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Anal. Chem. 1984, 56,379-381
Isotopic Analysis of Uranium and Plutonium Mixtures by Resonance Ionization Mass Spectrometry D. L. Donohue,* D. H. Smith, J. P. Young, H. S. McKown, and C. A. Pritchard
Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
The technlque of resonance lonlzatlon mass spectrometry (RIMS) has been applled to the measurement of U and Pu Isotope ratlos. The RIMS technique offers the advantage of high elemental selectlvlty compared to conventlonal thermal lonlzatlon mass spectrometry. Measurements of selectlvlty ratlos for U vs. Pu are presented at two different wavelengths of laser excltatlon. The preclslon obtalned for repllcate analyses of NBS standard reference materlals Is equal to that obtalned by single-fllament thermal lonlzatlon methods. Sample slzes of 10 ng for each element were adequate for measurement of the 240Pu/23gPu, z41Pu/230Pu,and 235U/238U Isotope ratlos. Sensitivity was enhanced by using a pulsed thermal atomization technique which resulted In a 10-fold Improvement In sample usage compared to continuous atomlzatlon. The RIMS technique Is, therefore, potentially useful In safeguards and nuclear materials accountancy, where preclse and accurate Isotope ratio measurements are Important.
One of the problems in isotopic analysis of P u and U is isobaric interferences a t masses 238 and 241. Both elements have isotopes at mass 238; it is a particular problem in P u analysis because that isotope is usually a major component of U and a minor one of Pu. For the specific case of spent reactor fuels, the U/Pu ratio is usually about 100, and complete chemical separation of the two elemenk is difficult and time-consuming. The resin bead method of sample preparation (1-3) has eliminated most of the chemistry required, but, for full exploitation of the technique, the elements are run sequentially from the same filament loading, making correction a t mass 238 essential. At the mass 241 position, buildup of %lAm from the decay of 241Pu(half-life 14.3 years) becomes significant after only a few weeks. It is not always possible to analyze samples quickly enough for 241Amto be of no consequence, and it is impossible to correct for its contribution to the P u signal because no Am isotope is available to monitor for this purpose. Because of its importance to safeguards, it is important to obtain as accurate an assay of the 241Puas possible. The recently developed technique of resonance ionization mass spectrometry (RIMS) offers the ability to perform the required isotopic analyses with little or no isobaric interference. RIMS has been described, and its use in eliminating isobaric interference in mass spectrometric analysis of Nd and Sm has been demonstrated (4). The technique can be applied to samples loaded onto typical thermal ionization filaments. For RIMS detection, neutral atoms evaporated from the filament are photoionized by a several-photon process operating via allowed transitions through intermediate resonant levels (4). In a previous report, a number of possible two-photon RIMS-active wavelengths were presented by which P u can be ionized by photons in the wavelength range of 431 to 451 nm (5). Uranium is expected to have a large number of lines in this region, so many that selectivity could be compromised. In this paper we describe a single color RIMS process for both P u and U that uses low-energy photons in the 590-nm region
of the spectrum. Nonresonant processes are essentially eliminated, and dye lifetime is significantly improved over that which would be expected in the 440-nm region.
EXPERIMENTAL SECTION Mass Spectrometer. A tandem mass spectrometer of ORNL design was used. It consists of two 90" magnetic sectors of 30 cm radius (6). The instrument is normally used for high sensitivity thermal ionization isotope ratio measurements of U and Pu requiring high abundance sensitivity (1part in 10% The detector is a 16-stageelectron multiplier (Model R515, Hamamatsu Corp., Middlesex, NJ) which can be operated in the pulse counting mode (voltage applied, 3700 V) or in a region of linear response for RIMS measurements (at 2500 V). Samples were prepared from NBS Standard Reference Materials; SRM U-500 was used for U and SRM Pu-947 for Pu. Approximately 5 ng of each element was adsorbed onto anion exchange resin beads (Dowex 1-X2) (1-3). Two resin beads were loaded into a canoe-shaped Re side filament of a triple filament assembly. The beads were then coated with a colloidal graphite suspension (Aquadag) and dried. The center