Determination of U isotopic ratios in environmental samples by ICP-MS

In e labo a o y No e De e mina ion o U iso opic a ios in en i onmen al samples by ICP-MS Shigeo Uchida,* a Ra ael Ga cõÂa-Teno io, b Keiko Tagami a and Manuel Ga cõÂa-LeoÂn c a En i onmen al and Toxicological Sciences Resea ch G oup, Na ional Ins i u e o Radiological Sciences, Anagawa 4-9-1, Inage-ku, Chiba-shi 263-8555 Japan. E-mail: s_uchida@ni s.go.jp b Dep . FõÂsica Aplicada, Uni e sidad de Se illa, A da Reina Me cedes, 2, 41012-Se illa, Spain c Facul ad de FõÂsica, Uni e sidad de Se illa, Apdo 1065, 41080-Se illa, Spain Recei ed 17 h Feb ua y 2000, Accep ed 17 h Ap il 2000 Published on he Web 14 h June 2000 Induc i ely coupled plasma mass spec ome y (ICP-MS) has been used o he de e mina ion o ace elemen s in a a ie y o ma e ials. Fo u anium, he concen a ion o 238 U can be measu ed wi h a de ec ion limi o less han 0.1 pp in a ew minu es. Howe e , because o he ex emely low 234 U concen a ions in en i onmen al ma e ials, i is necessa y o sepa a e U om he ma ices and o emo e in e e ing elemen s om he sample solu ion o measu emen o he 238 U/ 234 U a io by ICP-MS. In his s udy, a simple and apid sepa a ion me hod o U wi h an ex ac ion ch oma og aphic esin (TEVA esin) was de eloped o ICP-MS and he U iso opic a ios in phosphogypsum samples, collec ed in Spain, we e measu ed. The esin e ®cien ly e ained U in 6 M HCl medium and mo e han 98% o he U was easily elu ed wi h he ® s 30 mL o 0.1 M HNO 3 . The sepa a ed solu ions we e ee om mos o he ma ix elemen s and he concen a ions o U in he solu ions anged om 35 o 70 ppb. The 238 U/ 234 U iso opic a ios in he solu ions we e measu ed by ICP-MS. Mo eo e , due o he p esence o a highe numbe o 235 U a oms in compa ison wi h 234 U, he 238 U/ 235 U iso opic a ios we e also de e mined wi h high p ecision by ICP-MS. The esul s ob ained ag eed well wi h he a ios measu ed by alpha-spec ome y. The me hod o his de e mina ion is mo e apid han alpha-spec ome y, and is conside ed o be mo e sui able o en i onmen al moni o ing. In oduc ion Induc i ely coupled plasma mass spec ome y (ICP-MS) has been used wi h g ea success o he accu a e and p ecise de e mina ion o ace elemen s in en i onmen al ma e ials. Due o i s low de ec ion limi s, analy ical speed, lack o chemical in e e ence, and mul ielemen al capabili y, he me hod has been success ully applied o he de e mina ion o mo e han 30 elemen s in en i onmen al samples wi hou any chemical sepa a ions. 1±4 This echnique has p o ed o be powe ul enough o he de e mina ion o long-li ed adio- nuclides, such as 238 U, 237 Np, 232 Th, 99 Tc and 239 Pu, in en i onmen al samples. 5±12 P ecise de e mina ion o U and Th wi hou adiochemical sepa a ion can be achie ed o wa e samples, 13±15 o soil samples in which ea ed amoun s a e abou 0.1 g. 4 In ela ion o iso opic a io s udies, he mal ioniza ion mass spec ome y (TIMS) has been he con en ional me hod o high p ecision analysis, bu his echnique is ime consuming and expensi e. Recen ly, iso opic a io measu emen s using ICP-MS ha e been epo ed. 16±20 Howe e , he e is no much in o ma ion in he cu en li e a u e on he capabili y o ICP- MS o measu ing 238 U/ 234 U a ios in en i onmen al sam- ples. 11,12,21,22 This is mos ly due o he ex emely low 234 U mass concen a ion in hese ma ices. In ac , by assuming adio- ac i i y equilib ium in en i onmen al samples, he 238 U/ 234 U iso opic a io can be expec ed o be a ound 1.8610 4 . Ne e - heless, he