Anal. Chem. 1991, 63, 2705-2708 2705

(39) Choudhaty, G. G.; Hutzinger, 0. MechenlsHc Aspects of the Themre1 Formation of Halogenated Organlc Compounds Including Poly- ChlOrinatedDibenzo-p-dioxins; Qorden and Breach: New York, 1983

(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. n 167 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.; Von- nahme, 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.

(40) zhiub, W. M.; Tsang, W. In Human and Envkonmental Rlsks of Chb- rinated 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. In 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. (34) Tong, H. Y.; Arghestani, S.; Gross,' M. L.; Karasek, F. W. Chemo-

(35) Tong, H. Y.; Giblln, D. E.; Lapp, R. L.; Monson, S. J.; Gross, M. L.

(47) Altwicker, E. R.; Kumar, R.: Konduri, N. V.; Milllgan, M. S. Chemo- sphere 198% 78, 577.

Anal. Chem. 1991. 63. 1772.

sphere 1980, 20, 1935.

RECEIVED for review April 1,1991. Accepted September 13, was supported in part by the National

Science Foundation (Grants CHE-8620177 and DIR-9017262).

(36) Tong, H. Y.: Karasek, F. W. Chemosphere 1988, 75, 1219. (37) Olk, K.; Vermeuien, P. L.; Hutzinger, 0. Chemosphere 1977, 6. 455. (38) Lustenhwwer, J. w. A.; elk. K.; Hutzinger, 0.

501. i w o , 9 , 1991. This

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, In- cludlng 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 geo- chemistry 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 ap- propriate 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 de- scribed 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 envi- ronmental 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.

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 ca- pacity 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 La . ) 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 Na2C03 and SrC03 and analytical grade Th(N03)4 were all from Johnson Matthey Chemicals PLC. The zzsRa standard (NIST 4953 D) is known to a precision of &0.8% (2 SE).

Preparation of the 228Ra Spike. The 228Ra spike was pro- duced 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)4 was sufficiently high to render its initial cleanup unnecessary. The 228Ra spike was calibrated against three ac- curately 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)S04 precipitated after the sample had been heated and allowed to stand for -8 h. The sample was then centrifuged, and the precipitate slurried in H20 and transferred to a 1.5-mL centrifuge tube. The precipitate was again centrifuged and the washinglcentrifuging process was re- peated until the pH of the supernate was > -4.

Conversion of Sr(Ra)S04 to an acid-soluble compound is based on the classical Curie-Debierne method (6). The washed pre-

0003-2700/91/0363-2705$02.50/0 0 1991 American Chemical Society

2706 ANALYTICAL CHEMISTRY, VOL. 63, NO. 23, DECEMBER 1, 1991

cipitate was slurried with -1 mL of 0.5 M Na2C03 and 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 pro- cedure 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 -(I-2) X lo7 atoms, equivalent to (3.5-7) X Bq of 228Ra). 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 evap- orated 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 nec- essary 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-ex- change resin (BioRad 50W-X12,200-400 mesh) were cleaned with 4 M HN03 (-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 H20 (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 - 1 pg of a and 5 pg of Ba (taken from a standard solution), but unlike the procedure described above, both Ba and Ra were eluted with successive aliquots 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 <0.5%.

Mass Spectrometry. (a) Instrument Details. All mass spectrometry was performed on a Vacuum Generators 354 thermal

2 4 6 8 1 0 1 2

Column volumes Flgure 1. Elution curves for Ra and Ba using 0.01 M NH,EDTA, pH = 8.94. See text for details.

ionization mass spectrometer. This instrument is equipped with a Daly ion-counting detector, a fast scintillator (NE 102A, from Nuclear Enterprises), and Thorn EM1 9813B photomultiplier. An Ortec preamplifier was coupled to a Stanford Research Sys- tems SR400 gated photon counter. This system has an overall efficiency of >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 (<lo cps), a low-background ion- counting system is essential. The system used here has a re- producible background of -0.1 cps.

(b) Filament Design and Ionization Efficiency. Samples were loaded onto the central W filament of an outgassed triple Ta-W-Ta assembly, supported on a Cathodeon bead. Filament dimensions were 8 mm X 0.75 mm X 0.025 mm. Approximately 0.5 pL of a Ta-HF-H3P04 activator solution, similar to that described by Birck (IO) for Sr analysis, was loaded onto the center of the W filament. This was followed by the sample, dissolved in -0.5 pL of - 1 M HCl. The combined solution was dried gently by passing a current through the filament, and the filament finally glowed at dull red for 20-30 s. Typical ionization efficiencies (ions collected/atoms loaded) for samples loaded in this way were in the range 10-15% for standard solutions and were about a factor of 2 lower for samples, probably due to the presence of residual impurities.

