enon . These are mainly space charge eff ects (Col umbi c rep ulsions) in t he skimmer cone region, w hich result in prefere nt ial t ransmission of t he heavier ions (Platzner 1997), and mass-depend ent sensit ivit y of t he mass spect rometer. The effect of mass bias should be monit ored, and if necessary corr ected , by using a standard of know n isotop ic com posit ion close to th at of th e unknown. The standard should be analysed frequently during an analyt ical run t o mon itor change s in mass bias wi th t ime. Mass bias is also

Measured mas s range 204 Pb

203.905-204.041

206 Pb

205.906-206.043

207Pb

206.907-207.045

208 Pb

207.907-208.046

[ 1]

Ta ble 2. Detect io n limits (LOO) obt ained from 15 successive measureme nts on the blank sample and sensitivities obtained from calibration togeth er with the concen trations of t he dige sted NB5 standa rds. Sensitivity [cps (ng ml-' )"' ]

St. dev . of inte nsities Blank [cps]

Detectio n limit (LOD) [pg ml' ]

NB5-981

NBS-982

NB5-983

[pg rnl' ' ]

[pg mr ' ]

[pg mr ' ]

Pb204

905094.7

26.4

0.124

15.70

17.3

0.503

Pb206

903388.7

305

1.43

266

635

1356

Pb207

926895.3

264

1.21

243

297

96.5

Pb208

933902.1

631

2.86

577

63S

18.5

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BELlNDA FLEM, ANDREAS GRIMSTVEDT& NIGEL COOK

Table 3. Measured isotop e rati os and relative standard deviation of the mean of 15 consecutive measurements.

204 Pbl 20 6 P b

NBS-981

NBS-982

NBS-983

207

P bl

20 6 Pb

208 P bl 20 6 P b

Mean measured value

0.057925

0.92135

2.1987

Certificate value

0.059042

0.91 464

2.1681

Bias0/0

- 1.89 0/0

+ 0.733

+ 1.41

Experimental %RSD

0.08

0.05

0.07

Mean measured value

0.026576

0.46783

1.00973

Certificate value

0.02721 9

0.46707

1.0001 6

Bias0/0

- 2.36

+ 0.163

+ 0.957

Experimental%RSD

0.06

0.04

0.05

Mean measured value

0.000298

0.070906

0.013095

Certificate value

0.000371

0.071201

0.013619

Bias0/0

- 19.7

- 0.414

- 3.85

Experimental %RSD

0.68

0.05

0.1 9

stro ngly depend ent upon th e d ay-to -day tunin g of th e Ydefl ecti on lenses; th e mass bias w ill refl ect how we ll the Ydefl ecti on lenses we re tun ed on th e act ual day. Mass bias for th e isotop e rati os 204 PbP 06Pb, 207Pb/ 206Pb and 208PbP0 6Pb are giv en in Tabl e 3 for th e three sta ndard materials. The values are quite high, possibly suggest ing th at insuff icient tim e was spent on instrumental tuning and th at thi s mu st be borne in mind wh en unkn own sam ples are analysed. A typi cal run proc edure w ould include multiple det ermin ation of reagent blank s and referen ce st and ards alongside th e un known samp les. To correct systematic d eviati on s, corr ection factors are det ermined using exte rnal or internal sta ndards, w hich allow th e conversion of intensity rat ios into isotope rat ios. Ext ernal calibrat ion uses certified standards with known isotope rat ios, as the one used in thi s paper. The measured int ensity ratios ofth e sam p les t o be exam ined are cor recte d according to the followin g formula:

ax

lax sample

bx

Ibx sample

.z . Ibx

standard

nati on ettec t ot th e isot op e to be determin ed . I his has t o be checked in each individu al case. Extern al calibration is th erefor e normally used in preference t o inte rnal sta ndar disatio n (Finniga n 1998). It may happ en th at a value appears in a rat io determinati on w hich d epart s co nside rably from ot her values in t hat measurement series. If th e nu mber ofva lues is small, such an out lyi ng value can affec t th e standard deviati on t o such an extent th at it is reasonab le to conclude t hat th e mean rati o recalcul at ed wit hout th e out lying value w ould better express the tru e mean. Several stat ist ical crite ria for rejec t io n of out lying values exist, e.g., th e Chauve net reject ion crit eria (Platzner 1997).ln th e present st udy, howe ver, no rejec t ion of measurement s has been made, since th e aim is to d evelop a meth od for Pb isoto pe measurement s t o be used on a rout ine basis. Vanh aecke et al. (1996) repo rte d 206Pb/ 207Pb rat ios wit h relative sta ndard d eviati on (RSD) of 0.1% in a 20-40 J..lg 1-1 solutio n of Pb or w hen th e signal inte nsit y exceed s 200,000 cp s. The sta ndard NBS-981 has a Pb concent rat io n of 1.1 . J..lg 1-1 and by tunin g th e instrum ent t o an opt imal sig nal intensity , nearly 200,000 cps were obta ined for t he nuclides 206Pb and 207Pb. Values of precision obta ined in th e pre sent work are com parable w it h th ose of Vanh aecke et al. (1996), alt hough the isoto pic concent ratio ns are 20-40 t imes lower. Percenta ge RSD values are in the same order of magn it ud e; 0.2% RSD for sing le measurements and 0.05% RS D for th e me an (Tab le 3). The report ed precision is thu s full y acceptable for routine app licat io n. The det ecti on limits for th e isot op es are est imated to be bet ween 0.124 and 2.86 pg rnl' (Table 2). The lowest detection limit (0.124 pg rnl" ) is obtained for 204Pb, th e isot ope w hich usually has th e low est ab undance. In th e NBS-983 solut ion, 204Pb is present at a co ncent rat ion of 0.503 pg rn l'. i.e. 4 time s higher th an th e detect ion limit. The experim ent al RSD for th e ratio 204PbP06Pb in NBS-983 solut io n is, at 0.68%, quite high . Howe ver, since th e inst rument al intensity for 204Pb is approaching th e low er lim it of detecti on, RSD may be limited by count ing statisti cs. The st andard deviati on on th e co unt rate, based on counting stat ist ics giv en by Jen kins and De Vries (1970) is:

(J

l2J

lax standard

w here Z is th e cert ified isotope rati o and I is th e measured inte nsity . Elements wit h known isotope rati os, who se mass neighbour s th e mass of the elem ent to be determ in ed, can be used for inte rnal st andardisat ion (e.g. 203 TI/ 205TI can be used to d et erm in e th e isotope rat io 206PbP07 Pb).The kno wn isot ope rati o is th en used to correct for mass bias in th e ratio to be det ermin ed. The basic di sadvant age of inte rnal standardisatio n is link ed to th e co ndit io n th at th e mass di scrim inat ion effec t for th e inte rnal st andard mu st correspo nd t o th e mass di scrimi -

R

=

H -

T