knowledge o such a ios can gi e ele an in o ma ion on na u al sys ems. 23 I e eals, o ins ance, he exis ence o an h opogenic inpu s in aqua ic sys ems 24 o can help o da e geological samples h ough he U/Th da ing me hod. Also, he de e mina ion o he 238 U/ 235 U a io is use ul, o ins ance, o en i onmen al s udies a ound nuclea plan s. I is clea ly wo hwhile, he e o e, o in es iga e whe he ICP-MS could be applied o he de e mina ion o U iso ope a ios. T adi ional U de e mina ion by ICP-MS equi es almos no chemis y wo k. Howe e , he ex emely low 234 U mass concen a ion a en i onmen al le els makes i necessa y o ca y ou some p econcen a ion p ocesses, i.e. some chemical manipula ions, so as o su ®cien ly dec ease he limi s o de ec ion and be able o gi e ue U iso ope a ios. In ecen yea s, selec i e ex ac ion ch oma og aphic esins, such as TEVA, TRU and UTEVA esins, ha e been de eloped by EICh oM Indus ies Inc. (Da ien, Illinois, USA) o he sepa a ion o ac inides om solu ions. A no el and simple me hod o sepa a ion and concen a ion o U-iso opes om en i onmen al samples, based on he use o hese esins, has been de eloped and alida ed. 25±29 In pa icula , we a e in e es ed in TEVA esins om he iewpoin o U de e mina- ion by ICP-MS. The easons a e as ollows. (1) Adso p ion o U by TEVA esin unde sui able acid solu ion condi ions is highe han ha by UTEVA esin. (2) Deso p ion o U om TEVA esin by a weak ni ic acid solu ion is achie ed mo e easily han o he o he wo esins. This means ha TEVA esin can concen a e U in a small amoun o weak ni ic acid solu ion. I is also p e e able ha he ®nal solu ion is less han 0.5 M HNO 3 o easy de e mina ion by ICP-MS. (3) TRU esin shows he highes adso p ion cha ac e is ics o U, bu deso p ion by a small amoun o a weak ni ic acid solu ion seems o be di ®cul , because o he ela i ely high adso p ion ac o unde low pH acid solu ion condi ions. In his pape , we sepa a e and concen a e U in en i on- men al samples by using TEVA esins. La e ICP-MS will be used o de e mine he U iso opic a ios in some phospho- gypsum samples. DOI: 10.1039/b001332n J. Anal. A . Spec om., 2000, 15, 889±892 889 This jou nal is #The Royal Socie y o Chemis y 2000 Published on 14 June 2000. Downloaded by G al Uni e sidad Se illa on 26/02/2015 16:39:44. View A icle Online / Jou nal Homepage / Table o Con en s o his issue Expe imen al Samples Phosphogypsum samples (mos ly CaSO 4 ?2H 2 O), he by- p oduc o med in he p oduc ion o phospho ic acid om phospha e ocks, we e collec ed in Spain. They we e chosen o es he pe o mance o he chemical sepa a ion o U and he U a io de e mina ion because hey showed ela i ely high U concen a ions. The 238 U, 235 U and 234 U concen a ions had been p e iously de e mined by alpha-spec ome y, 30 which se ed o alida e he me hod. Reagen s P epacked columns o TEVA esin, including ca. 2 mL o he esin, we e used in he expe imen s. S anda d, washing and s ipping solu ions we e p epa ed wi h ni ic and hyd ochlo ic acids o ul a-analy ical g ade (Kan o Chemical Co. Inc., Tokyo, Japan). Deionized wa e (w17 MO) was used h ough- ou he wo k. Sample p epa a ion The samples we e p epa ed acco ding o a newly de eloped p ocedu e, which is shown schema ically in Fig. 1. A 0.1 g sample was placed in a poly e a¯uoe hylene (PTFE) beake co e ed wi h a PTFE co e and diges ed unde e¯ux wi h conc. HNO 3 o 2 h a 120±140 ³C on a ho -pla e. A e his s ep, he co e was emo ed and he sample e apo a ed o d yness. The esidue was hen dissol ed in o 10 