Anal. Chem. 1997, 69, 996-999
An Internal Standard Method in r Spectrometric Determination of Uranium and Thorium Radioisotopes Using Instrumental Neutron Activation Analysis Anthony R. Byrne* and Ljudmila Benedik
Jozˇ ef Stefan Institute, Jamova 39, P.O. Box 3000, SI-1111 Ljubljana, Slovenia
A new approach to r spectrometric determination of the radioisotopes of uranium and thorium was developed for environmental samples based on 238U and 232Th as internal standards. 238U and 232Th were accurately determined in the sample by simultaneous instrumental neutron activation analysis. Other aliquots of the sample were totally dissolved, uranium and thorium separated by anion exchange, and thin sources electroplated for r spectrometry. From the known mass concentrations of 238U and 232Th, their activity concentrations were obtained, and thus from the relative ratios of uranium (234U, 235U, 238U) and thorium (228Th, 230Th, 232Th) radioisotopes in the r spectra, the absolute isotopic activity concentrations were derived. The advantages of the procedure are that neither the chemical recovery of the radiochemical separation nor the counting efficiency of the r spectrometer is required; the use of internal standards eliminates the need for addition of expensive, calibrated, external radioisotopic tracers such as 232U and 229Th. The new approach was tested on some certified environmental reference materials and compared with the classical method using external radioisotopic tracers. It may advantageously be combined with the standard approach to obtain an independent set of data for quality control.
of equipment, the great bulk of such assays are performed by radiochemical separations, preparation of a “thin” (virtually massless) counting source, often by electrodeposition, and then R spectrometry, usually with a silicon surface barrier (SSB) detector and pulse height analyzer. Such classical procedures can be found detailed in the EML Procedures Manual,3 for example, or in numerous journal articles.4-17 An essential and common feature of these procedures is the addition of a radioisotopic tracer (or “spike”) of uranium and/or thorium at the beginning of the analysis, which evidently should be such as does not occur naturally in the sample; these radioisotopic spikes are usually uranium 232U and 229Th. In this way (provided radiochemical equilibrium is achieved), the chemical yield of the whole procedure can be calculated, and if the radiotracer is of known absolute radioactive concentration, the overall recovery factor inclusive of the counting efficiency is obtained. In the present work, a novel alternative method is proposed and tested, based not on addition of an external radioisotopic tracer but on the use of existing isotopes as internal standards. In the case of uranium this is 238U and for thorium it is 232Th; the mass concentrations of both of these nuclides can be readily determined with good accuracy and precision by instrumental neutron activation analysis (INAA), and thus from the specific
* Address correspondence to this author at the Laboratory for Radiochemistry, Department of Environmental Sciences, Jozˇef Stefan Institute. E-mail:
[email protected]. (1) McMahon, A. W. Appl. Radioat. Isot. 1992, 43, 289-303. (2) Banner, J. L.; Wasserburg, G. J.; Chen, J. H.; Moore, C. H. Earth Planet. Sci. Lett. 1990, 101, 296-312.
