Anal. Chem. 1995,67, 3814-3818
Evaluation of the Precision and Accuracy of a Uranium isotopic Analysis Using Glow Discharge Optogalvanic Spectroscopy C. M. Barshick,* R. W. Shaw, J. P. Young, and J. M. Ramrey
Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 3783 1-6375
The measurement precision and accuracywere evaluated for an isotopic analysis of uranium using diode laserexcited optogalvanic spectroscopy. The ratio of 235U/ (235U+ 238U) was measured using the 776 nm uranium isotope line for five samples ranging from depleted to enriched 235Ucomposition. The percent relative emor (accuracy) with respect to thermal ionization measurements ranged from 2.38% for a nominally 20% enriched 235U sample to slightly greater than 30% error for a depleted one. Run-to-run, day-to-day, and sample-tosample precision (repeatability)were measured using an enriched sample; the run-to-run precision ranged from &1.9% to &5.5% RSD, and the day-to-dayprecision was +2.6% RSD. The sample-to-sample precision, determined using three Merent cathodes, was found to be &3.5% RSD. The ratios showed no trends or biases, vaqying about a mean of 0.21 and ranging from a high of -0.23 to a low of 0.18. A search was conducted for a stronger atomic line and a matching higher power diode laser in an attempt to improve the signal-to-noiseratio for the depleted case. Using a 150 mW diode laser that produced nominally 42 mW chopped power at 831.84 nm, a depleted uranium oxide sample was found to contain 0.26% 235U, within 3.7% of the value found using thermal ionization mass analysis. Using this same laser, the run-to-run reproducibility improved to &7.8% from f13.6%. This level of accuracy and precision is s d c i e n t for screening applications, where preliminary information about the isotopic composition of a sample provides the incentive for additional analysis using more precise techniques. Several techniques have been developed for isotopic analysis, including decaycounting1S2and mass ~pectrometry,~-~ as well as methods that rely on the accessibility of optical transitions to provide isotopic selectivity (e.g., laser-iiduced fluorescence6and (1) Knoll, G. F. Radiation Detection and Measurement; John Wdey & Sons: New York, 1989. Overman, R T.; Clark, H. M. Radioisotope Techniques;McGrawHill Book Co., Inc.: New York, 1960. (2) El-Assaly, F. M. Methods of LuwLevel Counting and Spectrometry, Proceedings of an International Symposium on Methods of Low-Level Counting and Spectrometry, Berlin, Germany; Intemational Atomic Energy Agency: Vienna, 6 1 0 April, 1981. (3) Heumann, K. G. In Inorganic Mass Spectrometry; Adams, F., Gijbels, R, Van Grieken, R, Eds.; John Wiley & Sons: New York, 1988. (4) Dubois, J. C.; Retail, G.; Cesario, J. Int.]. Mass Spectrom. Ion Pmcesses 1992, 120,163-177. (5) Tumer, P. J. In Applications ofPhma Source Mass Spectrometry I t Holland, G., Eaton, A N., Eds.; Royal Society of Chemistry: Cambridge, 1993.
3814 Analytical Chemistry, Vol. 67, No. 20,October 75, 7995
optogalvanic spectroscopy7~4.It is this latter technique that we have investigated,focusing on the use of a dc glow discharge (GD) for atomization, a demountable discharge cell with a novel cathode arrangementgJOfor rapid sample exchange, and rugged, fieldable, high-power (> 30 mW) diode lasers to excite optical transitions. Our goal is a fieldable instrument that will provide uranium isotope ratio information; in this report, we describe the laboratory-based precision and accuracy results that influence the design. The GD is well suited for optogalvanic spectroscopy. Sample preparation is minimal, and the GD is amenable to a variety of sample types. Sputtered neutral species are produced in relatively large abundance (> 108 cm-3), providing a sufficient population for optical excitation to higher lying energy levels. Because the GD operates at pressures between 100 and 1500 Pa, the collision frequency is quite high (0.6 x 107-9 x lo7 collisions/s); this is suitable for optogalvanic spectroscopybecause ionizing collisions can bring about the change in discharge voltage necessary to produce a signal. By scanning the wavelength of a narrow bandwidth laser over a resonant transition, individual isotopes can be