mL o conc. HCl. The solu ion was ®l e ed o emo e insoluble ma e ials. The ®l e was insed wi h 10 mL o wa e which was added o he ®l a e. A e all hese p ocesses he ®nal solu ion o be passed h ough he TEVA esin was ca. 20 mL o ca. 6 M HCl. We conside ed ha his p ocedu e could no ha e comple ely diges ed he phosphogypsum. Howe e , we decided o use his we -diges ion me hod, because i was he one used o he U de e mina ion by alpha-spec ome y. 30 In o de o com- pa e he esul s by ICP-MS wi h hose ob ained by alpha- spec ome y, i was p e e able i he diges ion p ocedu es we e he same. Sepa a ion and concen a ion Isola ion o U and pu i®ca ion o he solu ion was done wi h a TEVA esin. The isola ion s ep is based on a s ong endency o he oxidised o m o U o be e ained on he esin column a high HCl mola i y. This a ®ni y disappea s a low acid condi ion, e.g., 0.1 M HNO 3 o 0.1 M HCl. 25 Thus, a e condi ioning he esins wi h 6 M HCl, he solu ions ob ained by we -diges ion o he samples, in which he acid s eng h was also 6 M HCl, we e passed h ough he columns, p oducing he elua e A. Nex , he esin column was washed wi h an addi ional 20 mL o 6 M HCl (elua e B) o elimina e o he elemen s ha ha e low ®xa ion on he esin. The U e ained was ®nally elu ed om he esin by passing h ee olumes o 30 mL o 0.1 M HNO 3 (elua es 1, 2 and 3). The ¯ow- a e was kep a abou 1 mL min 21 du ing he expe imen s. 0.1 M HNO 3 was selec ed as a s ipping solu ion, because Cl o igina ing om HCl o ms complexes wi h se e al elemen s, such as Au and Hg. When U iso opes we e measu ed by a quad upole ype ICP-MS, hese complexes, especially, 197 Au 37 Cl, 198 Hg 37 Cl and 200 Hg 35 Cl, ha e o be ejec ed in o de o a oid in e e ences. In his s udy, we ied o de e mine di ec ly he concen a ion o U iso opes in he ®nal solu ion by ICP-MS. Fo di ec measu emen o an elemen wi h an ICP-MS, a high acid solu ion is ha m ul o he de ec o . The 0.1 M HNO 3 solu ions can be di ec ly in oduced in o he ICP-MS, i o he ma ix elemen s in he ®nal solu- ions ha e been emo ed du ing sepa a ion and pu i®ca ion p ocedu es. Mo eo e , due o he low concen a ion o 234 Uin he samples, i is necessa y o make he coun ing ime as long as possible. High concen a ions o ma ix elemen s in he ®nal solu ions damage he in oduc o y pa h and de ec o o he ICP-MS. Bo h sepa a ion o U om o he elemen s and good eco e y o U a e necessa y o he de e mina ion o U iso opic a ios by ICP-MS. Measu emen s A Yokogawa (Tokyo, Japan) PMS-2000, quad upole ype ICP-MS, was used o he analysis o U iso opes in he samples. The ins umen al pa ame e s a e summa ised in Table 1. Unde hese analy ical condi ions, he oxide o ma ion o Ce was ound o be less han 2%. As an in e nal s anda d, Bi was used o compensa e o changes in he analy ical esul s du ing ope a ion. Also, a s anda d solu ion was p epa ed om Spex mul ielemen al plasma s anda ds (XSTC 13, Spex Indus ies, Edison, NJ, USA), and used o calib a ion pu poses. Th ee eplica e measu emen s we e done o e e y sample, looking o 2, 50 and 150 s coun ing imes o 238 U, 235 U and 234 U, espec i ely, in each un. Resul s and discussion E ec i eness o he sepa a ion me hod A ce ain amoun o each elua e (elua es A, B, 1, 2 and 3) ob ained om wo phosphogypsum samples (iden i®ed as A and B), was measu ed by ICP-MS. The amoun s o U ( 238 U) in each elua e a e lis ed in Table 2. I was clea ha only a negligible amoun o U was de ec ed in he d ain o he load solu ion and he washing solu ion o 6 M HCl (20 mL62). Almos