Anal. Chem. 1994,66,4154-4158
isotopic Analysis of Uranium Using Glow Discharge Optogalvanic Spectroscopy and Diode Lasers C. M. Barshick,* R. W. Shaw, J. P. Young, and J. M. Ramsey Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 3783 1-6375
A hollow cathode glow discharge has been coupled with tunable lasers for isotopically selective excitation of gaseous uranium atoms produced by cathodic sputtering. A CW argon ion laser-pumped titanium:sapphire ring laser and a semiconductor diode laser were employed. Optogalvanic detection of the discharge atom population allowed identification of 235Uat depleted, natural, and enriched abundances in uranium metal and uranium oxide samples based on the spectral signatures of two transitions. Isotope ratio measurements were precise from run to run (internal precision) to better than kl% relative standard deviation at lu; external precision (sample to sample) was on the order of &3%relative standard deviation at lu. As in mass spectrometry, the most accurate analysis will be obtained when a bias correction is employed to correct for day to day variations in the laser and discharge conditions. Conventional methods for measuring isotope ratios require sophisticated instrumentation well suited to an analytical laboratory. In particular, thermal ionization mass spectrometry, while a very accurate and technique for measuring isotope ratios, typically requires extensive sample preparation and is too cumbersome for any setting other than a laboratory. One exception to this generalization is the ion trap mass spectrometer that is currently being employed for field measurements of organic pollutants.4 Although a similar instrument has been proposed for measuring the concentrations and isotopic abundances of inorganic analytes in contaminated soil^,^,^ little has been done with this instrument other than proof of principle experiments in the laboratory. As an alternative to the use of conventional mass spectrometers, we have been pursuing glow discharge atomization coupled with high-resolution optogalvanic spectroscopy (OGS) for quan(1) Heumann, K. G. In Znovganic Muss Spectromety; Adams, F., Gijbels, R, Van Grieken, R, Eds.; John Wiley & Sons: New York, 1988. (2) Dubois, J. C.; Retail, G.; Cesario, J. 1nt.J.Mass Spectrom.Zon Processes 1992, 120, 163. (3) Tumer, P. J. In Applications oJPlasma Source Mass Spectrometty ZI; Holland, G., Eaton, A N. Eds.; Royal Society of Chemistry: Cambridge, U.K., 1993. (4) Wise. M. B.; Thompson, C. V.; Buchanan, M. V.; Meniweather. R.; Guerin, R Spectroscopy 1993,8, 14. (5) Duckworth, D. C.; Barshick, C. M.; Smith, D. H.J. Anal. At. Spectrom. 1993. 8, 875. (6) Duckworth, D. C.; Barshick, C. M.: Smith, D. H.; McLuckey, S. A. Anal. Chem. 1994,66, 92.
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Analytical Chemistry, Vol. 66, No. 23, December 7, 7994
tilication of uranium isotope ratio^.^ Further, we wish to make use of diode lasers for excitation. This approach has been considered by others! but we have focused on demountable discharge cellsg for practical isotope ratio measurements for realistic samples. While our eventual goal is a fieldable instrument, this report describes our laboratory-based results to date. A field instrument does not require the accuracy and precision of a laboratory mass spectrometer but must be sensitive to changes in sample isotopic composition, be free from interferences, and have sufficient precision for a decision to be made about the fate of the field sample, Le., does the sample warrant further laboratory analysis? The technique ideally should require a minimum of sample preparation and should at the same time be amenable to a variety of sample types. We are conducting experiments to determine whether the technique described herein meets these requirements for field use. In a glow discharge, the cathode is bombarded by energetic gas phase ions. This produces a popuiation of gaseous cathodic ions directly amenable to isotopic analysis using a mass spectrometer (glow discharge mass spectrometry, GDMS) .lo Neutrals are produced as well and are amenable to a variety of other techniques, including absorption," emission,12and laser-induced fl~orescence~~ spectroscopies. In the technique pursued here, the sputtered atoms are resonantly excited to a higher lying energy level, from which they may be ionized by collisions. The change in the discharge voltage brought about by disruption of the ionization equilibrium serves as an indicator of optical absorption and is the basis of optogalvanic signal c01lection.l~ Semiconductor diode lasers are coherent light sources that generally operate in the near-infrared portion of the electromagnetic spectrum. They are conveniently tunable by electronic means and are rugged en.ough for field use. Gagne and coworkers'j as well as 0thersl~8~~ have performed a number of fundamental spectroscopic investigations of uranium in this (7) Keller, R.A.; Engleman, R, Jr.; Zalewski. E. F. J. Opt. SOC.Am. 1979,69, 738. (8) Lipert, R J.; Lee, S. C.; Edelson, M. C. Appl. Spectrosc. 