Anal. Chem. 1985, 57, 1023-1026
l’O1
1023
groups acidified immediately and those acidified at 5 min, 6 h, 3 days, 7 days, and 14 days after sample collection. In general, all acidified samples had average lead recoveries within 10% of their initial control values throughout the study. Therefore, this study indicates that samples collected for lead analysis need not be acidified at the site of collection but must be acidified within 14 days after collection so as not to appreciably affect the experimental results. The data indicate that the analysis can be performed within a day after acidification provided acidification occurs within 14 days of collection. Also, due to some unknown interferences in water analysis, this study shows the need for either matrix matching or methods of standard addition when analyzing unknown water for lead.
p 1 100
,
0
I
10
I
I
I
1
A N A L Y S I S TIME FROM ACIDIFICATION
1.
1
40
30
20
(DAYS)
Figure 4. Percent recovery of immediately acidified group vs. nonacldifled group after correction for dally method varhbllity (20 M/L level): 0, not acldifled; A,acidified immediately.
ACKNOWLEDGMENT The authors wish to acknowledge the assistance of Keith Kelty and Melda Hirth in the preparation of the manuscript. Registry No. Pb, 7439-92-1; HzO, 7732-18-5.
LITERATURE CITED
4j
0.80
0
10
TIME o f
20 ANALYSIS
30
40
(DAYS)
Figure 5. Concentrationratio of control group, acidified immediately, to corresponding nonacldlfled group (70 pg/L level). R = [nonacidified]/[controi].
within the system and not of any true effect of acidification time.
CONCLUSIONS Due to the inherent variabilities within the analytical system it is not possible to make a clear distinction between those
(1) Issaq, Haieem J.; Zieiinski, Waiter L., Jr. Anal. Chem. 1974. 4 6 , 1328- 1329. (2) “Methods for Chemical Analysis of Water and Wastes” U S . Environmental Protection Protection Agency: Washington, DC, March 1979; EPA-600/4-79-020. (3) Guest, Ronald L.; Biutstein, Harry Anal. Chem. 1981, 53, 727-731. (4) Sharrett, A. Richey; Orheim. Robert M.; Carter, Ann P.; Hyde, John E.; Feinieib, Manning Envlron. Res. 1982, 28, 476-498. (5) Cox, David R. “Planning of Experiments”, Wiley Publications in Statistics; Wiiey: New York, 1966. (8) Bueiow, Ralph W.; Miiiette, James R.; McFarren, Earl F.; Symons, James M. J.-Am. Water Works Assoc. 1980, 72, 91-102. (7) “Analytical Methods for Atomic Absorption Spectrophotometry”, Perkin-Elmer Methods Manual; Perkin-Elmer Corp.: Norwaik, CT, Sept 1978. (8) “Analytical Methods for Furnace Atomic Absorption Spectroscopy”; Perkin-Elmer Corp.: Norwaik, CT, 1980; Pub1 332. (9) Henn, E. L. Am. SOC.Test. Mater. 1977, STf618, 54-64. (IO) Sokai, Robert R.; Rohif, F. James “Biometry, The Principals and Practice of Statistlcs in Biological Research”; W. H. Freeman: San Francisco, CA, 1969.
RECEIVED for review October 8,1984. Accepted January 22, 1985. The research described in this article hs been reviewed by the Health Effects Research Laboratory and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Determination of Thorium-228, Thorium-230, and Thorium-232 in Sediments by Anion Exchange and Nuclear Spectrometry S. R. Joshi Environmental Contaminants Division, National Water Research Institute, Canada Centre for Inland Waters, Environment Canada, P.O.Box 5050, Burlington, Ontario L7R 4A6, Canada A method is described for the determination of 228Thy 23”rhy in sediments. The analytlcai protocol employs direct and “%I determingtlon of 22% by high-resolution y-ray spectrometry. Isotoplc ratios are determined by a-particle spectrometric assay of the thorium fractlon purified by a slmpie anion-exchange technique. Three sediment and soli reference materials when analyzed by the described method gave results in general agreement wlth the certified values.
