Anal. Chem. 1991, 63,2705-2708 (24) Buser, H. R. Anal. Chem. 1988. 58, 2913. (25) Oehme, M.; Kirschmer, P. Anal. Chem. 1984, 56, 2754. (26) Miles, W. F.; Gurprasad, N. P.; Malls, 0 . P. Anal. Chem. 1985, 57, 1133. Larame6. J. A.; Arbogast, B. C.; Deinzer, M. L. Anal. Chem. 1988, 58, 2907. Schafer, W.: Balischmlter, K. Chemosphere 1988, 75, 755. Tong, H. Y.: Gross, M. L. Proceedings of the 36th ASMS Conference on Mass Spectrometry and Allied Topics; 1988; p 230. Sovocool, G. W.; Mltchum, R. K.; Tondeur, Y.; Munslow, W. D.; Vonnahme, T. L.; Donnelly, J. R. Biomed. Envkon. Mass Spectrom. 1988, 75, 669. Oberg, T.; Warman, K.; brgstrom. J. Chemosphere 1987, 76, 2451. Sovocool, G. W.; Donnelly, J. R.; Munslow, W. D.; Vonnahme, T. L.; Nunn, N. J.: Tondeur, Y.; Mltchum, R. K. Chemosphere 1989, 78, 193. Harless, R. L.; Lewis, R. G. ChemosDhere 1989, 78. 201. (34) Tong, H. Y.; Arghestani, S.; Gross,' M. L.; Karasek, F. W. Chemosphere 198% 78, 577. (35) Tong, H. Y.; Giblln, D. E.; Lapp, R. L.; Monson, S. J.; Gross, M. L. Anal. Chem. 1991. 63. 1772. (36) Tong, H. Y.: Karasek, F. W. Chemosphere 1988, 75, 1219. (37) Olk, K.; Vermeuien, P. L.; Hutzinger, 0. Chemosphere 1977, 6.455. iwo, 9, (38) Lustenhwwer, J. w. A.; elk. K.; Hutzinger, 0. 501.
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(40) zhiub, W. M.; Tsang, W. I n Human and Envkonmental Rlsks of Chbrinated Dioxins and Related compOunds; Tucker, R. E., Young, A. L., Gray, A. P., Eds.; Plenum Press: New York. 1983; p 731. (41) Shaub. W. M.; Tsang, W. Envkon. Sci. Technol. 1983, 77, 721. (42) Shaub. W. M.; Tsang, W. I n Chbrlnated Dioxlns and Dlbenrofwans /n the Total Envkonment II; Choudhary, G., Kelth, L. H., Rappe, C., Eds.; Butterworth Publishers: Boston, 1985; p 469. (43) Rghei, H. 0.;Eiceman, 0. A. Chemosphere 1985, 74, 167. (44) Dlckson. L. C.; Karasek, F. W. J . Chromtcgr. 1987, 389, 127. (45) Karasek, F. W.; Dickson, L. C. Science 1907, 237, 754. (46) Dlckson, L. C.; Lenoir, D.; Hutzinger, 0.;Naikwadi, K. P.; Karasek, F. W. ChemosDhere 1889. 79. 1435. (47) Altwicker, E. R.; Kumar, R.: Konduri, N. V.; Milllgan, M. S. Chemosphere 1980, 20, 1935.
RECEIVED for review April 1,1991. Accepted September 13, 1991. This was supported in part by the National Science Foundation (Grants CHE-8620177 and DIR-9017262).
Precise Determination of Femtogram Quantities of Radium by Thermal Ionization Mass Spectrometry Anthony S. Cohen* and R. Keith O'Nions
Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, U.K.
Femtogram quantltles of mRa (-3 X 10' atoms, or 4 X lo4 Bq) have been determlned In environmental materials, Includlng seawater, mineral samples, and silicate rocks, by thermal ionlzatlon mass spectrometry. Chemlcai separatlon technlqws suitable for all these matedab are described here. Overall, these techniques enable the abundance of 22'Ra to be determlned in samples of both seawater and silicates whlch are some 10' tlmes smaller than those required by conventional radloactlve countlng methods.
