Anal. Chem. 1983, 55, 976-977
976
Determination of Uranium-235Nranium-238 Ratio in Natural Waters by Chelex 100 Ion Exchange and Neutron Activation Analysis Ernest S. Gladney," Richard J. Peters, and Daniel R. Perrln Los Alamos National Laboratoty, MS K490, Los Alamos, New Mexico 87545
Determination of the level of environmental contamination from facilities handling depleted or enriched uranium can be made more accurately if the isotopic ratio is measured in addition to the total uranium concentration. In fact, the relatively high variability of uranium concentration in natural samples can often mask the contribution from the facility. Total uranium in natural waters has been measured by several methods (I-B), but only mass spectrometry is normally applied to the measurement of isotopic ratios (9). Alpha spectrometry is useful for determining 234U/238U and 235U/238U ratios in greatly enriched samples. However, samples of natural or depleted uranium isotopic abundance5 have such small amounts of 235Upresent that the difficulties of resolving the 235U4.58-MeV a peak from the tail of the much larger 234U 4.77-MeV peak limit both precision and sensitivity. To our knowledge, thermal neutron activation analysis has not been previously used for the determination of uranium isotopic ratios in natural waters. I t is potentially both an accurate and precise technique, partly because activities from both 236Uand 238U may be observed from a single irradiation. The former may be measured via fission product decay while the latter may be determined from the decay of 2.3-day 239Np 239Npreaction. We formed through the 238U(n,y) 239U(0-) chose the 1.38-day 143Cefission product so that activities having both similar half-lives and interference-free y rays of almost equal energy could be compared. The more intense activity from 1311was not utilized since iodine may be lost from samples during reactor irradiations.
EXPERIMENTAL SECTION Fifteen 3-L water samples believed t o contain only natural isotopic uranium were collected from a surface source within the Los Alamos area. These were spiked in groups of three with 250 Wg/sample of total uranium, using five different uranium isotopic compositions. The spiking level was selected to overwhelm the natural uranium concentration in the water (CO.1 ppb). National Bureau of Standards (NBS)Isotopic Uranium Standard Reference Materials were dissolved to make both the spikes and irradiation standards. Five 30-L water samples were collected from both surface and subsurface (Ogallala aquifer) sources in west Texas in the vicinity of a facility using depleted uranium. It was anticipated that the on-site samples might contain some evidence for contamination by depleted uranium. Short ion-exchange columns (4 cm X 1cm) of Chelex 100 were selected for the initial separation step because of the favorable uranium distribution coefficient on this resin and its ability to extract uranium from water at pH 4. The latter eliminates the need for large volumes of HCl (which frequently carries trace uranium), as would be required if AG-1x8 were used initially. The smaller samples were buffered with 10 g of ammonium acetate plus 25 mL of concentrated acetic acid, and the larger samples were buffered with double this amount. The samples were then siphoned through the Chelex 100 columns at 2 mL/min, providing for largely unattended sample processing. Following this initial separation, the uranium was eluted from the Chelex 100 columns with 100 mL of 1 N HC1. At this stage, the samples were largely free of extraneous cations, but contained far too much bromine for activation analysis. The 82Brthat would be produced via neutron activation is of approximately the same half-life as the isotopes of interest. The samples were therefore further purified by a standard hexone extraction procedure (IO),which removed almost all remaining interferences, but added small amounts of Na back into the
Table I. 235U/2sU Ratio in Spiked Water Samples from Los Alamos ( X lo-') sample no. 1A B C 2A B C 3A B C 4A B C 5A B
c
measured ratio individual mean 0.36 i 0.33 * 0.39 i 0.46 i 0.47 rt 0.47 i 0.72 i 0.72 i
t
s
std spike value
0.02 0.02
0.02 0.02 0.02 0.02 0.03 0.03 0.70 i 0.03 1.010 rt 0.03 1.016 i. 0.03 1.004 * 0.03 11.30 t 0.1 12.00 t 0.1 12.01 It 0.1
0.36
i
0.02
0.36
0.47
i
0.01
0.47
0.71 i. 0.01
0.72
1.010 t 0.006
1.01.4
11.77
t
0.35
11.36
samples. The residual Na was removed by anion exchange from 8 M HCl on 3 cm X 1cm columns of Dowex AG-1x8. Uranium was eluted from these colums with 3-mL of distilled water directly onto 4 cm X 4 cm polycarbonate f i i and dried under a heat lamp. The polycarbonate films were folded and loaded into small polyethylene vials for neutron irradiation. Standards were prepared by depositing 10 hg of NBS Isotopic Uranium Standard Reference Materials in 3 mL of water directly on polycarbonate films without any chemical processing. Samples and standards were irradiated simultaneously in groups of eight for 30 min at the Los Alamos Omega West Reactor in a thermal neutron flux of 1 X 1013neutrons cm-2 s-'. After 24 h of decay the polycarbonate films were transferred to clean containers and counted for 2000 s at a distance of 5 cm above vertically mounted, high-volume Ge(Li) detectors multiplexed to a 4096 channel pulse-height analyzer. System dead-times were below 8% in all cases. Data were transferred to Digital Equipment Corp. (DEC) RL02 disks for off-line data reduction on our DEC PDP 11/34 using our GAMS program (11). The peaks due to the 278-keV and 293-keV y-rays from the decay of 2.35-day 239Npand 1.38-day 143Ce,respectively, were found to be free of interferences and were used exclusively in the analysis. These two peaks were integrated in each spectrum and, after decay correction, a ratio of the peak areas was calculated for each sample and standard. These sample peak area ratios were compared to those of the known isotopic standards, and from these data the 236U/238U ratio was calculated for each sample.