excited and quantified. Diode lasers are well suited for this purpose. These semiconductor coherent light sources operate in the near-infrared (630 nm to 1.5 pm) and are conveniently tunable over a few nanometers by changing their temperature or drive current. Diode lasers are also rugged, an attractive feature when the goal is field deployment. Several hundred uranium transitions have been identified in the diode laser accessible range; Gagne and c o - ~ o r k e r s , ~ as~ - ~ ~ well as others,14-16have performed a number of fundamental spectroscopic investigations of uranium. Data concerning isotope shifts and relative emission intensities of many optical transitions14 are available. In this paper, we assess the relative optogalvanic strength of several transitions and identify suitable candidates for practical uranium isotopic analysis. We have limited our investigation to transitions that are accessible to commercially available (6) Cannon, B. D. Personal communication, 1995. (7) Keller, R A; Engleman, R; Zalewski, E. F. 1.Opt. Soc. Am. 1979,69, 738-742. (8) Idpert, R J.; Lee, S. C.; Edelson, M. C. Appl. Spectrosc. 1992,46, 13071308. (9) Barshick, C. M.; Shaw, R W.; Young, J. P.; Ramsey, J. M. Anal. Chem. 1994,66,4154-4158. (10) Tong, W. G.; Shaw, R W. Appl. Spectrosc. 1986,40, 494-497. (11) Demers, Y.; Gagne, J. M.; Dreze, C.; Pianarosa, P. j. Opt. Soc. B 1986,3, 1678- 1680. (12) Piyakis, K N.; Gagne, J. M. ]. Opt. Soc. E 1989,6, 2289-2294. (13) David, E.; Gagne, J. M. Appl. Opt. 1990,29, 4489-4493. (14) Blake, J.; Radziemski, L. J., Jr. J Opt. Soc. Am. 1976,66, 644-659. (15) Engleman, R; Palmer, B. A]. Opt. SOC.Am. 1980,70,308-317. (16) Palmer, B. A; Keller, R A; Engleman, R Informal Report No. LA-8251MS; Los Alamos Scientific Laboratory: Los Alamos, NM, 1980.
0003-2700/95/0367-3814$9.00/0 0 1995 American Chemical Society
r PROBE
-SAMPLE HOLDER
-QUARTZ TUBE
Figure 1. Sample configuration in the discharge cell
diode lasers and that will provide adequate sensitivity. Run-te run, day-today, and samplebsample repeatability are reported for a transition at 776.19 nm. A transition at 831.84 nm was also examined because of the availability of high-power diode lasers (>lo0 mW) in this region. Samples included machined uranium metal, uranium metal powder, uranium oxide, and uranium fluoride; both enriched and depleted W compositions were examined. EXPERIMENTAL SECTION Sample Preparation. Glow discharge cathodes were fabricated in the shape of hollow cylinders 4.85 nun in diameter x 2.54 mm in length, with an inner diameter of 2.38 mm. The lG% enriched n5Usample was machined from uranium metal, all other cathodes were formed by pressing powder in a hollow cathode die at 3.4 x lo7 Pa for 5 min. Although any pure conducting powder can be used as a binder, a 50/50 wt % mixture of silver powder (325 Mesh, 99.99+% Aldrich Chemical Co., Milwaukee, WI) and tantalum powder (325 Mesh, 99.98%Alfa Aesar Johnson Matthey, Ward Hill, MA) was used in this investigation. Silver was chosen because it is available in relatively high purity and is malleable; tantalum was added to the silver befause of its ability to getter oxygen. ?his gettering action helps to reduce the analyte oxides and hydroxides in the discharge." The cathode comprised a 50/50 wt % mixture of binder and sample (-0.25 g).
Special precautions are necessary when hnndling uranium due to its radioactiui&. Uranium is primarily an a emiffer, but it also emits law to moderate p-y e n m ; care must be taken to preuenf ingestion or inhalation and to minimize exposure. Uranium is also pyrophoric when finely divided, and anyone contemplatingresearch with uranium or uranium compounds is well advised to seek the help of a health physicist and industrial hygienist before beginning, Glow Discharge Source. The GD cell with demountable cathode has been described in a previous publication! F i r e 1 illustrates the position of the sample relative to its holder and vacuum insertion probe. The sample cathodes were held in a stainless steel ring welded to a 1.59 mm diameter rod. This rod is inserted in a sample holder that is shielded by a quartz tube to (17l Mei, Y.: Harrison, W. W. Spectmckim Acto 1 9 9 1 . 4 6 8 , 175-182.