INTRODUCTION The precise determination of Ra at the femtogram level is important in several disciplines ranging from isotope geochemistry to radiological protection. Chemical and mass spectrometric techniques have been described recently by Volpe e t al. (1) for the determination of small quantities of Ra (- 1 fg or 3 X lo6 atoms) in basaltic rocks. Thermal ionization mass spectrometry (TIMS) procedures, and appropriate chemical techniques, have also been developed in our laboratory for the measurement of small quantities (- 1 fg) of Ra. These have enabled us to measure the 2z6Ra abundance in as little as 35 g of seawater, as well as in a silicate mineral and basaltic rocks. Overall, the new techniques described below and in ref l offer considerable advantages over existing radioactive counting methods (2-5) which require sample sizes -lo3 times larger than those used here. The aims of this present contribution are (1) to describe our chemical methods for the extraction of Ra from environmental materials, including seawater and silicate rocks and minerals, in a form suitable for analysis by TIMS, (2) to provide details of the ion-counting TIMS requirements and methodology, (3) to present results on Pacific seawater, zircon (ZrSi04),and Icelandic basalt, and (4) to demonstrate that precise and repeatable results may be obtained.
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0003-2700/91/0363-2705$02.50/0
EXPERIMENTAL SECTION Apparatus. All beakers and vials used in this study were of PFA Teflon, cleaned for two -8-h periods in hot, high-purity 30% HN03, and washed with 18 MR deionized water. Ion-exchange columns were of two types. One type was of polypropylene (Poly-Prep columns supplied by BioRad Ltd.), with a resin capacity of 2 mL plus an integral reservoir of 10 mL. The second was made from heat-shrinkable PTFE with an i.d. of 3 mm, 0.15-mL resin capacity, and 1.5-mL reservoir. Both types of column were fitted with polyethylene frits. Reagents. Water and all acids were purified by subboiling distillation in quartz or PTFE stills. NH4EDTA (95%, from BDH L a . ) was prepared and cleaned as described in ref 1. It was then adjusted to pH 7.5 (solution A) and pH 8.94 (solution B) with high-purity NH3(aq). Specpure grade Na2C03and SrC03 and analytical grade Th(N03)4were all from Johnson Matthey Chemicals PLC. The zzsRastandard (NIST 4953 D) is known to a precision of &0.8% (2 SE). Preparation of the 228RaSpike. The 228Raspike was produced by separating Ra directly from a solution of -200 mg of Th(N03), in -5 mL of 7 M HN03 by anion exchange. The purity of Th(N03)4was sufficiently high to render its initial cleanup unnecessary. The 228Raspike was calibrated against three accurately weighed aliquots of the 216Ra standard by isotope dilution TIMS; the three determinations agree to better than 0.5%. Initial Ra Separation (Seawater). The first stage separation of a Ra-Ba fraction from seawater is conveniently performed by coprecipitation rather than ion exchange. Sr, rather than Ba, was used for the coprecipitation of Ra for reasons discussed later. Seawater samples of -35 mL were weighed accurately in PFA beakers. The sample was spiked with z28Ra,and -0.1 mL of a -0.25 M Sr solution (in -1 M HCl) was added, followed by - 2 mL of concentrated H2S04. Sr(Ra)S04precipitated after the sample had been heated and allowed to stand for -8 h. The sample was then centrifuged, and the precipitate slurried in H 2 0 and transferred to a 1.5-mL centrifuge tube. The precipitate was again centrifuged and the washinglcentrifuging process was repeated until the pH of the supernate was > -4. Conversion of Sr(Ra)S04to an acid-solublecompound is based on the classical Curie-Debierne method (6). The washed pre0 1991 American Chemical Society