(3) De Planque, G., Krey, P. W., Coordinators. EML Procedures Manual; U.S. Department of Energy Report HASL-300; Environmental Measurements Laboratory: New York, NY, 1990. (4) Sill, C. W. Health Phys. 1977, 33, 393-404. (5) Singh, N. P.; Ibrahim, S. A.; Cohen, N.; Wrenn, M. E. Anal. Chem. 1979, 51, 207-210. (6) Sill, C. W.; Hindman, F. D.; Anderson, J. I. Anal. Chem. 1979, 51, 13071314. (7) Singh, N. P.; Ibrahim, S. A.; Cohen, N.; Wrenn, M. E. Anal. Chem. 1979, 51, 1978-1981. (8) Fisenne, I. M.; Perry, P. M.; Welford, G. A. Anal. Chem. 1990, 52, 777779. (9) Singh, N. P.; Wrenn, M. E. Talanta 1983, 30, 271-274. (10) Ibrahim, S. A.; Wrenn, M. E.; Singh, N. P.; Cohen, N.; Saccomano, G. Health Phys. 1983, 44, 213-220. (11) Joshi, S. R. Anal. Chem. 1985, 57, 1023-1026. (12) Jiang, F. S.; Lee, S. C.; Bakhtiar, S. N.; Kuroda, P. K. J. Radioanal. Nucl. Chem., Art. 1986, 100, 65-72. (13) Singh, N. P.; Bennett, D. B.; Wrenn, M. E. Health Phys. 1987, 52, 769773. (14) Singh, N. P.; Bennett, D. B.; Wrenn, M. E. Health Phys. 1987, 53, 261256. (15) Fisenne, I. M.; Perry, P. M.; Decker, K. M.; Keller, H. W. Health Phys. 1987, 53, 357-363. (16) Doretti, L.; Ferrara, D.; Barison, G. J. Radioanal. Nucl. Chem., Art. 1990, 141, 203-208. (17) Parsa, B. J. Radioanal. Nucl. Chem., Art. 1992, 157, 65-73.
996 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
S0003-2700(96)00973-0 CCC: $14.00
Determination of the activity concentrations of the natural R emitting radioisotopes of uranium (234U, 235U, 238U) and/or thorium (228Th, 230Th, 232Th) in environmental samples by R spectrometry following their radiochemical separation is widely practiced in radiochemical laboratories. Such data are required in many studies related to radioecology; to exposure, uptake, and elimination of these radionuclides and their dosimetry; and to a variety of environmental, geological, geochronological, and other processes. Often, the isotopic ratios and the equilibrium or disequilibrium of the isotopes are of prime interest or represent an important methodological tool. Apart from the recent growth of mass spectrometry-based techniques,1,2 which are more expensive and demanding in terms
© 1997 American Chemical Society
activities their activity concentrations can be derived. Then, after radiochemical separation and R spectrometry, only the ratios or relative heights of the R peaks are required to calculate their absolute activities from the 238U and 232Th internal standards. The advantages of such a method are that expensive, calibrated radiotracers are not required, nor are the chemical yield and the counting efficiency needed. The external radiotracers involve some other potential disadvantages too, since decay products are sometimes produced (e.g., 228Th from 232U) and may require corrections or a prior cleanup step; further, if the resolution of the sources is suboptimal, tailing of the tracer peaks may deteriorate the detection limit of adjacent low-energy peaks. An additional advantage of the proposed internal standard method is that INAA is a reference method of proven accuracy for 238U and 232Th and thus provides an added element of quality assurance to the overall results. To test the proposed method and compare it with the classical, external tracer approach, several radionuclide reference materials were analyzed. The sequential separation procedure for U and Th radioisotopes used in this work was based, following total dissolution of the sample, on a simple sequential anion exchange procedure using a single resin column, electrodeposition on stainless steel disks, and R-spectrometry with SSB detectors. EXPERIMENTAL SECTION Instrumental Neutron Activation Analysis. About 300 mg of sample was weighed into polythene ampules, doubly encapsulated to avoid superficial contamination, and irradiated with a standard in our TRIGA Mark II reactor. The time of irradiation was 20-30 h in the rotary specimen rack at a neutron fluence of 2 × 1012n‚cm2‚s-1. The irradiated samples were cooled for 1 week and then transferred to measuring vials and counted on an HP Ge low-energy photon detector (LEPD) or a conventional HP Ge detector (239Np at 106.1 or 277.6 keV and 233Pa at 311.8 keV). As standards, NIST fly ash 1633a with certified U and Th contents of 10.2 ( 0.1 and 24.7 ( 0.3 mg‚kg-1, respectively, was used. (The use of a reference material as an irradiation standard is not normally recommended, but in this case, because of the high accuracy of the certification of 1633a and its suitability as a matrix, a convenient exception was made.) r Spectrometry. Tracer Solutions. 