filament, of highpurity rhenium ribbon (0.1 cm wide by 1.0 cm long), was used to produce thermal ions for initial mass spectrometric focusing and setup. A third filament of Ta ribbon was placed directly opposite the sample filament in a "box" arrangement. Sample filament temperatures during Pu analysis were 13OC-1400"C and 15OC-1600 "C for U using continuous heating (see below for pulsed conditions). Temperatures were measured with an optical pyrometer and are not corrected for emissivity. The use of a pulsed laser system with continuous atomization from the sample filament resulted in poor temporal efficiency. The laser duty cycle was 30 pulses s-l with 1 /.ts pulse length, resulting in a nominal temporal efficiency of 3 X lo-'. For this reason, it was decided to pulse the atomization process by supplying current pulses to the sample filament (7) in synchronization with the laser. The filament power supply (Model 6434B Hewlett-Packard, Palo Alto, CA) was connected to the filament through the circuit shown in Figure 1. Synchronization pulses from the laser were delayed by a variable timer circuit which resulted in pulsing the filament current from 1 to 13 ms before the next laser pulse. The width of the current pulse could be varied from 3 to 9 ms. With this system, an increase in sample efficiency of approximately 10-fold was obtained. To achieve this efficiency, the W wire support legs (0.05 cm diameter) of the sample filament were replaced with more massive Pt rods (0.15 cm diameter). This resulted in better heat conduction away from the filament between current pulses. Peak temperatures obtained were approximately100 "C higher than those used with continuous heating. Data were collected by peak jumping in a fashion similar to that used in thermal ionization work (8). As described previously ( 4 ) ,the ion multiplier signal from each pulse of RIMS ions was amplified by a preamplifier (nominal gain of 35) followed by a shaping amplifier (gain of 200), producing a Gaussian-shapedpulse for digitization by a 13-bitanalog-to-digital converter. The number of laser pulses collected for each isotope during an analysis is shown in Table I. The total number of ions collected in an analysis limits the ultimate precision obtainable. Therefore, measurements were made of the number of ions produced by each laser pulse. The photograph in Figure 2 shows the ion signal from the preamplifier for 2 q uwith 588.1-nm laser excitation (estimatednumber of ions = 70 i 10). Thus, in a normal analysis, 5 X lo5ions of 239pu would be collected, with an uncertainty of n1I2or 0.14%. The larger
0003-2700/84/0356-0379$01.50/00 1984 American Chemical Society
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ANALYTICAL CHEMISTRY. VOL. 58. NO. 3. MARCH 1984 LAYRE
untreated dye solution to more than IO h. Typical laser powen, were measured to be from 5 to 1 1 mJ for the experiments described herein; the laser bandwidth was 0.1 nm (3 cm-'). This bandwidth was chosen to be hroader than the expected isotope and hyperfine struclure of the transitions used. Thin was thought necessary IO reduce isotopic bias effects. When the laser was f& in the source region, the image size of the laaer was I nun: under the defocused conditions, the laser spot size varied from 2 to 4 mmz over the I cm length of the ion extraction slit. Due to the nature of the ion extraction field, it was not possible lo determine whether or to what extent saturation of resonance ionization was achieved. A small portion (4%) of the beam was directed into a 0.32-m spectrqzraph (Model H R 320, Instruments SA, Inc.. Metuchen, NJ) equipped with a silicon diode array readout (Model 1412 with Model 1218 Detenor Contrnller. EG&C PAR, Princeton. NJ). The optical spectrum from this was displayed on a high-speed storage oscilloscope (Model 7834. Tektrunix, Inc., Beavertnn. OR) with the maximum horizontal dis. persion. The wavelength readout of the spectrograph was calibrated against two Ne emission lines, 588.189nm and 585.249 nm. The wavelength of the laser could then be determined with an accuracy of *0.05 nm. In addition, tuning of the U line at 591.54 nm was verified by the optopalvanic effect in a U hollow cathode lamp (9). A computer routine was used to automatically scan and locate the laser wavelength based on the centroid of the optogalvanic signal for Ne at 591.96 nm. Several dyes covering other wavelength ranges were evaluated for use in this determination, Coumarin 480 and Coumarin 504 (Exiton. Dayton. OH). Useful Pu or U RIMS transitions were not found with either dye.
