Anal. Chem. 2008, 80, 6029–6033
Ultratrace Uranium Fingerprinting with Isotope Selective Laser Ionization Spectrometry Summer L. Ziegler* and Bruce A. Bushaw Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 Uranium isotope ratios can provide source information for tracking uranium contamination in a variety of fields, ranging from occupational bioassay to monitoring aftereffects of nuclear accidents. We describe the development of isotope selective laser ionization spectrometry for ultratrace measurement of the minor isotopes 234U, 235U, and 236U with respect to 238U. The inherent isotopic selectivity of three-step excitation with single-mode continuous wave lasers results in measurement of the minor isotopes at relative abundances below 1 ppm and is not limited by isobaric interferences such as 235UH+ during measurement of 236U. This relative abundance limit is attained without mass spectrometric analysis of the lasercreated ions. Uranyl nitrate standards from an international blind comparison were used to test analytical performance for different isotopic compositions and with quantities ranging from 11 ng to 10 µg total uranium. Isotopic ratio determination was demonstrated over a linear dynamic range of 7 orders of magnitude with a few percent relative precision and detection limits below 500 fg for the minor isotopes. The isotopic composition of uranium is important in a number of fields including environmental analysis after nuclear accidents and weapons tests,1–4 health physics and bioassay analyses,5 process control in the nuclear industry (front-end quality and effluent/reprocessing control),4,6 nucleosynthesis and cosmochronology,7,8 and treaty inspection and verification.4,9 In all these applications, isotopic fingerprinting can provide information on whether the uranium present is of anthropogenic or natural origin. The relative abundance of 235U against 238U is typically used to determine whether enrichment of natural uranium (0.725% 235U) * To whom correspondence should be addressed. E-mail: Summer.Ziegler@ pnl.gov. Fax: 509.376.5021. (1) Beasley, T. M.; Kelley, J. M.; Orlandini, K. A.; Bond, L. A.; Aarkrog, A.; Trapeznikov, A. P.; Pozolotina, V. N. J. Environ. Radioact. 1998, 39, 215– 230. (2) Boulyga, S. F.; Becker, J. S. Fresenius’ J. Anal. Chem. 2001, 370, 612– 617. (3) Boulyga, S. F.; Heumann, K. G. J. Environ. Radioact. 2006, 88, 1–10. (4) Zhao, X.-L.; Kilius, L. R.; Litherland, A. E.; Beasley, T. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 126, 297–300. (5) Wyse, E. J.; MacLellan, J. A.; Lindenmeier, C. W.; Bramson, J. P.; Koppenaal, D. W. J. Radioanal. Nucl. Chem. 1998, 234 (1-2), 165–170. (6) Boulyga, S. F.; Becker, J. S. J. Anal. At. Spectrom. 2002, 17, 1143–1147. (7) Cayrel, R.; Hill, V.; Beers, T. C.; Barbuy, B.; Spite, M.; Spite, F.; Plez, B.; Andersen, J.; Bonifacio, P.; Franc¸ois, P.; Molaro, P.; Nordstro¨m, B.; Primas, F. Nature. 2001, 409, 691–692. (8) Dauphas, N. Lunar Planet. Sci. 2005, XXXVI, 1126. (9) Donohue, D. L. J. Alloys Compd. 1998, 271–273, 11-18. 10.1021/ac800764j CCC: $40.75 2008 American Chemical Society Published on Web 07/09/2008
has occurred. The nuclear power industry enriches up to ∼3.5% in 235U (low enriched uranium), while weapons and some test reactors may enrich to >90% 235U (highly enriched uranium). The waste streams from enrichment processes yield depleted uranium with typically 0.2-0.3% 235U. In addition to the 235U/238U ratio, source information can also be gained from the relative abundance of minor isotopes 234U and 236 U. The long-lived isotope 234U (t1/2 ) 2.46 × 105 y) is produced from the decay of 238U and is 0.0055% abundant at secular equilibrium. However, 234U is enriched and depleted along with 235 Ususually to a greater relative extentsand as such can provide additional information on the enrichment process. The other longlived isotope of uranium, 236U (t1/2 ) 2.34 × 107 y), has been used in tracking uranium contamination from Chernobyl2,6 and other accident sites.1 236U occurs in nature only through activation of 235 U, either by self-activation or by cosmic-ray thermal neutrons; abundance in the general environment4 is estimated to be ∼10-14, and below 10-9 in uranium ores.10,11 In contrast, 236U is produced prodigiously in nuclear reactors. Spent fuel from commercial light water reactors (LWR)12 can contain up to a few tenths of a percent 236 U and the relative abundance may rise to greater than 10% for some test reactors.13 Thus, 236U can have an effective enrichment factor of greater than 107 and thus be a very sensitive measure of irradiated uranium, even when diluted by ubiquitous natural uranium.6,14 Measurement of the minor uranium isotopes at