1992,46, 1307. (9) Tong, W. G.; Shaw, R W. Appl. Spectrosc. 1986,40, 494. (10) King, F. L.; Harrison, W. W. In Glow Discharge Spectroscopies; Marcus, R. K, Ed.; Plenum Press: New York, 1993. (11) Ohls, K. Fresenius' 2. Anal. Chem. 1987,327, 111. (12) Broekaert, J. A. C. In Glow Discharge Spectroscopies; Marcus, R K., Ed.; Plenum Press: New York, 1993. (13) Van Dijk, C.; Smith, B. W.: Winefordner, J. D. Spectrochim. Acta 1982, 37B, 759. (14) King, D.S.; Schenck, P. IC;Smyth, IC C.: Travis, J. C. Appl. Opt. 1977,16, 2617. 0003-270019410366-4154$04.50/00 1994 American Chemical Society
Table 1- Isotopic Abundances (YO)of Samples Studied
sample enriched natural depleted
233U
~0.001
234U
0.063 0.005
235U
9.967 0.720
99.5
spectral range, and data concerning isotope shifts and relative intensities of many optical transitions are available. We have narrowed our present consideration to two particular uranium transitions originating at low-lying initial states and with wave lengths of 778.42 and 776.19 nm. In this paper, we report spectral and isotope ratio results for these two uranium transitions. The emphasis has been on demonstrating the usefulness of the technique for distinguishing samples based on their 235U content (e.g., depleted versus enriched). Isotope ratio measurements have been limited to preliminary studies of accuracy and precision to determine the utility of this method. EXPERIMENTAL SECTION
The preliminary spectroscopy was facilitated through the use of a commercial depleted uranium (0.3%235U) hollow cathode lamp (Model 3QQAY/U, Cathodeon, Ltd.) with a cylindrical metal cathode that is open at both ends. For all of the other studies, glow discharge cathodes were prepared in the shape of hollow cylinders 4.85 mm in diameter x 2.54 mm in length with an i.d. of 2.38 mm. These cathodes were fabricated by machining uranium metal into the desired shape or by pressing uranium Pa for 5 min. The powder in a hollow cathode diel8 at 3.4 x pressed samples comprised pure uranium metal powder and uranium oxide powder (natural and depleted compositions) homogenized with silver powder (325 Mesh, 99.99+%, Aldrich Chemical Co., Milwaukee, WI) in a 50/50 wt % ratio. Caution! Uranium metal is pyrophoric when finely divided. Because uranium is radioactive, special precautions are necessary. Uranium is an a emitter, and care should be taken to minimize ingestion and inhalation. Uranium also emits low to moderate levels of ,8 and y radiation, and care should be taken to minimize exposure. Anyone contemplating research with uranium is well advised to seek the help of a health physicist. Table 1shows the isotopic compositions of the samples used. The cathodes were held in a stainless steel ring that was welded to a 1.59 mm diameter rod (see Figure 1). The bulk of the sample holder was shielded with a quartz tube that was required to keep the discharge from arcing near the high-voltage feedthrough. A discharge formed over the entire cathode and holder ring. Uranium atoms were measured at the center of the cylinder bore, where the atom density is reported h i g h e ~ t . ~ ~ ? ~ ~ ~~~
(15) Demers, Y.; Gagne, J. M.; Dreze, C.; Pianarosa, P. J. Opt. SOC.B 1986,3, 1678. Piyakis, K. N.; Gagne, J. M.J. Opt. SOC.B 1989,6,2289. David, E.; Gagne, J. M. Appl. Opt. 1990,29, 4489. (16) Blaise, J.; Radziemski, L. J., Jr.1. Opt. SOC.Am. 1976,66, 644. (17) Engleman, R; Palmer, B. A.J. Opt. SOC.Am. 1980,70,308. Palmer, B. A; Keller, R A; Engleman, R Informal Report No. LA-8251-MS; Los Alamos Scientific Laboratory, Los Alamos, NM, 1980. (18) Marcus, R K. Ph.D. Dissertation, University of Virginia, Charlottesville,VA, 1986. (19) Caroli, S.; Alimonti, A. Petrucci, F. In Improved Hollow Cathode Lamps for Atomic Spectroscopy; Caroli, S., Ed.; Ellis Honvood Limited Publishers, Halsted Press: New York, 1985.
Figure 1. Demountable hollow cathode glow discharge Cell.
The discharge support gas for this study was argon, maintained at 500-900 Pa in the source. Flow rates were approximately 2-3 cm3(STP) min-I. During the analysis, the cathode was held at constant current, ranging from 20-50 mA. At these pressures and currents and with a 5 kQ ballast resistor in the circuit, the applied voltage ranged from 250 to 500 V. This relatively large discharge power range was necessary to facilitate signal collection for a variety of samples that included uranium metal enriched to 10%235Uand depleted uranium oxide with approximately 0.3%235U. Figure 1 is a drawing of the discharge cell showing the demountable cathode as an inset. Pumping was provided by a 0.4 L/s mechanical pump, and the ultimate base pressure was