In an ongoing Environment Canada project on the inputs and transport of actinides to Lake Ontario, a simple and 0003-2700/85/0357-1023$01,50/0
reliable method was required for measuring the levels of naturally occurring 228Th,230Th,and in sediments. Such information is necessary for computing very recent ( I ) and historical (2) sedimentation rates and for investigating other geochemical processes. Though several methods exist for the determination of various thorium isotopes, few deal effectively with the simultaneous determination of different thorium isotopes in sediments. Thomson (3)has recently reported a method for the determination of several uranium and thorium isotopes. His method involves addition of 232U/22sTh spike to the sample before its complete dissolution in a pyrosulfate fusion followed by radiochemical purification and a spec-
Published 1985 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
trometry. The principal advantage of the method is that the fusion step ensures that all the radionuclides are in solution. Although this is a very desirable feature of any radiochemical procedure being applied to environmental samples ( 4 ) , its applicability is severely limited by sample size. Also, this method is incapable of measuring 228Th. Other methods designed for the simultaneous determination of 228Th,23"Th, and 232Thin environmental samples use 234Th(5-8) or 22gTh (9,10)as isotopic tracers. Methods based on 234Thas isotopic tracer cannot be used in studies such as ours where one measures 238Uin the sample via its 633-keV 234Thdaughter. Methods employing q h as an isotopic tracer seemed suitable for adaptation, but it was discovered that 229Thhad significant contamination of 2?l?h and that observed recoil contamination could easily decrease detector sensitivity and reliability with prolonged use. In a previous investigation, a method for determining isotopic thorium in leachate samples from uranium mining and milling operations was developed (11). The method involved direct measurement of 228Thin the sample by y-ray spectrometry followed by extraction and purification of the thorium fraction using high molecular weight amines and anion-exchange techniques, respectively, prior to a-particle spectrometry to determine isotopic ratios. The method was found to give satisfactory results for leachate samples high in sulfate ion concentration. However, some interference was occasionally noted in the determination of 228Th. In view of the inherent limitations and operational difficulties associated with methods based on isotopic tracers, it was decided to investigate the utility of direct y-ray measurement technique for analyzing these thorium isotopes in sediments. The method described in the present article takes advantage of several well-established and simple analytical procedures and integrates well with the existing in-house procedures for the determination of actinides in sediments. EXPERIMENTAL SECTION Instrumental Methods. y-Ray intensities were memured with Aptec hyperpure germanium detectors. The 583.1-keV y-ray of 1was measured in a large volume, closed-end coaxial detector, while the 238.6 keV 212Pby-ray was measured with a detector in planar configuration. The active area and depth of the latter detector were 1500 mm2 and 10 mm, respectively, while the resolution for 122 keV was 720 eV. The detectors were shielded by 10 cm of pre-World War I1 lead on all sides. Amplified and shaped pulses from these detectors were analyzed by a Nuclear Data 6620 nuclear spectrometer. The efficiency of the planar detector was determined at four different sample thicknesses by using the Canadian Certified Reference Materials Project (CCRMP) standard BL-5, a uranium ore, obtained from the Canada Centre for Mineral and Energy Technology (12). The denser reference material was diluted with several times its weight of the sediment in order to minimize self-absorption effects which are dominant in the energy range 0-300 keV. The validity of the calibration data at lower activity levels was ascertained by using the less-active CCRMP standards BL-3, DH-la, and DL-1. A homemade standard consisting of a U308/NaHC03mixture of known 238Ucontent and sediment samples individually spiked with 210Pb(46.5 keV), 241Am(59.6 keV), 57Co(122 keV), 228Th (239.6 keV), and 22eRa(186and 352 keV) were also used to validate the efficiency measurements in the low-energy range. The sediment spiking procedure involved addition, with stirring, of a known amount of radiotracer to wet sediment (typically 40 g) in order to facilitate uniform distribution of tracer in the sediment. The sediment was then allowed to go to dryness at room temperature before grinding, using a porcelain mortar and a pestle, and transfer to counting vials. The accuracy of transfer to vial was established by subsequent counting of dry (NaHC03) and wet washings of mortar and pestle. The efficiency of the coaxial detector was also determined at four different sample thicknesses using sediment samples individually spiked with lMCe(133.5 keV), 226Ra(186 and 352 keV), 125Sb(427.9 keV), 13'Cs (661.6 keV), and