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cipitate was slurried with -1 mL of 0.5 M Na2C03and the slurry transferred to a 3-mL screw-top PFA vial. The vial and its contents were heated to 100 OC for a few hours in order to fully convert Sr(Ra)SO, to Sr(Ra)COp After conversion to Sr(Ra)COs, the contents of the vial were centrifuged, the supemate discarded, and the remaining precipitate washed twice with HzO. The precipitate was dissolved in -0.5 mL of 3 M HCl, transferred to a PFA vial, and evaporated to dryness. In order to separate Sr from Ra, the residue was taken up in 0.5 mL of 3 M HCl and put twice through the second part of the cation-exchange procedure described below. The Ra-bearing fraction was then ready for the Ra-Ba separation. Initial Ra Separation (Silicates). For zzsRa abundance determinations in silicates,samples of up to -40 mg were weighed accurately and then spiked with a known quantity of %( u s d y Bq of 228Ra). -(I-2) X lo7 atoms, equivalent to (3.5-7) X Sample dissolution and the separation of a Ra-Ba fraction was broadly similar to the method of Volpe et al. (I),but the procedure described below differs in a number of respects. The initial separation of a Ra-Ba fraction involved a two-stage purification on a BioRad Poly-Prep column containing 0.6 g of cation-exchange resin (BioRad AG 5OW-X12, 200-400 mesh). After conditioning with 4 X 0.5 mL of 3 M HCl, samples were loaded onto the column in -2 mL of 3 M HCI and were washed in with 4 X 0.5 mL of 3 M HC1 followed by a further 3 mL of 3 M HCl. Ra (together with Ba) was eluted with 5 x 0.5 mL of 4 M HN03. The Ra-bearing fraction was converted to C1- and was reloaded in 0.2 mL of 3 M HC1 onto the cleaned cation column. After washing in with 4 X 0.5 mL of 3 M HCl, a further 6 mL of 3 M HCl was passed, and the Ra (plus Ba) was collected with 5 X 0.5 mL of 4 M HN03. The Ra-Ba fraction, which also contains a large proportion of the rare-earth budget, was evaporated to dryness, in preparation for the Ra-Ba separation. The Ra yield of this first stage was assessed by measuring the recovery of Ba by ICP-MS, which was >98%. Radium-Barium Separation. The procedure developed for the separation of Ra from Ba and the rare earths (essential for the efficient ionization of Ra during TIMS)is, like that of Volpe et al. (I), based on two previously published techniques (7,8). While our procedures are broadly similar, there are two important differences of detail. Firstly, we have used columns of small aspect ratio which are operated under gravity and do not need to be pressurized; two column passes are required to obtain the necessary Ra purity. Secondly, the final Ra-EDTA separation was effected simply by washing out the EDTA with water; the final column separation used by Volpe et al. (I)for this purpose appears unnecessary. Small Teflon columns (capacity 0.15 mL) holding cation-exchange resin (BioRad 50W-X12,200-400 mesh) were cleaned with 4 M H N 0 3 (-1.5 mL), converted to the NH4+ form with 1 M NH,Cl (0.15 mL, then -1 mL), and finally conditioned with solution A (4 X 0.15 mL). The Ra-Ba fraction from the first stage was dissolved in -0.15 mL of solution A, loaded onto the column, and washed in with 3 x 0.15 mL of solution A. Rare earths and any residual Ca, Sr, Al, and Fe were removed in the loading and washing effluents. Ba was then eluted with 4 X 0.15 mL of solution B, whilst the Ra remained on the column. The EDTA was washed out with H 2 0 (5 X 0.2 mL), the NH4+was displaced with 2 M HCl (3 X 0.2 mL), and the Ra was finally recovered with 1.4 mL of 6 M HCl. The Ra-bearing solution was evaporated to dryness, redissolved in 0.15 mL of solution A, and then put through this procedure a second time to yield high-purity RaCIz suitable for analysis by TIMS. Elution curves for Ra and Ba using the 0.15-mL cation columns are shown in Figure 1. These were obtained by using a mixture of 1pg of and 5 pg of Ba (taken from a standard solution), but unlike the procedure described above, both Ba and Ra were eluted with successivealiquots of solution B in order to determine the efficiency of the Ba-Ra separation. Ba was measured by ICP-MS, while the Ra-bearing aliquots were spiked with a known quantity of p8Ra after collection. They were processed to remove the EDTA and the Ra was then determined by TIMS. The total recovery of Ra for this stage was >98%, while the amount of Ba carried over into the Ra fraction was 90% and is h e a r at count rates up to 3 x 106 counb s-l (cps), demonstrated using the set of Central Bureau for Nuclear Measurement (CBNM) EEC uranium standards (see ref 9 and Cohen et al., in preparation). Because useful count rates for Ra+ in TIMS may be very low (
