Anal. Chem. 1986,58,3215-3218
3215
Registry No. Sucrose, 57-50-1.
LITERATURE CITED
I
Small Bore LiquU Chrqmatography Columns ; Their Properties and Uses; Scott, R., Ed.; Wlley: New York, 1983. Knox, J.; Gilbert, M. J. Chromatogr. 1979, 786,405. Ruzlcka, J. Anal. Chem. 1983, 55, 1038A. Jorgenson, J. Science (Washington, D.C.) 1984, 226, 254. Pawliszyn, J. Anal. Chem. 1985, 58, 243. Goiay, M. I n Gas Chromatography 7958; Desty, D.,Ed.; Butterworths: London, 1959. Schardin, H. Ergeb. Exacten Naturwiss , 1942, 20, 303. Muller, R. H. I n Advances in Electrochemistry and Electrochemical Engineering; Muller, R., Ed.; Willey-Interscience: New York, 1973; Voi. 9, pp 326-353. Pawiiszyn, J.; Weber, M. E.; Dignam, M. J.; Mandelis, A,; Venter, R. D.; Park, S.-M. Anal. Chem. 1986, 58, 239. Munk, M. I n Liquid Chromatography Detectors; Vickery, T., Ed.; Marcel Dekker: New York, 1983. HaNey, M.; Stearns, S.Anal. Chem. 1984, 56, 837. Harvey, M.; Stearns, S. J . Chromatogr. Sci. 1983, 27, 473. Ruzicka, J., Hansen, E. Flow In/ection Analysis; Wiley: New York, 1981. Pinkei, D. Anal. Chem. 1982, 5 4 , 503A. Zarrln, F.; Dovichi, N. J. Anal. Chern. 1985, 57, 2690. Hershberger, L. W.; Callis, J. 6.; Christian, G. D. Anal. Chem. 1979, 57,1444. Van Vliet, H. P. M., Poppe, H. J . Chromatogr. 1985, 346, 149. Pawliszyn, J., in preparation. Pawllszyn, J., detailed description is available on request. Fournier, D.; Boccara, A.; Badoz, J. Appl. Opt. 1982, 27, 74. Pawilszyn, J. Rev. Sci. Instrum., in press. Bauer, H.; Lewin, S. I n Physical Methods of Organic Chemistry; Welsserger, A., Ed.; Interscience: New York. 1960; Voi. 1, Part 2. CRC Handbook of Chemistry and Physics, 64th ed.; CRC: Boca Raton, FL, 1985. Ciouser, D. E.; Craven, T. S. U S . Patent 4 185490. Wells, G. J. Chromatogr. 1985, 379, 263. Pawliszyn, J.; Weber, M.; Dignam, M.; Mandelis. A,; Venter, R.; Park, S.-M. Anal. Chem. 1988, 58, 236. Synovec, R.; Yeung, E. Anal. Chem. 1985, 57, 2162.
I
0
O
1IME
Figure 10. Computer-simulatedchromatogram of a sample consisting of three components: total number of calculated points, 1920; retention times, 560/900/1500; mass ratio, 1:2:3. (A) Chromatogram without the noise. (B) Chromatogram with the superimposed Gaussian 0.3). (C) First numerical integral of the chromatogram noise ( S I N (S/N 3). (D) Second numerical integral of the chromatogram ( S I N
-
- -
15).
The total amount of sample detected was in the nanogram range. However, by using an additional excitation beam, one can simultaneously detect the absorbing components of the analyzed mixture. The detection limit in that case is several orders of magnitude lower compared to the nonselective mode of operation (5). By use of this sensor in its dual mode of operation, the direct assignment of chromatographic peaks would be possible on the basis of the extinction coefficient at a given wavelength, since both absorption and concentration information will be provided simultaneously.
ACKNOWLEDGMENT I thank J. Harris for suggesting the use of the sheath flow technique.
RECEIVED for review February 24,1986. Accepted August 18, 1986. This project was supported in part by USU Faculty Research Program.
Simultaneous Determination of Americium and Plutonium by Liquid Scintillation Counting Using a Two-Phase Cocktail John J. Miglio Los Alamos National Laboratory, Los Alamos, New Mexico 87545 A method for the simultaneous determlnatlon of 238Puand 241Amby llquld scintlllatlon counting uslng a multlchannel instrument has been developed. A two-phase cocktail is employed that separates mPu from 241Am.Plutonlum is oxMlzed to the +4 state whlle americium remalns in the +3 state. This difference in oxidation states allows the extractlon of plutonium by ~(P-ethylhexyl)hydrogen phosphate (D2EHPA) Into the toluenebased phase of the liquid sclntlllatlon cocktail. The amerlclum remalns in a 0.15 M "0,-dioxane-based cocktail. Differences In sclntlllatlon efflclency between phases give rise to spectral peaks that do not overlap and also permlt the simultaneous detennhratlon of the two isotopes whose energies of emltted a particles differ by only 14 keV. Recoveries for each Isotope average better than 95 % even when the ratio of one isotope to the other is 1O:l.