(c) Hydrocarbon Interferences. At the very low count rates used for the TIMS analysis of Ra from small samples, hydrocarbon background within the Ra mass range is potentially a major problem. These hydrocarbon interferences may often be observed at most masses in the vicinity of the expected Ra peaks a t an intensity (- 1-100 cps) such that their presence would severely restrict the size of sample that could be measured.

Hydrocarbon interferences were eliminated completely in this work by careful handling of filament assemblies and source components and by the use of a liquid N, cryotrap positioned within the source chamber. By thermal cycling of the triple-fi- lament assembly at temperatures below those of significant Ra evaporation, the filament area was cleaned of organics by dis- placement and cryotrapping. For all results reported here, hy- drocarbon interferences were reduced to around background levels (-0.1 cps).

(a) Running Conditions. After the hydrocarbon interference at 225 amu had been reduced to C0.5 cps in the manner described above, the central filament current was increased until either mRac or %a+ was observed. Samples of - 10' atoms of Ra from either the 22BRa standard or the neRa spike would easily provide beams of (2-3) X lo2 cps, which were large and stable enough to allow the ion beam to be optimally extracted and focused. Where beam sizes for the silicate or seawater samples were below 100 cps and the sample lifetime potentially relatively short, direct focusing on a Ra beam was not attempted. Instead, extraction and focusing utilized Ba+ derived from residual Ba within the load. This was found to give an excellent approximation for the optimum conditions for Ra+.

Data collection proceeded by integrating each ion beam for 7.5 s, allowing 1.5 s for settling during magnet switching. The beam centering routine and background measurement (25-9 integration) were performed every 6 min. The interference at 225 amu was

4 7 c I I ." 17.3 17.1

d 16.9 00

- - Mean -

Table I. Isotopic Composition of the Ra Spike"

sample size

1 -3 4.7164 f 0.0354 2 -3 4.7103 0.0438 3 -3 4.7002 * 0.0499 4 -3 4.7010 * 0.0439 combined datab 4.7082 f 0.0212

analysis (lo6 atoms of 2"Ra) 228Ra/228Ra

"Uncertainties are at 95% confidence limits, based on ratio The combined result is the measurement counting statistics.

weighted mean of the four individual analyses.

monitored immediately before and after data collection and was usually indistinguishable from the typical background count level of 0.1 cps.

(e) Mass Fractionation. An assessment of Ra mass frac- tionation was made by collecting data in blocks of 20 nsRalmFta ratios during the course of an extended analysis. Approximately 0.25 pg of =Ra was spiked with -4 X 10' atoms of nsRa; 22eRa+ was run at 4000-8000 cps. Each block of 20 ratios represented -8 min of data collection or (4.5-9) X lo4 counts of 2zsRa+ per block. The results for each block of 20 ratios are shown in Figure 2, together with the overall mean for the data set. Block 3 has been excluded because the errors are significantly larger than those of the other blocks, probably resulting from a 50% decrease in beam intensity a t this stage of the analysis.

These results demonstrate that instrumental fractionation does not produce any significant bias over the course of an analysis. Furthermore, we note that the external SD of the 11 means (0.45%) is very close to the typical internal SE (mean = 0.48%) for the 11 data blocks.

RESULTS A N D DISCUSSION Composition of the zzsRa Spike. Four independent de-

terminations of the neRa spike composition were made, each using -1 X lo7 atoms of PSRa. The quantity of % analyzed in each determination was -1 fg, equal to -3 x loe atoms or -4 X lo4 Bq. The =Ra+ beam size was generally - 5 M cps, and data were collected for 3000-4000 s. The results are presented in Table I. The precision of each determination is - 1'70 (95% confidence limits). Combination of the four data seta yields a mean zzsRa/226Ra = 4.708 & 0.21 (95% confidence), representing a combined precision of f0.45%.

Abundance Determinations. The mRa abundance was determined in two -35-mL aliquots (DP1 and DP2) of deep Pacific seawater (Table 11). The calculated zz6Ra abundances are 2.854 X los atoms/kg (DP1) and 2.941 x lo8 atoms/kg (DP2). The difference between the two results is 3%, which is slightly greater than the expected uncertainty based on counting statistics and is probably the result of an unusually short analysis time for DP1. The precision for DP2

15.9 15.7

4NALYTICAL CHEMISTRY, VOL. 63, NO. 23, DECEMBER 1, 1991 2707

Table 11. Ra Abundance Data for Seawater and Silicates"

seawater sample abundance sample size, g (lo8 atoms of 22BRa/kg)