6oCo(1173.2 and 1332.5 keV). An EG&G Ortec silicon surface barrier detector (R-series) was used for a-particle spectrometry with a window of 450 mm2,a sensitive thickness of 100 Mm, and a resolution of 18.5 keV for an infinitely thin, 5.486 MeV 241Am source. The detector was mounted in a vacuum chamber coupled to a Nuclear Data 66 pulse-height analyzer. Direct communication between Nuclear Data 66 and 6620 units permitted an efficient handling and storage of a-particle spectrometry data. The shape of filter holders used for mounting the sources resulted in a source-to-detector distance of 1 cm. Such a large path length, coupled with the fact that the detector area was considerably smaller than the source area, normally affords lower counting efficiency and larger low-energy tail due to large-angle scattering into the detector. The adverse effect of large-angle scattering, however, is probably compensated by the improved resolution as the thickness of the deposit is also reduced. Reagents and Column Preparation. All the reagents used were of analytical grade and the solutions were prepared in distilled deionized water. All samples and solutions were stored in polyethylene bottles. 22eThwas obtained from Oak Ridge National Laboratory; all other standardized radioactive solutions were obtained from Amersham Canada Limited. One hundred grams of Dowex 1-X8 resin (100-200 mesh, chloride form; BioRad Laboratories, Richmond, CA) was poured into a l-L beaker prior to the addition of 500 mL of water. The slurry was stirred for about 5 min and, when the bulk of resin had settled, the water was slowly decanted along with the fines. The washing was repeated several times and resin (about 20 g) poured into a glass column and washed with 10-12 column volumes of 2 M NaOH and then with 10-12 column volumes of water. The resin was converted to the nitrate form by washing with 8 M HN03 until a silver nitrate test of the wash showed the absence of chloride. After four to five samples were processed, the resin in the column was replaced by a new batch. Procedure. The dried, powdered, and weighed (usually 30-40 g) sample was stored sealed in a 55-mm diameter polystyrene counting vial for at least 25 days and then the y-spectrum accumulated for up to 2.5 X lo5 s by placing the vial directly on the planar detector. The events in the 238.6-keV l12Pbphotopeak were summed and Compton background subtracted. The net sample count rate was obtained by subtracting background count rate, measured with an empty vial, from the sample count rate. The 2 2 q hconcentration was then calculated from the net sample count rate and the detector efficiency data. The sample was then transferred to a standard reflux unit using 8 M HN03 and sodium nitrite (1 g) and enough 8 M HN03 added to soak the sample completely. The sample was refluxed for 2 to 3 h and filtered hot through Whatman 42 papers. The filtrate was retained and the sample leached again using 8 M HN03. The process was repeated once more and all the leached fractions were combined, cooled to room temperature, and then allowed to pass through the anion-exchange column at a flow rate of about 1 mL/min. The column was washed with three column volumes of 8 M HNO, to remove interfering elements. Thorium was eluted with 3 to 4 column volumes of 10 M HC1. The eluate was reduced to near dryness and organic matter destroyed by dropwise addition of 50% H202to warm solution. The sample source for a-spectrometric assay of thorium isotopes was prepared by lanthanum fluoride coprecipitation technique (13)similar to that described by other workers (14, 15). The a-particle spectrum was accumulated until reasonable counting statistics were obtained. The 230Thand 232Thconcentrations were computed using the ratios of LY peaks of these isotopes to that of 228Thand the 228Thconcentration obtained by y-ray spectrometry. RESULTS AND DISCUSSION Although 228Thdoes not emit easily detectable y-rays itself, a daughter, "'Pb, emits a measurable 238.6-keV (44.6% relative to 22sTh)y-ray and another (208T1) emits a 583.1 keV (30.25% relative to z28Th)and a 2614.5 keV (35.93% relative to 2zsTh) y-ray. Both these radionuclides have half-lives shorter than that of 224Ra,the first decay product of 228Th, and thus grow in with the 224Rahalf-life of 3.64 days. Determination of 2z8Thconcentrations in aqueous or solid samples can be accomplished by measurement of the emission rate
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
CHANNEL NUMBER
1025
U
Figure 1. Partial y-ray spectrum of Lake Ontario sediment sample spiked with 228Th. Energies are in kiloelectronvolts.
E P
3
1 160-
8 120
7 -0
200
400
600
800
1000
1200
CHANNEL NUMBER
Flgure 3. a-Particle spectra of thorium separated from (a) Rocky Flats soil standard and (b) Lake Ontario sediment core section. Energies
in ’megaelectronvolts.