234Th tracer solution was prepared from 5 g of natural uranium nitrate as described in the EML Procedures Manual.3 An electrodeposited source was prepared to check the absence of R emitters. A calibrated 232U tracer solution (169.49 mBq‚g-1) was purchased from the U.S. Department of Commerce, National Institute of Standards & Technology. The original solution of 232U contained decay products (228Th) which needed to be removed (by anion exchange) before use in a sequential procedure for both uranium and thorium radioisotopes. A calibrated 230Th tracer solution (624 ( 6 Bq‚g-1) was purchased from AEA Technology (Harwell, UK). Radiochemical Procedure. Anion exchange columns of height 10 cm, diameter 1 cm were prepared from Dowex 1X8, 100-200 mesh Cl- form resin, washed well, converted to the nitrate form with 0.5 M HNO3, and finally washed with 8 M HNO3 just before use. Approximately 1.5-2 g of sample was weighed in a 100 mL Teflon beaker already containing 234Th and 232U tracer solutions,
dissolved in HNO3, HNO3 + HClO4, and HClO4 + HF and heated on a sand bath to near dryness. (Caution: when using perchloric acid, all recommended safety precautions must be observed, e.g., ref 18). The residue was dissolved in hydrochloric acid, and 50 mg of Fe3+ was added (none for sediments). Thorium and uranium radioisotopes were coprecipitated with iron hydroxide after addition of ammonia solution to pH 8. The precipitate was separated by centrifugation, washed, and then dissolved in 40 mL of 8 M HNO3. The solution was then transferred to the top of the anion exchange column and allowed to flow through at about 1 mL‚min-1. The column was then washed with 50 mL of 8 M HNO3. Thorium radioisotopes (228Th, 230Th, 232Th, and 234Th) were eluted from the column with 50 mL of 7 M HCl and collected in a clean beaker. Uranium radioisotopes (234U, 235U, 238U, and 232U) were eluted from the column with 50 mL of 1 M HCl and collected in another clean beaker. The solutions in both beakers were evaporated to dryness, dissolved in 1 mL of 6 M HCl, and evaporated to small volume. The radioactive sources for R spectrometry were prepared according to Puphal and Olsen,19 by electrodeposition on 19 mm diameter stainless steel disks, active diameter 17 mm, from 5.7% ammonium oxalate in 0.3 M HCl solution at pH 1-1.5. Thorium radioisotopes were electroplated for 4 h at 300 mA, and uranium radioisotopes for 2 h at 300 mA, using a Pt anode. Counting. R activities of uranium and thorium radioisotopes were measured with silicon surface barrier detectors (Tennelec and Nucleus), connected to an EG&G ORTEC Maestro MCA Emulator multichannel analysis system. Chemical Yield and Counting Efficiency. The chemical yield of the separated thorium radioisotopes was measured from the γ activity at 63.3 keV of added 234Th by measuring the stainless steel disk after electrodeposition on an HP Ge low-energy photon detector connected to a Canberra 90 multichannel analyzer system, in comparison to an evaporated aliquot of the 234Th tracer. The chemical yield of the separated uranium radioisotopes was determined from the ratio of the R count rate of the 232U spike on the disk after electrodeposition to that of a source prepared directly from the spike solution. The counting efficiency of the SSB detectors was measured from an electroplated disk of 230Th prepared from the tracer solution of known radioactive concentration. 234Th was added to measure the efficiency of electrodeposition (found to be 100%). RESULTS AND DISCUSSION It should be emphasized that it was not the purpose of this work to develop and recommend improved separation, source preparation, and counting procedures for R spectrometry of uranium and thorium radioisotopes, nor to optimize INAA of these elements. Rather, as a conceptual paper, it was to propose an alternative internal standard technique and to test it on a number of environmental reference materials for which certified or information values are available for uranium and thorium radioisotopic concentrations. For comparison, analyses were performed simultaneouslyson the same sample aliquotssusing the classical external tracer approach. The proposed internal standard method depends on determination of the mass concentrations of 238U and 232Th by INAA, using (18) Bock, R. A. Handbook of Decomposition Methods in Analytical Chemistry; T & A. Constable Ltd.: Edinburgh, 1979. (19) Puphal, K. W.; Olsen, D. R. Anal. Chem. 1972, 44, 284-289.