RESULTS A N D DISCUSSION
Flgure 2. Oscilloscope trace showing the number of 23sPuions produced in a single RIMS hsar pulse. Vemca sensBMty was 0.1 V m" with 500 ns cm-' horizonla1 sensnivity
Table I. Laser Pulses Collected during RIMS Analysis
isotope 'ISpU
"OPU "1pu
mu r3qJ
laser pulses cycles runs per total laser per cycle per run analysis pulses 32 23 10 1360 23 10 29 440 128 32 23 10 1360 32 30 10 9 600 32 30 10 9 600
number of pulses collected for Uopu was necessary to obtain the same number of ions, reflecting its lower isotopic abundance. Assuming a sample size of 10 ng, this represents an efficiencyof 1 ion collected for every lo' atoms loaded, which reflects the temporal overlap efficiency (approximately lo4) combined with the geometric overlap of the laser beam with the sample atom cloud Laser System. The laser used in this study was a tunable, flash-lampexcited dye h e r (Model CMX-4, LCD/Milton Roy Co., Sunnyvale, CA). It was operated at 30 Hz and at maximum tlnah lamp voltape, 7.8 kV. The laaer beam passed through a 3ocm focal length lens and was then directed through a window into the mass spectral source region located 36 em from the lens. It was then directed to pass c l m to the sample f h e n t as previously described (4). RIMS data were collected with the laser either focused or defocused in the source region; however, it was found that more ions per pulse could be collected in the defocused condition. Rhodamine 6G dye (Eastman, Rochester. NY) a t a mncentration of 1.3 x le M wan used in the laaer; the dye solvent was 4 L of a metbanol-water mixture (54-50 ~ 0 1 %to) which was added 2 X mol of cyclmctatetraene, COT. Prior to the addition of COT, N, was bubbled through the dye solution to remove dinsolved 0, The use of COT and N, bubbling prolonged the useful half-life of the dye power from about 2 h for the
A number of RIMS-active transitions were found for both Pu and U in the region of 580 to 605 nm. As was the case for the RIMS spectra of several lanthanides (10).it is obvious that ionization cannot occur by a two-photon resonant process originating from ground-state atoms, there not being sufficient energy in two photons of this wavelength range to photoionize such a t o m Therefore. if the RIMS transitions o k r v e d result from allowed two-photon routes. they must involve initial statea of energies >12OOO cm-l. Other, more complex, routes involving more than two singlecolor photons are also posible. By use of published Pu (11) and U (12,13 spectral line lista. a computer search for reasonable two-photon routes to ionization did not yield useful information. The wavelength chosen for the RIMS analysis of U, 591.54 nm. is reported to be a three-photon, two-intermediate level photoionization process ( 1 4 ) . This process occurs in U with a mismatch between the first and second transitions of less than 1.0 em+. A similar three-photon process can be assigned to the wavelength chonen for Pu analysis, 5RR.04 nm. In this latter case, the initial state of Pu ia7Fi,at 2203.6 a-' The .first transition is to an odd state at 19203 cm-l: the second transition is to an even state at 36204.4 cm-'. There is a 3-cm-l mismatch in this process. Several other RIMS-active wavelengths were also evaluated for U and Pu but suffered from insensitivity and/or unacceptable analytical bias. Further study of several RIMS transitions for U and P u i s under way and will be the subject of a later publication (15). The excellent elemental selectivity of RIMS has been previously demonstrated (4, 16.17). Preliminary work with U and Pu has indicated that a large number of optical transitions are accessible lor a two-photon single-color process in the blue region of the spectrum (5). This is to be expected due to the efficient production of low-lying excited states of both U and Pu by the thermal atomization process (18). This fact, coupled with the large number of higher energy levels of these elements. results in so many pceaible RIMS tramitions that the elemental selectivity is degraded. Use of higher wavelengths (590 nm region) in this study excludes the poasibility of such two-photon transitions from ground-state or low-lying excited levels. The reault is a much less complicated
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
Table 11. U and u'F Isotope Ratios by RIMS a 3 9Pu analysis 240Pu/
1
2 3 4 5 av std dev RSD NBS certified a
24
PuI2 9Pu
U/'"U
0.2456 0.2471 0.2461 0.2460 0.2464
0.0353 0.0362 0.0361 0.0351 0.0354
1.0246 1.0335 1.0367 1.0216 1.0236
0.2464 0.0006 0.24% 0.2414
0.03 56 0.0005 1.4% 0.0341'
1.0280 0.0067 0.65% 0.9997
Corrected to date of analysis.
