Anal. Chem. 1990, 62, 1271-1274 (13) Savickas, P. J.; LaPack, M. A.; Tou, J. C. Anal. & e m . 1080. 67, 2332. (14) CRC Hendbodc of Chemlsby and phvsics, 64th ed.; CRC Press, Inc.: Boca Raton. FL, 1983; pp F-5 and F-38. (15) Pasternak, R. A.; Schimscheimer, J. F.; Heller, J. J . Porn. Sci. 1070, 8, 467. (16) Ziegei, K. D.; Frensdorff. H. K.: Blair, D. E. J . Pobm. Sci. 1089, 7 , 809. (17) Analytical Sciences Mass Spectral Data Base. The Dow Chemical
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Co., MMiand, MI, internal communication. (18) Bbr, M. E.; Kohiato, T.; Cooks, R. G. Anal. Chim. Acta, in press. (19) Medical Materials Buslness Product Form 51-772589; Dow Corning Corp.: MMland, MI.
RECEIVED for review October 4, 1989. Revised manuscript received February 26, 1990. Accepted March 2, 1990.
Mass Spectrometry of Technetium at the Subpicogram Level Donald J. Rokop,* Norman C. Schroeder, and Kurt Wolfsberg Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
We have developed a method to determine 210' atoms of @% or @'Tcand 5 X IO' atoms of @@Tcby negatlve thermal lonlzatlon mass spectrometry. Interferences from Isobaric lmpurlties or hydrocarbons are equlvalent to 5 X IO6 atoms of technetlum or less. Lanthanum oxlde Ion enhancers In conJunctlonwith Ca( NO3), are added to slngle, zone-refined rhenlum fllaments to achleve lonlzatlon efflclencles that are >2% for the formatlon of Tc0,-.
INTRODUCTION As an essential part of our program to measure the fluence of high-energy solar neutrinos over the past several million years (I, 2), we have developed a high-efficiency mass spectrometric technique to determine technetium isotopic compositions using negative ions. In the molybdenum-technetium solar neutrino experiment, we anticipate levels of IO6 atoms cof' @ lg and g7Tc and 10l2 to 1013atoms of gsTc in purified samples isolated from lo7 kg of molybdenite ore. Previously developed methods (3) for the mass spectrometry of technetium with positive ions have detection limb of 1 pg (6 X log atoms). This limit is mainly due to the high ionization potential of 7.3 V for technetium. In addition, isobaric impurities produced by positive ion methods are so large that these techniques cannot be used a t the low levels required for this experiment. Kastenmayer (4),Heumann et al. (9,and Delmore (6) have previously shown negative thermal ionization as a possible means of performing high ionization efficiency mass spectrometry on selected elements, including technetium. Delmore proved that pertechnetate ions could be formed, while Kastenmayer and Heumann developed methods for microgram samples, sizes that are still lo3 to lo6 times larger than our experiment allows. The attraction of negative thermal ionization is that technetium is formed and measured as the Tc04- ion while molybdenum, the isobaric impurity which is most common and difficult to remove, is preferentially formed as the Moo3- ion. A disadvantage of this method is that the only available spikes, 98Tc and 9 7 T ~ form , ions with *'O and l80a t the gsTc04- position. This limitation establishes a measurement limit of a few million atoms of V c . However, this is not a serious constraint for the solar neutrino experiment.
EXPERIMENTAL SECTION Instrumentation. We use a tandem magnetic mass spectrometer with pulse counting and movable Faraday cage detectors
(7,8)both at the intermediate (between magnets) position and in front of the multiplier. The ion source is a modified Nier thin-lens source (9)designed for thermal ionization with "2"axis focusing. The tandem instrument is necessary to discriminate against large scatter tails of Reo4- ions that are produced by the ionization of the rhenium filaments and by residual hydrocarbons. The Reo; signals are between lo4 and 10" A. Full-peak resolution with a 50% correction in peak height for oxide contributions from 9 7 Tand ~ 98Tc (i.e., 97T~1e03180 and By extrapolation, 4 X lo6 atoms can be measured to *lo%. The 9 7 Tspike ~ concentration was measured by preparing five mixtures with NBS 99Tc SRM 4288. The prepared 9997 mass ratio was varied by e factor greater than 100 to test linearity in this sample size range; see Table 111. Approximately 1pg of total technetium was loaded per analysis. The results show that good linearity and precision of measurement for this sample size are obtained and that isotopic equilibration has been established. The error stated is on an individual measurement. One-nanogram loads of the simulated solar neutrino mixture were measured 8 times to obtain an average 162:163 weight ratio of (1.129 f 0.024) x lo4. The measurement precision, f2.4%, looked very good, but the measured value differed from the prepared value of 0.98 f lo4 by 15%. After looking for and not finding a hitherto undetected isobaric interference at the 162 mass position, we decided to look more closely a t
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because the sample size requirement may be reduced to as low as a few grams. Such studies include detecting and following the movement of anthropogenically produced q c contaminants in the environment and studying the migration of 99Tc produced by natural processes in and around uranium ore deposits. While these applications are critical for current environmental and nuclear waste storage considerations, future applications of the method discussed above may be even more important. The Tc0,- ion has a number of properties that make it useful as a future monitor of nuclear waste repositories: a high abundance in fission product wastes, a long half-life, and a high mobility in oxidizing environments in many geologic media.