DP1 (SW) 37.51 2.854 * 0.042 DP2 (SW) 37.86 2.941 f 0.033

silicate sample size, abundance

mg (le atoms of mRa/g) - -

84-6 (B) 84-29 (B) H-81 (1) (BA) H-81 (2) (BA)

first load second load first load second load first load second load first load second load

20.89 20.89 17.35 17.35 7.60 7.60 7.53 7.53

2.413 f 0.024 2.361 f 0.037 1.767 f 0.021 1.737 f 0.021

11.97 f 0.23 11.65 f 0.14 12.07 f 0.19 11.84 f 0.16

"Uncertainties: as for Table I. Samples are as follows: B = basalt, BA = basaltic andesite, SW = seawater.

Table 111. zzsRa/z28Ra Isotope Ratio in a Zircon"

sample sample size, mg 226Ra/226Ra

ESPl -8 524.3 f 4.9 ESP2 -8 523.2 f 6.8

Uncertainties: as for Table I.

is just over 1% at 95% confidence. % abundances were determined for two basalts (84-6 and

84-29) and for one basaltic andesite (H-81) from Iceland. Sample H-81 was analyzed in duplicate. The final Ra fractions from each sample were split in two, and each split was mea- sured independently. The eight results are shown in Table 11. No corrections have been made for the intrinsic 228Ra of the sample, because in every case the amount of 2zsRa added from the spike was - lo3 times greater. Differences between the replicates of the two basalt samples 84-6 and 84-29 and between the four determinations of the basaltic andesite H-81 are all less than 2.6%. Indeed, the mRa abundances all agree within the uncertainties based on the measurement counting statistics of the 2z6Ra/22sRa ratios. Typical uncertainties in the 226Ra/228Ra ratios are 1-2% a t 95% confidence.

22sRa/228Ra in Zircon. Ra was extracted from two -8-mg aliquots (ESP1 and ESP2) of a 3000000-year-old zircon sample (containing -200 ppm U) from the Massif Central, in order to determine the natural zz6Ra/228Ra ratio for this sample. The U content of each aliquot (- 1.6 pg) is such that each should contain -0.6 pg of 226Ra, assuming secular equilibrium for the 23sU decay series. 2z6Ra/22sRa ratios measured for ESPl and ESP2 are given in Table 111. The two determinations (524.3 f 4.9 and 523.2 f 6.8) are in ex- cellent agreement, the 95% uncertainties being close to 1 %. The zz8Ra+ beam size was 25-30 cps during data collection.

Contamination a n d Blank Levels. The determination of 226Ra blank levels is made potentially difficult by (1) the necessity to measure a very small total number of nsRa atoms and (2) the fact that 18% of the Ra in the spike is 226Ra. Nevertheless, an attempt was made to measure directly the 2zsRa procedural blank for the entire silicate separation me- thod; the blank was spiked with a known amount of 228Ra before the solution from the final Ra-Ba separation was evaporated to dryness. The spiked blank composition was indistinguishable, within the measured uncertainties, from that of the spike. However, a theoretical maximum blank con- tribution may be calculated by assuming that there is a real difference between the composition of the spiked blank and the composition of the spike itself. A difference equal to 4 X SE of the spiked blank measurement would represent a

2708 ANALYTICAL CHEMISTRY, VOL. 63, NO. 23, DECEMBER 1, 1991

blank contribution of 1 x 106 atoms of %, or approximately 3% of a 1-fg Ra sample.

Isotopic Composition of the 228Ra Spike. The z2aRa/ 22sRa ratio of the spike is governed by the 232Th/23@I'h ratio of the T h compound from which it was separated, the purity of the T h compound, and the time lapsed since the T h com- pound was manufactured or purified. The 232Th/230Th ratio of the T h compound will itself have been determined by the Th/U ratio of the original material from which the T h com- pound was produced. If, for example, the original material had a Th /U ratio = 3, then, under the assumption that the T h and U decay series were in a state of secular equilibrium, the ( % T h / T h ) activity ratio will be -1. Consequently, the 226Ra and 22aRa production rates will be almost the same in any T h compound manufactured from this material. In practice, the best way of discovering whether any given T h compound contains Ra of an isotopic composition suitable for use as a spike is to measure it.

Choice of Chemical Separation Methods. The high concentration of Na present in seawater samples must be taken into account during their analysis. Although the sep- aration of Ra could have been effected by cation exchange, this would have involved the use of much larger reagent volumes. Therefore, Ra coprecipitation using a carrier was chosen as being a more suitable method. The two carriers which are normally used, Ba and Pb, are both unsuitable here. The presence of Ba suppresses the ionization of Ra from a thermal filament, and as i t is notoriously difficult to separate i t from Ra chemically, further Ba addition would be coun- terproductive. P b is unsuitable because it is complexed in the presence of C1- and does not precipitate on the addition of H2S04. For these reasons, Sr was used as the carrier for Ra.