The determination of a-emitting radionuclides by liquid scintillation counting has been a common practice in numerous
laboratories for many years. This method of analysis is not only rapid but also essentially 100% efficient. There are, however, two disadvantages that limit the utility of the method. The instrument background is much higher than with other methods of counting, such as (Y spectroscopy, and the method is relatively insensitive to differences in energy of the emitted radiation. For example, 238Puand 239Pu,whose a particle energies differ by 344 keV, are easily resolved by (Y spectroscopy but are not sufficiently resolved by liquid scintillation counting for quantitative work. Although 344 keV appears to be a significant energy difference, these two isotopes differ in energy by less than 7 7'. The energy difference between 238Puand 241Amis only 14 keV. When both are present in a sample, a single spectral peak is obtained, thus preventing the simultaneous determination of both isotopes. Previously, if both isotopes were to be determined by liquid scintillation counting, two steps were required. One step involved the determination of the count rate of one isotope by use of a suitable extractant
0003-2700/86/0358-3215$01.50/0 0 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
cocktail, and the second step required the determination of the total count rate. The count rate of the second isotope was determined by difference. This method is most suitable when the activities of both isotopes are approximately equal. Prior chemical separation of the two elements is another technique that has been employed, but it suffers from the disadvantage of variable chemical recoveries. The simultaneous determination of americium and Plutonium by liquid scintillation counting can be accomplished by using a two-phase cocktail. Although 23sPuand 241Amhave nearly identical a particle energies, they exhibit different oxidation states. Americium primarily exhibits a +3 state, whereas plutonium is easily oxidized from the +3 state to the +4 state. This difference in oxidation states is commonly employed to separate the two elements. A suitable cocktail for extracting Pu(IV) from aqueous solutions had been previously developed (1) and was chosen for one of the phases. This cocktail employs toluene as a solvent and exhibits minimal miscibility with water. A modification of Bray’s cocktail (a),which is p-dioxane based, was used as the other phase. The advantage of Bray’s cocktail is its ability to accept relatively large amounts of aqueous solutions and still function as a scintillator. When sufficient aqueous solution is added to a solution of the two cocktails, a two-phase system results with scintillators present in both phases. The resulting partitioning of 238pu in one phase and =lAm in the other phase allows both isotopes to be counted simultaneously if the scintillation efficiency is different for the two phases. If the scintillation efficiency of each phase is not sufficiently different, a dye can be added to one phase to cause a shift in the spectrum of one isotope. To make maximum use of sufficiently separated spectra, a scintillation counter must have at least two independent channels. By appropriate adjustment of the gain and discriminator settings, a single isotope can be counted in each channel. The counting efficiency in either phase does not have to be 1009’0,but it does have to be constant. Counters are commercially available that permit the simultaneous determination of americium and plutonium by liquid scintillation counting.