(a)
a
R
(D
R I6a 800
J
CHANNEL NUMBER
Flgure 2. Partial y-ray spectra of (a) 228Th-spikedand (b) unspiked Lake Ontario sediment samples obtained with the planar detector. Energies are in kiloelectronvolts.
of one or all of the above y-rays after a suitable growth period using a high-resolution, low-background detector. The 241keV y-ray from 224Racannot be used due to interference from the 242-keV photopeak of 214Pb. A potential problem in this y-ray spectrometric measurement technique is the likely change in geometry due to the release of 2’%n from the sample and its subsequent dispersal in the sample container. Experiments using 228Th-spikedsamples layered with epoxy glue did not reveal any discernible differences in concentrations of 2 q h derived from nonlayered samples. Other researchers have obtained similar results from the behavior of longer-lived zzzRnin the y-spectrometric measurements of 2zsRain water
(16) and marine phosphorites (17). All the current measurements in our laboratories are, therefore, performed on nonlayered samples. The results presented in this article were also obtained using nonlayered samples. The partial y-ray spectrum (at 1 keV per channel) of a sediment sample, spiked with zz8Thin equilibrium with its decay products, obtained with the coaxial detector is shown in Figure 1. Obviously, the highest sensitivity will be attained by using the 238.6-keV photopeak; the 2614.5-keV 208T1y-ray (not shown) provides only poor sensitivity. The 238.6-keV photopeak, however, is subject to interference from the 242keV photopeak of 214Pbfor samples containing significant amounts of zzsRa. In such cases it is more practical to employ the 583.1-keV photopeak to calculate zz8Thconcentrations. In the earlier investigation (11)we used 238.6-keV photopeak to derive 228Thconcentrations in leachate samples. The error observed in several zz8Thmeasurements was attributed to incomplete separation of 228Thfrom 210Po.Subsequently, with the availability of a planar germanium detector, it was discovered that those leachate samples have in fact significant amounts of 226Raand the error was more likely due to the inability of the Ge(Li) detector used in that study to separate the 238.6-keV photopeak from the 241- and 242-keV peaks to an acceptable degree. Figure 2a depicts the partial y-ray spectrum of 228Th-spikedsediment, used in Figure 1,obtained with the planar detector at 0.25 keV per channel, while Figure 2b shows the spectrum of an unspiked Lake Ontario sediment core section of comparable ambient 228Thand z2sRacontents of about 0.04 and 0.05 dps/g dry sediment, respectively. An inspection of the spectra reveals that the 238.6-keV photopeak
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ANALYTICAL CHEMISTRY, VOL. 57, NO.
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Table I. Results of Measurements on NBS Soil and Sediment Standards certified activity,”
“Rocky Flats Soil” (SRM-4353)
“8Th 230Th 232Th “Peruvian Soil” (SRM-4355) 228Th 2a0Th z3zTh ”River Sediment” 228Th (SRM-4350B)‘ 230Th 232Th
measured activityb
*
7.08 f 0.36 7.24 0.13 4.43 0.26 3.23 A 0.17 6.93 f 0.35 7.07 f 0.28 4.22 f 0.21 4.30 i 0.11 3.97 f 0.20 3.62 & 0.18 4.30 0.21 4.45 f 0.21 3.79 0.10 3.35
* *
2.95
3.32
*
2.45 f 0.18 3.93 f 0.21
Errors, when reported, include those from counting statistics and other analytical protocols. Errors are based only on counting statistics of f l u . ‘Uncertified values given by NBS for this
standard. is easily resolved from the 241- and 242-keV photopeaks on the detector in planar configuration (which affords a resolution about 30% superior to that manifested in the low-energy region, by coaxial detector when operated under similar conditions). The key to an accurate measurement of thorium isotopes by a-particle spectrometry is to obtain thorium fraction free from major sediment matrix components, such as silica, iron, aluminum, and other a-emitters. The described radiochemical procedure meets these requirements. The major matrix elements, the alkaline earths which include the radium isotopes, 241Am,210Pb,and zlOBiare not retained on the nitrate column (18,19). Elution with 10 M HC1 selectively removes thorium but not 210Po,Pu(IV), and U(V1) (20, 21). The a-particle spectra (Figure 3) of various thorium isotopes, isolated from a Lake Ontario sediment core section and a standard reference material (NBS