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Table 1. Comparison of the Activities of Uranium and Thorium Radioisotopes Found in Reference Materials (in Bq‚kg-1) by the Classical Added Tracer Method and the Proposed Internal Standard Method
sample
radioisotopes
river sediment NBS SRM 4350B
Th-228 Th-230 Th-232 U-234 U-235 U-238 Th-228 Th-230 Th-232 U-234 U-235 U-238 Th-228 Th-230 Th-232 U-234 U-235 U-238 Th-228 Th-230 Th-232 U-234 U-235 U-238
radionuclides in Baltic Sea sediment IAEA-300
stream sediment IAEA-313
radionuclides in sea plant IAEA-307
a
INAA
resultsa internal standard added tracer 34.8 ( 2.3 (4) 29.2 ( 1.8 (4)
33.2 ( 2.0 (3) 32.6 ( 1.4 (4) 1.7 ( 0.1 (4) 31.0 ( 3.0 (3) 73.0 ( 1.6 (4) 94.3 ( 8.1 (4) 74.0 ( 3.7 (4) 67.0 ( 2.3 (4) 2.4 ( 0.2 (4) 63.0 ( 1.8 (4) 339 ( 4 (4) 281 ( 11 (4) 322 ( 12 (4) 218 ( 11 (4) 8.3 ( 1.0 (4) 231 ( 14 (4) 1.8, 1.7 1.6, 1.2 1.5 ( 0.1 (4) 16.1, 15.8 16.4 ( 0.7 (4)
33.2 ( 1.4 (4) 27.2 ( 2.7 (4) 32.4 ( 3.0 (4) 36.5, 34.2 2.0, 1.9 32.8, 31.7 72.8 ( 2.9 (4) 90.4 ( 6.6 (4) 73.4 ( 4.0 (4) 68.4, 66.3 1.65, 1.60 68.5, 64.9 322 ( 7 (4) 273 ( 8 (4) 313 ( 7 (4) 221, 234 6.8, 7.6 216, 228 2.0, 1.7 1.7, 1.4 1.8, 1.6 17.4, 16.9 16.9, 16.0
certified or information valueb 33.5 29.5 33.2 33.2 1.7 30.8 64 88 72.4 69* 2.75 64.7* 317* 224* 3.2
14
(Standard deviation; number of determinations in parentheses. b Certified values are marked with an asterisk.
γ spectrometry of the radionuclides neutron capture and β decay:
239Np
and
233Pa
induced by
238
U(n,γ)239U (t1/2 ) 23.5 min) 98 239Np (t1/2 ) 2.355 d)
232
Th(n,γ)233Th (t1/2 ) 22.3 min) 98 233Pa (t1/2 ) 27.0 d)
This technique is well established and widely used as a reference method in reference material (RM) certification.20 In the present work, normal reactor flux irradiations were used; for lower uranium and thorium contents, epithermal neutron irradiation can be advantageously used to improve the accuracy and limit of detection.21 The results obtained by INAA for some environmental RMs, shown in Table 1, demonstrate excellent agreement with certified or information values.22-25 After total dissolution of fresh (non-irradiated) portions of the samples, a simple sequential separation of uranium and thorium was performed. To compare the proposed internal standard technique with the normal external radiotracer technique, known amounts of 232U and 234Th radiotracers were added to each sample aliquot before dissolution, so that both methods of determination (20) Gladney, E. S.; Burns, C. E.; Perrin, D. R.; Roelandts, I.; Gills, T. E. Concentration Data for NBS Biological and Environmental Standard Reference Materials; National Bureau of Standards: Washington, DC, 1984. (21) Ehmann, W. D.; Vance, D. E. Radiochemistry and Nuclear Methods of Analysis; J. Wiley & Sons: New York, 1991. (22) Certificate of Analysis, NBS River Sediment, SRM 4350B, Sept 9, 1981, Washington, DC 20234. (23) Reference Sheet, IAEA-300, Radionuclides in Baltic Sea Sediment, Report No. IAEA/AL/064, Marine Environmental Laboratory, IAEA, Monaco, September 1994. (24) Report on the intercomparison run IAEA-313, Ra-226, Th and U in Stream Sediment, IAEA/AL/037, Vienna, January 1991. (25) Reference Sheet, IAEA-307, Radionuclides in Sea Plant, Report No. IAEA/ AL/014, Marine Environmental Laboratory, IAEA, Monaco, October 1989.