ionization spectrum with the potential for high selectivity. Measurements of the selectivity ratios for U, Pu, and Am were made at the wavelengths used in this study. At the wavelength used for Pu (588.04 nm), a selectivity ratio of 150 (239Pu/235U) at approximately equal thermal ion signals was obtained; for the U wavelength (591.54 nm), a ratio of 3400 (235U/239Pu) was obtained. The selectivity ratio for Pu with respect to Am was measured from a mixture of NBS-947 Pu with separated 243Am. Conventional thermal ionization yielded a 239Pu/243Am ratio of approximately 1,but with RIMS a ratio of 15000 was obtained at 588.04 nm. These values are adequate for most analyses involving these elements but are not as high as those obtained elsewhere (16) for other elements. Differences in experimental conditions which may allow nonselective processes such as charge or excitation exchange or electron bombardment to occur would also affect selectivity ratios. Table I1 lists measured isotopic ratios for U and Pu obtained from five separate sample loadings; the values are uncorrected for any bias. Since the neutral atoms are being produced thermally, one would expect biases of the same order as those observed in single-filament, thermal ionization mass spectrometry (0.3-0.5% per mass). The biases observed for U (0.8%)and for Pu (2.1%) are somewhat larger. The bias for U at 591.54 nm depends to a small extent on laser power. This may result from an ac Stark effect, which would distort the normal isotopic structure of both the bound-bound transitions involved; this will be addressed in another publication (15). Of more concern in this study is the external precision. Pulse counting thermal ionization isotopic analyses generally have precisions on the order of f0.5%; to be considered a viable alternative, a new technique must approach this level of reproducibility. The results in Table I1 show precisions of f0.24% for the 240Pu/239Pu ratio and 0.65% for the 235U/23sU ratio, which are comparable to those obtained by the more
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conventional technique. Thus, in addition to virtual elimination of isobaric interferences, the new technique offers the potential of being at least as precise as the conventional thermal ionization technique. The precision shown above is sufficient for performing isotope dilution analyses to determine the element concentration in a particular sample. This involves spiking the sample with enriched isotopes at a known concentration followed by RIMS analysis at the appropriate wavelength for U or Pu. Precision and accuracy of better than f l % are expected.
ACKNOWLEDGMENT The authors thank C. E. Bemis, Jr., R. N. Compton, J. C. Miller, and J. D. Fassett for their assistance in this work. Registry No. 235U,15117-96-1; 239Pu,15117-48-3; 240Pu, 14119-33-6;241Pu,14119-32-5. LITERATURE CITED (1) Walker, R. L.; Eby, R. E.; Pritchard, C. A.; Carter, J. A. Anal. Left. 1 ~ 4 7,, ~ 3 3 - 5 7 4 . (2) Walker, R. L.; Carter, J. A.; Smith, D. H. Anal. Lett. 1982, 14, 1603-1 612. (3) Smith, D. H.; Walker, R. L.; Carter, J. A. Anal. Chem. 1982, 5 4 , 827A-832A. (4) Donohue, D. L.; Young, J. P.; Smith, D. H. I n t . J . Mass Spectrom. Ion Phys. 1982, 43, 293-307. (5) Donohue, D. L.; Young, J. P. Anal. Chem. 1983, 55, 378-379. (6) Smith, D. H.; Christie, W. H.; McKown, H. S.; Walker, R. L.; Hertel, G. R. Int. J . Mass Spectrom. Ion Phys. 1972178, IO, 343. (7) Fassett, J. D., NBS, personal communication, May 1983. (8) Smith, D. H.; McKown, H. S.; Christie, W. H.; Walker, R. L.; Carter, J. A. "Instructlon Manual for ORNL Tandem Mass Spectrometer"; ORNLITM-5485; Oak Ridge National Laboratory: Oak Ridge, TN, 1976; pp 54-65. (9) Miron, E.; Smilanski, I.; Llran, J.; Lavl, S.; Erez, G. I€€€ J . Quantum Electron. 1979, Q€-15, 194-196. (10) Young, J. P.; Donohue, D. L. Anal. Chem. 1983, 55, 68-91. (11) Striganov, A. R. Report IAE-2965, Kurchatov Institute of Atomic Energy, Moscow, USSR, 1978. (12) Blake, J.; Radziemski, L. J. J . Opt. SOC.Am. 1978, 66, 644. (13) Paisner, J. A.; Worden, E. F.; Johnson, S. A.; May, C. A,; Solarz, R. W. J . Opt. SOC.Am. 1978, 66, 846. (14) Chen, H.; Borzilieri, C. J . Chem. Phys. 1981, 7 4 , 6063. (15) Donohue, D. L.; Smith, D. H.; Young, J. P., unpubllshed results. (16) Miller, C. M.; Nogar, N. S.; Gancarz, A. J.; Shields, W. R. Anal. Chem. 1982, 5 4 , 2377-2378. (17) Fassett, J. D.; Travis, J. C.; Moore, L. J.; Lytle, F. E. Anal. Chem. 1983, 55, 765-770. (18) Fassett, J. D.; Moore, L. J.; Travis, J. C.; Lytle, F. E., unpublished results.
RECEIVED for review August 29, 1983. Accepted December 8,1983. Work sponsored by the US. Department of Energy, Office of Basic Energy Sciences, under Contract W-7405eng-26 with the Union Carbide Corporation; J. P. Young received partial support from the Office of Health and Environmental Research.