SRM-4353, Rocky Flats Soil), attest to the radiochemical purity afforded by the simple procedure. The accuracy of the method was checked by analyzing several sediment samples spiked with known amounts of 228Th. The results showed that the measured concentrations in these samples were within about 8% of the added amounts. In addition, three NBS environmental soil and sediment reference materials were analyzed for 22?l’h, 230Th,and 232Th.All three reference materials were found to have densities similar to the material used for calibrating the detector. Thus, the self-absorption of y-rays is assumed to be of minor importance in these comparisons. Though no values are given by the NBS for 22aRaconcentrations in these standards, zz8Thand 232Thwere assumed to exist in equilibrium in all three cases based on their reported levels and that of zzsAc for one standard (Rocky Flats soil). Results from these measurements, using the 238.6-keV photopeak for determining zz6Thconcentration, are given in Table I. These data show that most of our measurements agree with the certified values within
1 u counting error. The observed deviations are associated in both cases with the 230Thvalues. It is quite possible that the matrices holding natural thorium and uranium (parent of 230Th)in these standards, especially the Rocky Flats soil, behave differently in the leaching procedure employed. This may result in somewhat less efficient leaching of 23nThcompared with 2z8Thand 232Th.However, this is probably compensated to some extent by the capacity of the leaching procedure employed to handle much larger samples and the resultant reduction of uncertainty in the final results due to natural variability. This is perhaps the greatest source of error in methods based upon total dissolution of smaller samples. The method has been successfully employed in measuring concentrations of 228Th,230Th,and 232Thin over 100 sections of sediment cores retrieved from locations in western Lake Ontario. From start to completion, the data for each section are obtained in about 6 working days including counting time. The lengthy (up to 2.5 X lo5 s) y-ray measurements in our samples are actually necessitated by the extremely low levels of 241Am(0.1-0.3 dpm per gram) which we measure simultaneously in these samples by analyzing the 59.6-keV y-emission of this radionuclide. Thus, in cases where measurement of only thorium isotopes is desired, the y-ray counting time can be substantially reduced.
ACKNOWLEDGMENT The author thanks E. Kokotich and S. P. Thompson for technical assistance. Registry No. 228Th,14274-82-9;230Th,14269-63-7; 232Th, 7440-29-1.
LITERATURE CITED (1) Koide, M.; Bruland, K. W.; Goldberg, E. D. Geochlm. Cosmochlm. Acta 1973, 37, 1171. (2) Krishnaswami, S.; Lal, D. I n “Lakes Chemistry, Geology, Physics”; Lerman, A,, Ed.; Springer-Verlag: New York, 1978; pp 153-177. (3) Thomson, J. Anal. Chlm. Acta 1982, 142, 259. (4) Joshi, 6. R.; Durham, R. W. Chem. Geol. 1978, 18, 155. (5) Sill, C. W. Anal. Chem. 1974, 4 6 , 1426. (6) Percival, D. R.; Martin, D. B. Anal. Chem. 1974, 4 6 , 1742. (7) Kolde, M.; Bruland, K. W. Anal. Chim. Acta 1975, 75, 1. (8) Sill, C. W. Anal. Chem. 1977, 49, 618. (9) Wrenn, M. E.; Singh, N. P.; Ibrahim, S. A,; Cohen, N. Anal. Chem. 1978, 50, 1712. (IO) Singh, N. P.; Ibrahim, S. A,; Cohen, N.; Wrenn, M. E. Anal. Chem. 1979, 57, 207. (11) Durham, R. W.; Joshi, S . R. J . Radloanal. Chem. 1979, 52, 181. (12) Faye, G. H.; Bowman, W. S.;Sutarno, R. CANMET Report 79-4, 1979. (13) Joshi, S. R. J . Radioanal. Chem., in press. (14) Lieberman, R.; Moghissi, A. A. Health Phys. 1988, 15, 359. (15) Sill, C. W.; Wllllams, R. L. Anal. Chem. 1981, 53, 412. (16) Michel. J.; Moore, W. S.;King, P. T. Anal. Chem. 1981, 53, 1885. (17) Kim, K. H.; Burnett, W. C. Anal. Chem. 1983, 55, 1796. (18) Hyde, E. K. ”The Radiochemlstry of Thorium”; National Academy of Sciences-National Research Councll, NAS-NS-3004, 1960. (19) Gibson, W. M. “The Radiochemistry of Lead”; National Academy of Sciences-National Research Council, NAS-NS-3040, 1961. (20) Coleman, G. H. “The Radiochemistry of Plutonium”; National Academy of Sciences-National Research Council, NAS-NS-3058, 1965. (21) Gindler, J. E. “The Radiochemistry of Uranium”; National Academy of Sclences-National Research Council, NAS-NS-3050, 1962.
RECEIVED for review October 5, 1984. Accepted January 3, 1985.