keep the discharge from arcing near the high-voltage feedthrough. A discharge forms over the entire cathode and holder assembly and is most intense at the center of the cylinder bore, where the atom density is reported to be highesP it was in this region that uranium atoms were measured. Sample manipulation was accomplished by means of a probe inserted through an O-ringseal. The discharge current was held at a constant 20 mk With a 50 kQ ballast resistor in the circuit, the applied voltage was 500 V. The vacuum chamber housing served as the counter elech-ode. The cell was maintained nominally at 1050 Pa of argon with a gas flow rate of 3 cm3(STP) &-I. To reduce further the level of oxides and hydroxides in the discharge, a liquid nitrogen-filled cooling ring was positioned near the cathode; this aids in the removal of water vapor from the discharge.'J3 h e r Spectroscopy. A continuous wave (CW) argon ion laser-pumped titanium-sapphire ring lasela was used for exploratory spectroscopy and to provide the data on relative optogahanic signal intensities for the transitions investigated. Two single longitudinal-mode AlGaAs diode lasers were used in this study: a Model ML64110N (Mitsubishi Electric Corp., Tokyo, Japan) with a nominal room temperature output of 30 mW at 777 nm and a Model SDL54mHl (SDL Inc., San Jose, CA) with a nominal room temperature output of 150 mW at 834 nm. These diode lasers were used in conjunction with commercial control electronics? For the 776 nm speed study, the diode laser was maintained at 25.0 "C and 130 mA for an approximate power at the sample of 8 mW (400 W/cm?. For the 832 nm study, the diode laser was maintained at 18.8 "C and 136 mA for an approximate power at the sample of 42 mW (2100 W/cmz). The diode laser frequency was scanned using a sawtooth signal generator to vary the drive current;g a 7.5 mA current scan (corresponding to 21.9 GHz) was used for the 776 nm line, and a 14 mA current scan (corresponding to 15 GHz) was used for the 832 nm line. Wavelengths were determined using a wavemeter (Model WA-20 VIS; Burleigh Instruments, Inc., Fisher, NY) with readout of *0.001 nm. The CW laser light was chopped at loo0 Hz with a mechanical chopper (Model SR540;Stanford Research Systems, Inc., Sunnyvale, CA). The ac component of the cathode voltage was monitored with a digital oscilloscope (Model 2246A; Tekh-onix Inc., Beaverton, OR) and a lock-in amplitier (Model SR510 or SRSSO; Stanford Research Systems, Inc.; 1 s time constant). In the titanium-sapphire laser experiments, the output f"the I d in amplitier was digitized using the same computer that was used to control the laser. The lock-in ampliiier used for the diode laser experiments (Model SR850) included data collection and storage capabilities. Data were collected in ASCII form and processed off-line using commercially available software WIeidaGrapk Synergy S o h , Reading, PA). The lasers used in this investigation were Class 4 (titaniumsapphire) and Class 3b (diode) devices. Safe operating practices consistent with these classihtions were followed. RESULTS AND DISCUSSION Measurement Accuracy. Isotope ratio measurements using a magnetic sector mass spectrometer have the advantage that all (18) Caroli, S.; Alimonti. A; Petrucci. F. In I m p m d Hollolo Cnthodc Lnnrgs fm Atonic SPecWoscopx Caroii. S., Ed.;EUis Homwd Limited Publishers. Hdsted R e s New York, 1985. (19) Ohorodnik, S. K; DeGendt, S.; Tong, S. L,Harrison. W. W. I. A d . At Spedmn. 1993.8, 859-865. (20)Ohomdnik. S. IC; Harriwn. W. W. A m l Clem 1993.65.254-2544.
Analytical Chemishy, Vol. 67, No. 20,October 15, 1995
3815
Table I.Isotoplc Abundance8 of Samples Studied (Atom %)
sample
234U
235u
236u
238U
0.0012 f 0.0002 0.0027 f 0.0002 0.0054 f 0.0004 0.064 f 0.001 0.1246 f 0.0003
0.268 f 0.005 0.490 f 0.003 0.716 f 0.005 10.054 f 0.022 20.013 f 0.020
0.0030 f 0.0002
99.728 f 0.005 99.507 f 0.003 99.279 f 0.005 89.824 f 0.022 79.651 f 0.021
233u
powder depleted uranium metal powder depleted uranium tetrafluoride (UF4) powder natural uranium metal machined solid enriched uranium metal powder enriched uranium oxide (U308) NIST SRM U-200
0.3%235Urelative abundance. The quality of the measurements suffered from lack of signal strength at the 776 nm (12 879.981 cm-I) line. One potential solution was to increase the laser power (as in the case of the spectnun in Figure 4A, excited using the 500 mW titaniumsapphire laser). Because higher power diode lasers were not readily available for the 776 nm transition, an alternative transition was sought. Several parameters restricted the search for a different line. Transitions accessible to diode lasers (630-1000 nm) were required for which high-power (>30 mw) devices were commerciallyavailable. Transitions that had a relatively large 235U isotope shift (> 10 GHz; see Table 3) and a relatively large strength (stronger than the 776 nm line) were preferred. Table 3 shows the transitionsI6that were considered as optogalvaniccandidates, along with the relative emission intensitiesI6and isotope shifts.15
,
I .