all U in he samples was concen a ed in he ® s s ip solu ion o 0.1 M HNO 3 . Mos o Fig. 1 Expe imen al scheme o he sepa a ion o U in phosphogypsum sample. Table 1 Ope a ing condi ions o ICP-MS R powe 1240 W Coolan gas ¯ow a e 13 L min 21 Auxilia y gas ¯ow a e 1.2 L min 21 Ca ie gas ¯ow a e 0.83 L min 21 Dis ance om load coil o ape u e 5.4 mm Dwell ime 300 ms o 234 U, 100 ms o 235 U and 4 ms o 238 U Numbe o sweeps 500 Replica ion 3 890 J. Anal. A . Spec om., 2000, 15, 889±892 Published on 14 June 2000. Downloaded by G al Uni e sidad Se illa on 26/02/2015 16:39:44. View A icle Online he ma ix elemen s passed h ough he column wi h he 6 M HCl solu ion, wi h he excep ion o Fe. Some Fe was elu ed wi h he 6 M HCl solu ion (elua e A), bu he es beha ed in a simila manne o U. The concen a ion o Fe in elua e 1 was mo e han 100 ppm. This migh be due o he o ma ion o a s ongly adso bed nega i ely cha ged i on complex in he 6 M HCl condi ions. The samples used in his s udy con ained a ela i ely high concen a ion o Fe. We hough ha some Fe± Cl complex ions in he in oduc o y solu ion we e adso bed on he esin, bu he complex ions exceeded he capaci y o he esin o adso p ion and we e elu ed wi h he solu ion (elua e A). The ions adso bed on he esin hen we e deso bed wi h 0.1 M HNO 3 solu ion (elua e 1). Ou esul s show ha he TEVA esin e ®cien ly e ained U in 6 M HCl medium, while i was easily elu ed wi h he ® s 30 mL o 0.1 M HNO 3 . The concen a ion o U iso opes in he solu ion can be de e mined di ec ly by ICP-MS, because majo elemen s we e emo ed om he solu ion by he esin. In ou expe imen , we ®nally collec ed U in 30 mL o 0.1 M HNO 3 , bu we hough ha U could be concen a ed in he s ip solu ion, o which he olume is less han 30 mL. The esul s ob ained p e iously in o he aliquo s o he same samples by alpha-spec ome y a e lis ed in Table 3. 30 The esul s ag ee well o bo h samples A and B. I can be deduced ha no app eciable ac ion o U emained in he column. Consequen ly, he solu ions o 0.1 M HNO 3 , co esponding o elua e 1, con ain mo e han 98% o he U o iginally p esen in he sample, and a e ee om mos o he ma ix elemen s excep Fe. These solu ions con ain a ound 50 ppb o U. The e o e, his sepa a ion me hod can be used o he de e mina ion o he 238 U/ 234 U and 238 U/ 235 U iso opic a ios by ICP-MS. Iso opic a ios o U in phosphogypsum samples The U iso ope amoun s we e measu ed by ICP-MS in he pu i®ed solu ions (elua es 1) ob ained om h ee mo e phosphogypsum samples (samples C, D and E). These expe imen al esul s a e compiled in Table 4. The esul s ob ained o a blank sample a e also included. The blank sample unde wen exac ly he same chemical sepa a ion p ocedu e as he phosphogypsum samples. F om hese esul s, we d ew wo conclusions: (1) e y good accu acy was ob ained in he measu emen o di e en U iso opes (in all he cases, he s anda d de ia ions we e lowe han 4%); and (2) 234 U in hese samples was clea ly de ec ed, wi h coun ing alues a leas 10 imes highe han ha in he blank sample. These da a made us con®den in calcula ing he iso opic and ac i i y a ios o U iso opes. The esul s a e summa ised in Table 5. A e age alues de e mined in a se o aliquo s o he same phosphogypsum samples by alpha-spec ome y a e also gi en. 30 F om he esul s ob ained p e iously, a secula equilib ium was ound be ween 234 U and 238 U, and he 235 U/ 238 U ac i i y a io was 0.047, which is expec ed in na u al samples. We concluded ha , a he U concen a ions exis ing in he ®nal