ACKNOWLEDGMENT
163
162
161
161
162
163
Flgwe 3. Mass spectra obtained from a 1-ng load of NBS ' q c SRM 4288. Small peak is '%O,-. No isobarics are visible at mass 161.
NBS SRM 4288 q c . When four 1-ng samples were loaded and carefully examined (see Figure 3), an average 162:163 weight ratio of 1.27 X lo-' was measured. Because no other peaks were observed in the spectra and because the ratio was constant with time, temperature, and repeated loadings, we now assume 9sTc to be present in the NBS SRM 4288 99Tc. The gsTc may be present from its independent formation in fission or from reactions such as 99T~(n,2n)~Tc. When this correction is applied to the measured ratio for the simulated solar neutrino mixture, it becomes 1.002 X lo4, within 2% of the prepared value. Thus the 6 X 106 atoms of q c , loaded on the filament, were measured to f2.4%. As can be seen in Figure 9, mTc is easier to measure due to both the absence of isobaric interferences and the greater displacement from the scatter tail of the large 99Tc peak. Figure 2, which shows ionization efficiency vs Ca(NO& loading, demonstrates how variations in the level of one constituent on the filament surface can greatly influence the ionization efficiency. Calcium nitrate is only one of many constituents that have a possibility of being present. At this level of analysis, where only a small number of atoms are being measured, all possible constituents must be carefully considered for their effect on the measurement.
CONCLUSIONS Samples of technetium from 1 ng down to less than 1 fg (6 X lo6 atoms) can be successfully analyzed with ionization efficiencies of >2% with negative thermal ionization. Our method gives measurement limits for the three long-lived technetium isotopes, 9 7 T ~98T~, , and q c , that are in the few-million atom range. Such sensitivity should greatly simplify 9syc determinations in a variety of environmental studies
The authors wish to thank D. B. Curtis and E. A. Bryant for their support and helpful discussions. We also thank R. E. Perrin and J. C. Banar for designing the building the filament sintering/degassing chamber and also for helpful discussions on the mass spectrometry. R. C. Hagan provided scanning electron micrographs for the determination of particle sizes. RePistrv No. 9 q c . 32025-58-4: 97T~.15759-35-0: q c . 14133-76-6 Ca(NO&, 10124-37-5; La203, 1312-81-8; neutrino; 12587-66-5.
LITERATURE CITED (1) Cowan, G. A.; Haxton, W. C. Science 1982, 276, 51. (2) Wolfsberg, K.; Cowan, G. A.; Bryant, E. A,; Daniels, K. S.; Downey, S. W.; Haxton, W. C.; Niesen, V.; Ncgar, N. S.; M k , C. M.; Rokop, D. J. Solar Neutrino and Neutrino Astronomy; Cheny, M. L.. Lande, K., Fowler, W. A., Eds.; American InstiMe of physics: New York, 1985; pp 196-202. (3) Walker, R. L. Radioelement Analysis, Progress and problems; Lyons. W. S., Ed.; Proceedings of the Twenty-third Conference of Analytical Chemistry in Energy Technology, Oct 9-1 1, 1979; Ann Arbor Science: Ann Arbor, MI, 1980 pp 377-383. (4) Kastenmayer, P. Doctoral Thesis, University of Regensburg. Regensburg, West Germany, 1984. (5) Heumann, K. G.; Schindlmeier, W.; Zeininger, H.; Schmidt, M. Fresenius' ZAnal. Chem. 1985, 457, 320. (6) Delmore. J. E. Idaho National Engineerlng Laboratory, personal communication, 1986. (7) Rokop, D. J.; Perrin, R. E.; Knobeloch, G. W.; Armijo, V. M.; Shields, W. R. Anal. Chem. 1982, 54, 957. (8) Alei, M.;Cappis. J. H.; Fowler, M. M.; Frank, D. J.; Goldblatt. M.; Guthals, P. R.; Mason, A. s.; Mills, T. R.; Mroz. E. J.; Norris, T. L.; Perrin, R. E.; Poths, J.; Rokop, D. J.; Shields, W. R. Atmos. Environ. 1987, 21, 909-915. (9) NBS Technical Note 428; Shields, William R., Ed.; U.S. Government Printing Office: Washington, DC, 19137. (10) Zief, M., Mitchell, J. W. Contaminabbn Conlrolin Trace Element Analysis; John Wiley 8 Sons: New York. 1976; p 92. (1 1) Schroeder, N. C.; Wolfsberg, K.; Rokop, D. J. Unpublished work. (12) Bbgl, W. and echmann, K. Inorg. Nucl. Chem. Lett. 1974, 70, 697-705. (13) Muller,--H.-W.: Chmielewska, D. Nucl. Data Sheets 1988, 4 8 , 663-752. (14) Deimore. J. E. J. Phys. Chem. 1987, 91, 2883-2886. (15) IUPAC, Element By Element Review of Their Atomic Weights; Pergamon Press Ltd.: Oxford, 1984.
RECEIVEDfor review on January 22, 1990. Accepted March 16,1990. This work was supported by the U.S. Department of Energy Office of High Energy and Nuciear Physics (Division of Nuclear Physics) and Office of Basic Energy Sciences (Division of Engineering and Geosciences).