EXPERIMENTAL SECTION Apparatus. Scintillation counting was done with a Packard Tri-carb Model 3320 liquid scintillation counter. All counting was done using 20-mL low-potassium glass vials. Two-phase samples were shaken for at least 0.5 h at approximately 200 excursions/min on an Eberback Model 6000 platform shaker to ensure the extraction of the plutonium-238 into the D2EHPA phase. Reagents. p-Dioxane, suitable for use in liquid scintillation counting, was purchased from EM Science, Gibbstown, NJ. Ethylene glycol was purchased from MCB Manufacturing Chemists, Inc., Cincinnati, OH. Methanol, low in acetone, was purchased from J. T. Baker Chemical Co., Phillipsburg, NJ. Reagent grade nitric acid was purchased from VWR Scientific, San Francisco, CA. A solution of yellow dye, FD&C No. 5, was prepared from Schilling brand food coloring by diluting 0.1 mL of the food coloring with 1.0 mL of deionized water. Insta-Gel was purchased from Packard Instrument Co., Downers Grove, IL. All other chemicals were purchased from Eastman Kodak Co., Rochester, NY. The toluene and all scintillators were “scintillation grade”. The bis(2-ethylhexyl) hydrogen phosphate was technical grade. All chemicals were used without further purification. The D2EHPA cocktail was prepared by dissolving 100 mL of D2EHPA, 5 g of p-terphenyl, and 0.05 g of 2,2-p-phenylenebis(5-phenyloxazole)(POPOP) in 900 mL of toluene. Bray’s cocktail was prepared by dissolving 60 g of naphthalene, 4 g of 2,5-diphenyloxazole (PPO), and 0.4 g of POPOP in 20 mL of ethylene glycol, 100 mL of methanol, and 880 mL of p-dioxane. Procedure. Plutonium solutions were analyzed for americium content by liquid scintillation counting in Insta-Gel, which is suitable for counting both plutonium and americium, and by
counting in D2EHPA, which extracts and counts only plutonium. Both cocktails gave identical counts, indicating no americium contamination of the plutonium solution. Americium solutions were analyzed for plutonium content by extracting an americium solution with DPEHPA cocktail, removing the aqueous phase, replacing it with fresh 4 M HNO,, and counting for plutonium. No plutonium was detected. Isotopic compositions of all Plutonium and americium solutions were further determined by (Y spectroscopy and found to be >99.5% pure. Volumes of all solutions of plutonium and americium were measured with Eppendorf pipets. Solutions of radionuclides were pipetted into glass scintillation vials and evaporated to dryness on a hot plate at low heat. A heat lamp was used to aid in the evaporation by preventing condensation on the upper walls of the vial. One milliliter of concentrated HNOBwas added and the solution was evaporated to dryness again. The addition of HNO, and the evaporation process were repeated. The purpose of adding HN03 was to ensure the formation of Pu(1V). Except as noted, the residue was dissolved in 3 mL of 0.15 M HNO, followed by 10 mL of Bray’s cocktail and 0.8 mL of the D2EHPA cocktail. The D2EHPA cocktail was added in two steps. The first step was the addition of 0.5 mL of cocktail followed by swirling to form a one-phase system. The second step was the addition of the remaining 0.3 mL of cocktail. Vials were shaken for at least 0.5 h prior to counting. Two phases were usually present at room temperature and always present at the lower temperature of the counting chamber in the liquid scintillation counter. The DPEHPA phase was approximately 2 mL in volume and occasionally was slightly yellow. This increase in volume is the result of partial miscibility of Bray’s cocktail in the D2EHPA phase. Before counting, vials were allowed to stand for at least 2 h in order to ensure complete separation of the phases, which separated very slowly. The D2EHPA phase was less dense than the Bray phase. Plutonium was counted at 2 % gain with the window set from 100 divisions to 500 divisions. Americium was counted at 20% gain with the window set at 25-1000 divisions. For each isotope, plots of number of counts as a function of window setting were obtained by counting in successive windows, with each window 20 divisions wide. The data were plotted as smooth curves by connecting the midpoints of each window. For all plots the number of counts was normalized to the highest number recorded for each spectrum. The effect of acid concentration on counting efficiency and the extraction of 241Aminto the D2EHPA phase was determined by dissolving evaporated solutions of americium, as described above, in HNOBsolutions of varying molarity, adding the cocktails, shaking the vials, and counting the solutions at 10-8070 gain at intervals of 10%. Recoveries of americium and plutonium were determined by counting mixtures of the two isotopes in solutions of 0.15 M HN03 The activity ratio of the isotopes varied from approximately 1:l to over 1 O : l with a lower level of activity of 350 cpm. In addition, activity ratios of 1:l were investigated at the 80 cpm level. The effect of yellow dye, FD&C No. 5, on the spectra was determined by adding 0.050 mL of dye solution to a two-phase system containing americium and plutonium. The dye was added after the cocktails had been added and after the vials had been shaken for 1 h. The spectra were determined and plotted as described above.
RESULTS AND DISCUSSION Ten milliliters of the dioxane cocktail and 0.8 mL of the DBEHPA cocktail were completely miscible. As a nitric acid solution was added to the cocktail solution, a point was reached when two phases formed. The volume of HNO, necessary to form a two-phase system was found to be dependent on the acid concentration and the temperature. As the acid concentration was increased, less HNO, solution was required to form a two-phase system. At a concentration of 0.05 M HNO,, 0.9 mL of acid was required for the formation of two phases at room temperature. If only 0.8 mL of 0.05 M H N 0 3 was used, one phase was present at room temperature, although two phases formed in the refrigerated compartment of the liquid scintillation counter, which was maintained at 4 “C.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
1
020
3217
t \
WINDOW (energy-)
MOLARITY OF HNO3
Flgure 3. Plutonium-238 spectra in D2EHPA phase at 2 % gain: (0) no dye added and
Flgure 1. Fraction of 241Amcounted at 2 % gain and a 100-5OOdivision window as a function of molarity of added "Os. 1oor
(e)FDBC No. 5 dye added.