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Figure 1. (a) R Spectrum of uranium radioisotopes from IAEA-300 radionuclides in Baltic Sea sediment. (b) R Spectrum of thorium radioisotopes from IAEA-313 stream sediment.
could be applied simultaneously. The simple radiochemical separation used provided uranium and thorium fractions which, after electrodeposition, gave sufficiently pure R spectra, as shown in Figure 1. The two approaches were compared for reference materials comprising marine and fresh water sediments and a sea plant
material, as shown in Table 1. As evident, good agreement between the two methods and with the certified or information values was obtained for uranium and thorium radioisotopes, demonstrating their good accuracy and precision. In evaluation of the new internal standard technique, it is clear that it has a number of practical advantages. No external tracers of known radioactive concentration are needed, as stated above, since the chemical yield and detector counting efficiency are not required. The results are simply calculated from the relative R peak areas and the internal standard radioactive concentration, as obtained from INAA. It should be noted that not even the sample mass is required. In the classical method, if the added tracers are not of known absolute activity concentrations, the correction factors for chemical recovery and counting efficiency must be determined separately. The use of tracers has some other disadvantages. In the case of 232U, it results in addition of the unwanted decay product 228Th, which should be removed prior to use if a joint assay of uranium and thorium radioisotopes is to be made. This purification step also destroys the calibrated nature of the solution. Further, addition of tracer may deteriorate the limit of detection and accuracy of an adjacent, lower energy peak in the R spectrum when the energy resolution of the source, i.e., the source purity (thickness), is suboptimal. This is the case with 229Th, which has its main R energy of 4.84 MeV rather close to the peak of 230Th at 4.69 MeV. For this reason, we preferred γ counting of 234Th for determination of the chemical yield of thorium. Also, the use of an internal tracer should be more reliable than an external spike when samples are difficult to dissolve, or when there is some problem in achieving radiochemical equilibration (e.g., different valency states). Though the chemical yield is not needed in the internal standard method, as a check on the proper functioning of the radiochemical separation, it may be calculated from the measured R activity of the internal standard and the counting efficiency.
Regarding the disadvantages of the proposed approach, INAA of U and Th in the samples is, of course, required, and this takes up to 10 days because of the need to allow the irradiated samples to decay, reducing interferences from shorter-lived radionuclides, particularly 24Na (half-life 15 h). This means, however, that direct (on-site) access to a reactor is not required, since transport from distant reactor facilities can be arranged. Handling of radioactive samples is involved, but doses to personnel after the necessary 1 week decay period are low. It should be emphasized that INAA is a method of proven accuracy and that, therefore, the determinations it provides of the mass and, hence, activity concentrations of 238U and 232Th enhance the quality of the analytical procedure. Indeed, since INAA results can be obtained at a cost which is very low compared to the overall cost of R spectrometric analysis, it is cost-effective and analytically intelligent to do so and thus obtain, as in this work, a set of independent analytical data for all the uranium and thorium radioisotopes of great value for quality control. This may, in fact, be the most worthwhile application of the proposed method: not necessarily to entirely replace conventional tracer-added R spectrometry of U and Th radioisotopes but to provide independent, supplementary quality control data from the INAA values at minimal extra cost. ACKNOWLEDGMENT We thank the Ministry of Science and Technology, Slovenia and the National Institute of Science and Technology, Washington, DC, for financial support.
Received for review September 23, 1996. December 11, 1996.X
Accepted
AC960973A X
Abstract published in Advance ACS Abstracts, February 1, 1997.
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