The subsequent removal of the Sr carrier was achieved by conventional cation-exchange chromatography, as was the initial separation of Ra from silicates, as this method is both simple and effective. For the final Ra purification, a survey of Ra-Ba separation techniques (4, 7,8,11,12) suggested that cation exchange using EDTA as eluant gave the highest h - B a separation factor, a conclusion also reached by Volpe et al. (I). The method of Nelson (8) was adapted as described here to remove both EDTA and NH4+ and yields a fiial Ra fraction which ionizes readily.

Advantages of Ra Determinations by TIMS. (1) 226Ra. The major advantage of measuring 226Ra by TIMS using methods described in ref 1 and those herein, rather than by traditional counting methods, is that TIMS is capable of far greater sensitivity. This advantage stems from two facts: firstly, the specific activity of 2z6Ra (A = 1.35 X lo-" s-l) is such that relatively large numbers of nsRa atoms are required in order to measure the activity; secondly, the ionization ef- ficiency of Ra by TIMS, as reported in this study and by Volpe e t al. (I), is high (-10%).

The following example serves to illustrate these points. In a traditional counting analysis with 20% counting efficiency, a precision of 1% (95% confidence) would be achieved on -2 pg (-5.3 X lo9 atoms) of 22sRa in - 1 month. The only way to determine smaller samples would be by longer counting times, and in practice this would not be readily possible. In contrast, the determination by TIMS of as little as 1 fg of 228Ra to a precision of - 1 % , as reported here, takes only - 1 h of data collection. The theoretical counting time for a sample of this size, measuring its activity, would be 211 years. (2) 228Ra. Because the specific activity of z2aRa in -260

times greater than that of 228Ra, the advantages of measuring 228Ra by TIMS are not as readily apparent if only the factors mentioned above are considered. However, the direct analysis of 228Ra by traditional counting methods is difficult (4, 5) because it is a soft P emitter. For this reason, it is usually determined by counting its daughter P'Ac, tl12 = 6.1 h) or its granddaughter ( T h , t l I 2 = 1.9 years) or by counting 2uRa ( t l / 2 = 3.6 days), the daughter of 228Th ( 4 , 5 ) . None of these methods is both straightforward and sensitive and thus it is likely, as shown here and by Volpe et al. (I), that TIMS might also be of much value for the determination of 228Ra.

ACKNOWLEDGMENT We thank Nick Belshaw for generous help with the ion-

counting mass spectrometry and the electronics and com- puting. Karl Gronvold kindly provided the Icelandic samples, Hany Elderfield the Pacific seawater, and Michel Condomines the zircon. ICP-MS analyses were performed a t the NERC facility a t Egham with the generous help of Kym Jarvis and colleagues.

LITERATURE CITED Volpe, A. M.; Olivares, J. A.; Murrell, M. T. Anal. Chem. 1991, 63, 913. Michel, J.; Moore, W. S.; King, P. T. Anal. Chem. 1981, 53, 1885. Mathbu, G. G. Annual Technical Report COO-2185-0; Lamont-Doherty Geological Observatory: New York, 1977. Vdovenko, V. M.; Dubasov, Yu.V. The Analyfical Chemlshy of Radyum; John Wiley and Sons: New York, 1974; p 198. Ivanovich, M.; Harmon, R. S. Uranium Series Disequilibrium: Applica- tions to Environmental Problems. Oxford University Press: Oxford, U.K., 1982; p 571. Curie, M.; Debierne, A. Compt. Rend. 1910, 751, 523. Duyckaerts. G.; Lejeune, R. J . Cbromtogr. 1980, 3. 56. Nelson, F. J . Chromatcgr. 1984, 76, 403. Lycke, W.; de Bievre, P.; Verbruggen, A.; Hendrickx, F.; Rosman, K. Fresenius' 2. Anal. Chem. l98& 331, 214. Birck, J. L. Chem. Geol. 1988, 56, 73. Weigel, F., Ed.; Gmelin tiandbuch der anorganischen Chemie: Radi- um: Springer-Verlag: Berlin, 1977; Vol. 11. p 435. Korkisch, J. Modern Methods for the Separation of Rarer Metal Ions; Pergamon Press: Oxford, U.K., 1969.

RECEIVED for review May 3, 1991. Accepted September 13, 1991. This work was supported by NERC and the Royal Society (Department of Earth Sciences Contribution No. 1963).