1 .o v)
#-*--q
F
3
000
0.6
0.6
W
5
2
4 0.4
z
60c
P
0.2
0.0
100
200
300
400
WINDOW (energy
500
600
700
4)
Flgure 4. Americium-241 spectra in Bray's phase: (0) no dye added, 20% gain, and (0)FD&C No. 5 dye added, 3 0 % gain.
96 Gain Flgure 2. Percent 241Amcounted in a 25-1000-division window as a function of gain at various nttric acid concentrations: (0)0.05 M, (U) 0.10 M, (0)0.15 M, (A)0.20 M, (0) 0.25 M, (A)0.30 M.
Figure 1 shows the fraction of 241Amcounted in the D2EHPA phase as a function of added acid concentration. A t an added acid concentration of 0.05 M HN03, approximately 20% of the americium appears to be extracted into the D2EHPA phase. When the added acid concentration is increased to 0.15 M H N 0 3 approximately 2% of the americium is extracted. Although less than 1% of the americium is extracted when the HN03 concentration is 0.30 M, the scintillation efficiency of the dioxane-based phase decreases so that a maximum of 90% of the americium is counted. This decrease in efficiency is due to a shift in the energy spectrum toward lower energy. Such a shift in the spectrum requires an increase in the gain setting to maximize to total count. This increase in gain results in a much broader spectrum. Thus, the overall counting efficiency of 241Amin Bray's cocktail is the result of opposing factors; increasing the nitric acid concentration increases the americium concentration in the dioxane-based phase but also decreases the overall efficiency of the scintillation process. Figure 2 shows the percentage of 241Am counted in the Bray phase as a function of acid concentration and gain setting. The low percentage of americium-241 counted in the americium channel at low acid concentrations and low gain is attributable to the increased concentration of americium-241 in the D2EHPA-based phase or a shift in the spectrum. A t low acid concentrations and high gains, the still lower percentages counted are caused by a shifting of the spectra beyond the counting window. At high acid concentrations the percentage of 241Amcounted in the Bray phase is relatively independent of gain, exhibiting a maximum between 40% and
60%. The lower percentages counted at the higher acidities are the result of a lower efficiency of the scintillation process and a broad peak extending beyond the counting window. A nitric acid concentration of 0.15 M was chosen for the determination of mixtures of americium and plutonium in mixtures because of the high percentage of americium counted a t gains between 10% and 30%. Also, the slightly higher recoveries at higher gain settings indicated a flatter curve of percentage of americium counted in Bray's solution as a function of percent gain. The counting rate obtained in 0.20 M HN03 decreased more rapidly at higher gains than did the counting rate in 0.15 M "OB. The order of addition of reagents is important. The experiment to determine the relationship between percentage of americium recovered and gain at different acid concentrations was repeated using a different order of addition of reagents. In this experiment the D2EHPA cocktail was added before the Bray cocktail. The results were erratic: paired samples yielded americium recoveries as divergent as 79.2% and 52.7 %. When the order of cocktail addition was reversed no such problems were encountered. In another experiment the 0.8 mL of Bray's solution was added all at once and then shaken. In some instances the recoveries were approximately 3% lower than values obtained by adding the Bray cocktail in two portions as indicated above. The energy spectrum recorded at 2% gain (Figure 3) exhibits a significant number of counts in the region of 50-90 divisions. These counts are due to 241Amand can be eliminated by counting 238Puin a window of 100-500 divisions. 241Amcan be counted in the second channel with a window of 25-1000 divisions and a gain of 20%. Figure 4 shows the 241Amspectrum. Under these conditions the spectra of both americium and plutonium are relatively sharp. If the gain of the 241Amchannel is increased to 40%, the americium spectrum becomes significantly broader and is shifted partially out of the window; however, approximately 87% of the disintegrations can still be recorded. A t gains greater than 4070,
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Table I. Recovery of Plutonium and Americium from Solutions Containing Both Isotopes element
cpm added
activity ratio Pu/Am
av '70recovered
Pu Am
6660 339
102.0 f 0.3" 100.3 f 9.0"
Pu Am
344 4130
98.1 i 0.gb 100.4 f O.@
0.083
Pu Am
344 339
99.6 f 0.2" 101.7 f 0.1"
1.01
Pu Am
6660 4130
102.3 f 0.5" 98.2 f 0.5"
1.59
Pu Am
201 169
95.8 f 0.9' 103.3 f 3.4'
1.19
96.3 f 2.0' 105.2 i 1.2'
1.19
80.5 67.9
Pu Am
n = 2, f average deviation.