-e
A
.mu
.F 600 VI
.-0
E
400
3 %
200
0 0 -15
-10
-5 0 Frequency (GHz)
5
10
Figure 5. Uranium optogalvanic spectrum (average of five scans) recorded for the 831.84 nm line using a diode laser with a power density of -2.1 x lo3 W/cmz.The sample was the same as that used for Figure 4, a depleted uranium metal powder (0.27% 235U).The discharge conditions were 1050 Pa of argon, 20 mA, and 500 V.
Our experimentally determined optogalvanic intensities for these transitions are also shown. To generate these data, three 10 GHz scans were recorded over the 238U line using the titaniumsapphire laser (-500 mW at the sample). As anticipated, there is no strong correlation between relative emission and optogalvanic signal intensities. Of the eight transitions examined, the one at 832 nm (12 018.3089 cm-') was the most promising. The optogalvanic signal intensity was 2.5 times greater than that at 776 nm for the same laser power, and a 150 mW diode laser was commercially available in the 834 nm region. In addition, the magnitude of the isotope shift was approximately the same as for the line at 776 nm. Figure 4B shows a spectrum for a depleted 235Uoxide sample at 832 nm, obtained with use of the titaniumsapphire laser attenuated to 38 mW (the anticipated power of the 832 nm diode laser). The 235Usignal is clearly observed, and the signal-to-noise ratio is comparable to that for the 500 mW, 776 nm case. Precision and Accuracy of a Measurement at 832 nm. Figure 5 shows a depleted uranium metal spectrum recorded using a 150 mW SDL diode laser operating with a nominal output power of 42 mW at the discharge cathode. The spectrum is similar to the one shown in Figure 4B, an anticipated result given Analytical Chemistty, Vol. 67,No. 20, October 75, 7995
3817
that the line widths of the lasers are at least an order of magnitude narrower than the Doppler-limited spectral line width. The signalto-noise ratios are comparable. The measured 235U/C35U 23*U) ratio was 0.0026, within 3.7% of the thermal ionization mass analysis value. For enriched samples, precision was not expected to improve significantly for the 832 nm line. The increase in signal-to-noise ratio, however, should have an impact on the measurement precision for a depleted sample. The run-to-run reproducibility, as defined previously, did improve to &7.8%from f13.6% RSD (la for 11 measurements). This is the direct result of the more powerful diode laser and the stronger optogalvanic transition (compared to the 776 nm line).
+
CONCLUSIONS Precision and accuracy of a uranium isotope ratio measurement have been evaluated for diode laser-excited optogalvanic spectroscopy. Samples that were enriched, natural, or depleted in 235U were easily distinguishable from one another. Determined with use of a 150 mW diode laser at 832 nm, the measured 235U/(235U 238U) ratio for a depleted uranium metal sample was 0.0026, within 3.7% of the thermal ionization mass analysis value. The
+
3818 Analytical Chemistiy, Vol. 67, No. 20, October 15, 1995
run-to-run reproducibility was f7.8%,a 2-fold improvement from the measurement obtained with use of a 30 mW diode laser at 776 nm. These results are sufficient for many applicationswhere preliminary data concerning isotopic composition of a sample provide the basis for deciding whether further investigations are warranted. ACKNOWLEWMENT The authors acknowledge the help of J. B. Treen of the Office of Radiation Protection, Oak Ridge National Laboratory, for his assistance in sample preparation and D. H. Smith of the Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, for performing the thermal ionization mass spectrometry. Research was sponsored by the U S . Department of Energy, Office of Research and Development. Oak Ridge National Laboratory is managed by Lockheed Martin Energy Systems, Inc., for DOE under Contract DE-AC05840R21400. Received for review May 24, 1995. Accepted July 20,
1995.B AC9505029 @Abstractpublished in Advance ACS Abstracts, September 1, 1995.