RESULTS AND DISCUSSION The results of our measurements on the spiked surface water samples are shown in Table I. Good accuracy and precision were achieved for all five of the 235U/238Uratios, ranging from depleted to moderately enriched (0.0036-0.1136). The poorest precision (fa%) was obtained on the samples with the least 236U.This is to be expected since the 293-keV peak from 143Ceis about an order of magnitude smaller than the 278-keV peak in natural urenium samples. The overall precision of this measurement was in line with the f 6 % counting statistics uncertainty for the 293-keV line. The precision for the most enriched sample was less than expected, since the 143Cepeak was quite prominent and should have yielded the most precise data. Nevertheless, the accuracy and
0003-2700/83/0355-0976$01.50/00 1983 American Chemical Soclety
Anal. Chem. 1983, 5 5 , 977-978 ~
-
.
bracketed the isotopic range of the samples. All data reported here were obtained by interpolation between two standards.
Ratio in Natural Water Samples Table 11. 235U/238U from West Texas a sample no. 1
2 3
4 5 a
measured ratio ( x lo-')
source on-site surface off-site surface yegional surface regional surface regional groundwater
ACKNOWLEDGMENT We thank W. Purtymun for collecting the west Texas water samples, W. Eberhardt, M. Muller, and D. Noveroske for assistance with the chemistry, and the staff of the Omega West reactor for assistance with the neutron irradiations. Registry No. 235U, 15117-96-1;238U,7440-61-1; Chelex 100, 11139-85-8;water, 7732-18-5.
* 0.03 0.68 * 0.03 0.72 f 0.03 0.72 i 0.03
0.65
0.70
Note: natxral 235U/238U = 0.72 X 10".
___-.___
977
* 0.03
-
precision achicved a t all levels are quite sufficient for environmental monitoring purposes. The results of our uranium isotopic measurements on large volume, unspilred water samples from west Texas are shown in Table 11. (Onlyin the case of the on-site surface water sample can a possible deviation from natural isotopic composition be seen. This body of water receives storm runoff from areas which may contain small amounts of depleted uranium. Sources far removed from the facility and deep ground water 13how natural isotopic abundances. For greatest precision and accuracy, it is necessary to approximately match the samples and standards for both total activity and isotopic composition. Imprecise results have been obtained when sample and standard activities differed by more than a factor of 2. Even though our calibration curve is linear over an isotopic ratio range of 1000, the most precise results were achieved by simultaneous irradiation of standards that
LITERATURE C I T E D Stelnnes, E. Radiochem. Radloanal. Lett. 1973, 16, 25. Korllsch, J.; Godl, L. Anal. Chim. Acta 1974, 7 1 , 113. Hathaway, L. R.; James, G. W. Anal. Chem. 1975, 4 7 , 2035. Deutscher, R. L.; Mann, A. W. Analyst (London) 1977, 102, 929. Brits, R. J. N.; Smit, M. C. B. Anal. Chem. 1977, 4 9 , 67. Aider, J. F.; Das, B. C. At. Absorpf. Newslett. 1978, 17, 63. Gladney, E. S.; Curtis, D. B.; Perrin, D. R.; Owens, J. W.; Goode, W.E. "Nuclear 'Techniques for the Chemical Analysis of Environmental Materlals"; 1980, Los Aiamos Sclentlflc Laboratory Report LA-0192MS. Pakaius, P. Anal. Chim. Acta 1980, 120, 289. Chen, J. H.; Wasserburg, G. J. Anal. Chem. 1981, 5 3 , 2060. Harley, J. W., Ed. "EML Procedures Manual"; Report HASL-300, Environmental Measurements Laboratory, 1972. Gladney, E. S., Ed. "Envlronmental Surveillance at Los Alamos: Analytical Technlques, Data Management, and Quality Assurance", 1982; Los Alamos National Laboratory Report, in press.