measu ed solu ions (in he ange 35±70 ppb), he 238 U/ 234 U iso opic a ios can be measu ed wi h good accu acy. These a ios ag ee wi h hose p e iously de e mined by alpha- spec ome y. Due o he p esence o a highe numbe o 235 U a oms in compa ison wi h 234 U, he 238 U/ 235 U iso opic a ios can also be de e mined wi h high p ecision by ICP-MS. The esul s show he capabili y o he ICP-MS me hod de eloped in his wo k o he de e mina ion o he 238 U/ 234 U a io a he concen a ion le els desc ibed. Conce ning he measu emen ime, he ICP-MS me hod can be conside ed supe io o o he adi ional adiome ic echniques, e.g., Table 2 U anium esul s ( 238 U in ng) in he ® e elua es ob ained om wo phosphogypsum samples by he applica ion o he adiochemical me hod in Fig. 1 Sample A U/ng U in elua e (%) Sample B U/ng U in elua e (%) Elua e A 7.56¡6.95 0.47 Elua e A 9.8¡6.9 0.68 Elua e B 8.72¡5.50 0.50 Elua e B 5.3¡6.4 0.37 Elua e 1 1603¡8 98.86 Elua e 1 1415¡9 98.77 Elua e 2 0.94¡0.08 0.06 Elua e 2 2.0¡1.0 0.15 Elua e 3 1.30¡0.7 0.08 Elua e 3 0.38¡0.10 0.03 To al 1622¡12 100.01 To al 1433¡13 100.0 Table 3 238 U speci®c ac i i ies (mBq g 21 ) de e mined in wo phospho- gypsum samples by ICP-MS and alpha-spec ome y a Sample 238 U/mBq g 21 ICP-MS Alpha-spec ome y Sample A 179¡2 170¡7 Sample B 161¡2 170¡8 a ¡:1ss a is ical e o by ICP-MS and 1scoun ing e o by alpha-spec ome y. Table 5 Mass and ac i i y u anium iso opic a ios ob ained in he measu emen s o h ee phosphogypsum samples. The a e age alues o hese iso opic a ios, de e mined by alpha-spec ome y, a e also lis ed Sample 238 U/ 234 U mass a io 238 U/ 235 U mass a io 234 U/ 238 U ac i i y a io 235 U/ 238 U ac i i y a io C 18601¡681 144¡4 1.00¡0.04 0.044¡0.001 D 18884¡847 144¡3 0.98¡0.04 0.044¡0.001 E 18967¡588 147¡4 0.98¡0.03 0.043¡0.001 Alpha-spec ome y a 18550¡370 139¡3 1.00¡0.02 0.047¡0.002 a The a e age alues de e mined om a se o o he aliquo s o he same phosphogypsum samples by alpha-spec ome y. Table 4 Analy ical esul s ob ained in he de e mina ion o he U- iso opes in h ee phosphogypsum samples and a blank sample a Sample 238 U coun s 235 U coun s 234 U coun s 2 s 50 s 150 s C 797778¡14592 138372¡3220 3573¡102 D 799421¡11815 138903¡2285 3538¡134 E 1424194¡27592 241231¡5345 5978¡135 Blank 403¡15 204¡13 358¡9 a ¡:1ss a is ical e o by ICP-MS. J. Anal. A . Spec om., 2000, 15, 889±892 891 Published on 14 June 2000. Downloaded by G al Uni e sidad Se illa on 26/02/2015 16:39:44. View A icle Online alpha-spec ome y. In ac , o ob ain he same accu acy by alpha-spec ome y a coun ime o a leas 72 h is necessa y, assuming 100% eco e y yield and 0.1 g o phosphogypsum. This ®gu e mus be compa ed wi h 10 min, which is he ypical coun ing used along his wo k. Besides, i he de e mina ion o he 238 U/ 235 U a io is also o in e es , ICP-MS can ob ain i accu a ely because he adioac i i y o 235 U is clea ly lowe han ha o 234 U in he samples. Acknowledgemen s One o he au ho s (RGT) g a e ully acknowledges he ®nancial suppo o he Science Technology Agency o Japan o a scien i®c isi o he Ins i u e o Radiological Sciences in Nakamina o. RGT is also e y g a e ul o all he s a o his ins i u e o hei help and wa m hospi ali y. Re e ences 1 Y. Iga ashi, C. K. Kim, Y. Takaku, K. Shi aishi, M. Yamamo o and N. Ikeda, Anal. Sci., 1990, 6, 157. 2 T. Yoshida, S. Yamasaki and A. Tsumu a, J. Mine . Pe ol. Econ. Geol., 1992, 87, 107. 3 S. Yamasaki, A. Tsumu a and Y. Takaku, Mic ochem. 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