19.6
n = 4, f 1 SD.
n = 3, f 1 SD.
the americium spectrum extends beyond the window and the fraction of americium counted decreases. At the suggested gain settings, 2.37% of the plutonium present counts in the americium channel, and 2.58% of the americium counts in the plutonium channel. These values were determined at counting rates between 4000 and 7000 cpm. The contribution of plutonium to the americium counts and the contribution of americium to the plutonium counts were determined by counting each isotope individually with both cocktails present. The contribution from counts by one isotope to the counts of the other is the result of the distribution of each isotope between the two phases and/or counting of an isotope across the phase boundary. The latter occurs when an atom near the phase boundary decays and the emitted (Y particle passes across the phase boundary. The addition of 0.050 mL of yellow dye solution caused an appreciable shift in the %lAmspectrum toward lower energies without a significant shift in the plutonium spectrum. Visually, the dye appeared to be distributed in the dioxane phase only. The amount of dye used will determine the magnitude of the spectral shift. This technique should also be applicable to other isotope pairs when the spectra overlap. The shifted spectra are also shown in Figures 3 and 4. Table I shows the recoveries of each isotope from solutions containing both 23sPu and 241Am. In all cases the counting results are corrected for the contribution of the other isotope. The equations used to calculate these results are
where Cc is the counts per minute calculated. The activity of solutions containing both 238Puand 241Am can be calculated from these two simultaneous equations in two unknowns. The initial concentrations of all solutions were determined by liquid scintillation counting in either Insta-Gel or DPEHPA. The results indicate both 238Puand 241Amcan be quantitatively determined even when the activity of one isotope is 10 times the activity of the other. In addition, amounts of both isotopes can be determined when each is present a t the 80 cpm level at an activity ratio of 1:l. Initial experiments in which plutonium/americium residues were dissolved in 3.5 mL of 0.15 M HN03, 10 mL of Bray's solution, and 1 mL of D2EHPA gave plutonium recoveries of 100% and americium recoveries of 83% for solutions containing 6600 cpm of plutonium-238 and 4200 cpm of americium-241. A gain of 50% was necessary to maximize the americium counts. Results were somewhat erratic and the spectra peaks were somewhat broader. Reducing the volumes of added H N 0 3 and D2EHPA had the effect of improving scintillation efficiency and producing sharper spectral peaks. Background counts at the conditions suggested for optimum counting of plutonium and americium were typically 20 cpm a t the plutonium settings and 41 cpm at the americium settings. The high backgrounds observed are due to the relatively high location of Los Alamos National Laboratory at 2200 m.
CONCLUSIONS Americium and plutonium can be simultaneously determined by liquid scintillation counting with a recovery of over 95% for each isotope. The spectra are sufficiently separated with no overlap of the peaks. Although plutonium-238 and americium-241 were used in this study, there is no reason this technique cannot be extended to other isotope pairs that exhibit different oxidation states and partition suitably in a two-phase cocktail system. For example, curium and californium exist primarily in the +3 oxidation state and should behave like americium. Plutonium-239 will behave like plutonium-238. The gain settings used in counting these other isotopes may be different because of the differences in energy, but the changes should be slight. In addition, the use, if necessary, of a very small amount of a dye, such as yellow food coloring, can shift the spectra of the isotope in the dioxane phase by color quenching. Although a dye could be added to the D2EHPA phase, this would have the effect of reducing the separation between the spectral peaks and would not be beneficial in this instance. With other pairs of isotopes this approach might be beneficial. The liquid scintillation counter settings used in this study may need to be adjusted slightly for other instruments, but the principle remains the same. ACKNOWLEDGMENT I thank Thomas Whaley for his suggestions in the preparation of this manuscript. Registry No. DBEHPA, 298-07-7; 238Pu,13981-16-3; 241Am, 14596-10-2.
and
Am
LITERATURE CITED
where Cm is the counts per minute measured, C' is the counts per minute added, and f is the fraction of isotope counted in the other window. (The subscripts P u and Am refer to plutonium and americium, respectively.) These equations can be rewritten in the form (CmPu
+ C'PufPu
- C'AmfAm)
=
ccPu
(3) (4)
(1) Keough, R. F.; Powers, G. J. Anal. Chem. 1970, 4 2 , 419-421. (2) Bray, George A. Anal. Blochem. 1960, I , 279-285.
RECEIVED for review June 13,1986. Accepted August 20,1986. This work was supported by the Division of Biological and Environmental Research and was performed at the Los Alamos National Laboratory, which is operated under auspices of the U.S. Department of Energy, Contract W-7405-ENG-36.