RECEIVED for review September 20,1982. Accepted December 27,1982. Thirr work was performed under the auspices of the U.S. Department of Energy.
Modified Wide-Band Dye Laser Tunable over 5 cm-l with 0.08 cm-' Bandwidth David S. Bomlse' and Richard A. Keller" Los Alamos National Laboratory, Chemistry Division, University of Californla, Los Alamos, New Mexico 87545
Single frequency dye lasers represent the state of the art in tunable highly monochromatic light sources. Unfortunately, for most research problems of interest to chemists, their complexity, cost, and limited wavelength scan range (1 cm-l for the Coherent 599 and Coherent 699-21) are serious drawbacks. Furthennore, the single frequency laser line width, -3 X cm-l, is narrower than required for most optical spectroscopy experiments where Doppler and pressurebroadened line widths are -0.03 cm-l. We wish to report a simple procedure using the Coherent 590 dye laser which yields a scan range a t least five times greater than available with single frequency lasers and permits wavelength measurements with the Burleigh wavemeter. The wavemeter offers several distinct advantages over a conventional monochromator for lalser frequency measurement. Most important, the wavemeter accuracy is one part in lo6 (high resolution). In addition, the wavemeter does not have to be scanned manually to locate the input laser frequency and so yields much more reproducible readings than could be attained w t h a monochromator. Although our method does not yield single frequency operation, the laser line width is sufficiently narrow (50.08 cm-l) to suit many laboratory needs. This procedure should find application in a variety of experiments requiring dye laser tunability in excess of 1 cm-l. EXPERIMENTAL SECTION A Coherent 590, continuous wave (CW) dye laser was pumped at 4.2 W by the visible (all lines) output of a Spectra Physics 165
argon ion laser. The dye solution was Rhodamine 6G ( 5 X M in ethylene glycol). Both the three-plate birefringent filter and the 0.5-mm etalon (-20% R coating on both sides) were purchased from Coherent. The etalon was mounted on a two-degree-of-freedomtilt stage equipped with micrometer controls. With the etalon in the laser cavity, the vertical and horizontal angles were adjusted at least 1.2' (both axes) from normal to assure that the etalon did not act as a laser output mirror. Once the angle in the horizontal plane was set, it was kept fixed; wavelength waEi changed by adjusting the vertical tilt away from the laser beam axis. The dye laser bandwidth was examined with a Spectra Physics spectrum analyzer, Model 470, which has a 0.27 cm-' free spectral range. Output was displayed on an oscilloscopeand photographed. A glass microscope slide used as a beamsplitter diverted a portion of the dye laser output to a Burleigh Model 20 wavemeter used in the high-resolution mode. According to the manufacturer, the wavemeter has a maximum acceptable laser bandwidth of 0.066 cm-l on the high resolution setting, yet it did accept the 0.08 cm-l wide laser output. The stated accuracy of the wavemeter is +0.01 A at high resolution. At wavelengths -6000 A, this accuracy is equivalent to -0.03 cm-'. We infer the wavemeter precision is no worse than its accuracy. Although the spectrum analyzer results imply a dye laser bandwidth 1.5 times this value, frequency meawrements made with the etalon and Lyot filter adjusted for maximum power were steady to *0.03 cm-'. As the etalon was scanned (see below) and the output power dropped, scatter in the frequency measurements first increased to 0.06 cm-l when the power had diminished by 30% and then increased again to 0.09 cm-I when the power fell to 20% of the maximum value.
Present address: Exxon Research & Engineering Co., P.O. Box 45, Linden, N J 07036.
R E S U L T S AND DISCUSSION A Coherent 590 CW dye laser has cavity modes separated by -0.009 cm-l. This corresponds to -111 modes per c d .
0003-2700/83/0355-0977$01.50/0
0 1983 American Chemical Soclety