ultratrace levels is a challenge for conventional mass spectrometry methods, such as inductively coupled plasma mass spectrometry (ICPMS), thermal ionization mass spectrometry (TIMS), or accelerator mass spectrometry (AMS). Detection limits are largely determined by hydride formation (235UH+ at 236U+) and mass-peak tailing,4,15,16 as well as by uranium contamination introduced during required sample preparation chemistry. Detection limits for 236U have been measured at ∼10-7 relative abundance for ICPMS,3,15 10-5 are also measured.14 Isotope selective laser ionization spectrometry (ISLIS) presents a fundamentally different approach from conventional mass spectrometry. In conventional mass spectrometry, all analytes present are ionized and a mass-selective device separates out ions of specific M/e value to be counted. In contrast, ISLIS methods create ions only from specific targeted isotopes, resulting in inherent isotopic selectivity and dramatically reduced isobaric interference effects. The work reported here uses triple-resonance excitation with high-resolution, single-mode continuous wave (cw) lasers to yield extremely sensitive and selective detection of targeted uranium isotopes, without the need for mass spectrometric analysis. We demonstrate ISLIS measurement of the uranium fingerprint in four uranyl nitrate standards through participation in an international blind comparison study.18 After certification, the blind standards were used to evaluate overall analytical performance. EXPERIMENTAL SECTION Samples and Treatment. Sample Descriptions. The Regular European Interlaboratory Measurement Evaluation Program (REIMEP-18) supplied blind (isotopic composition) samples nominally as 2.5 mg of total uranium in 0.5 mL of 0.5 M nitric acid.18 The four samples were identified as, “depleted to low enriched uranium with total activity of 0.18; the lower value for 235 U is because of nearer time proximity of the two 235U measurements within the cycle. (3) Operator noted errors include observed laser instabilities, electrical noise spikes, or oven current setting errors. Manual rejections are quite rare and affected less than 2% of all the measurement cycles in the work reported here. The raw isotope ratios for a given sample are taken as the weighted mean for all retained cycles, where weights are derived from quadrature addition of the counting statistic uncertainty and an (estimated) 3% instrumental uncertainty. Raw ratios for 234U and 236 U (relative to 238U) are expected to be approximately correct while 235 U/238U ratios should be lower because the starting F ) 19/2 Analytical Chemistry, Vol. 80, No. 15, August 1, 2008
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hyperfine component contains 5/26 of the total ground-level population, based on the number of magnetic substates. Raw ratios measured for 235U are multiplied by 26/5 to account for this known hyperfine partitioning. 235U values exhibit further reduction due to oscillation strength correction. Additional small deviations are expected for all isotopes because of true mass-based biases (e.g., Doppler widths and laser interaction times), differences in laser power and pointing, and uncertainty in isotope shifts. Final values are normalized to values obtained from reference samples of known isotopic composition to correct for recovery rate. The reference samples are measured during the same experimental session and bracket the sample measurements. RESULTS Measurement of Isotope Shifts. The excitation scheme shown in Figure 2 was developed and optimized using 238U, but did not include isotope shift measurements.20 Thus, three-dimensional raster scans were performed to obtain precise isotope shifts for each minor isotope in all three excitation steps, as described for 234U in an earlier excitation scheme.19 Measured transition isotope shifts are given in Table 1. Listed uncertainties are twice the standard error for three to four repeated measurements. REIMEP-18 Blind Measurements. The series of four blind standards (A-D) supplied as part of the REIMEP-18 campaign were measured over a period of several days. Initial calibration measurements were performed on a natural uranium standard (329N), and both 234U and 235U raw values were found to agree relatively well with the expected abundances after correction for recovery rate (both ∼10% low). However, there was an apparent and unexpected presence of 236U at an abundance of ∼1.5 × 10-6. We proceeded to perform a measurement on the REIMEP-A sample, which appeared also to be of (near) natural isotopic abundance. Unlike the 329N standard, the 236U abundance of the REIMEP-A sample was below detection limits and was estimated as
