existent on this column. The retention times for the components also are constant regardless of sample composition. As little as 2 pg of MMH or NZH4in a 3-pl water sample was detectable, resolved, and produced symmetrical peaks on column A. This indicates that only normal solvent-solute interactions are responsible for the separation on this column. Other columns such as F and G showed appreciable adsorption of H20,MMH, and N z Hwhen ~ the sample size was below 0.5 ,ul. Column Stability. 2-Hydrazinopyridine is a strongly alkaline material and reacts rapidly with COZ. Columns such as A, B, or C are stable for 6 to 10 days if some helium flow is maintained at all times; however, they deteriorate
rapidly if exposed to the atmosphere for several days. Thermogravimetric analysis of 2-hydrazinopyridine indicates that significant vaporization losses occur above 80" C. Solvents with higher boiling points would be an advantage; however, chemical similarity should be maintained. ACKNOWLEDGMENT
The authors are grateful to J. T. Nakamura and H. H. Martens for editing the original manuscript. RECEIVED for review February 13, 1967. Accepted May 26, 1967. This work was supported by the United States Air Force at the Rocket Propulsion Laboratory, Edwards, Calif.
Determination of Americium-241 in Pure Plutonium Using Extraction with Trioctylphosphine Oxide and Gamma Counting Joseph Bubernak, Marion S. Lew, and George M. Matlack Unicersiry of California, Los Alamos Scientific Laboratory, Los Alamos, N . M . RECENTWORK ON ELECTROREFINING methods at Los Alamos has resulted in the production of ultra pure plutonium containing less than 5 ppm of americium. For the determination of americium at this level, direct counting techniques such as combined alpha and gamma counting ( I ) proved to be inaccurate. This was due t o the high plutonium contribution to total gamma activity, thereby requiring the use of a large correction factor, and to the effect of uranium-237 gamma activity present as a decay product of plutonium-241. Several chemical methods are available for the separation of americium from plutonium and uranium. One method involves oxidation of plutonium to the hexavalent state with ceric or dichromate ion, followed by the carrying of americium(II1) on a precipitate of lanthanum fluoride ( 2 ) ; plutonium (VI) and presumably also uranium(V1) remain in solution. Another method involves adsorbing plutonium(1V) from 8M nitric acid onto anion exchange Dowex-1 resin and eluting the americium(II1) through the column with additional acid (3). Alternatively, plutonium(1V) can be extracted from 1M nitric acid into 0.5M thenoyltrifluoroacetone (TTA) in xylene, leaving americium(II1) in the aqueous phase (4). Experience in this laboratory has shown that the lanthanum fluoride precipitation and TTA-extraction methods give insufficient decontamination from plutonium to allow measurement of americium by alpha or gamma counting. In the anion exchange method incomplete separation of americium from uranium-237 prohibited the use of gamma counting, while alpha counting was unsuitable because of salts present from adjusting the valence state of plutonium. (1) J. Bubernak, M. S. Lew, and G. M. Matlack, ANAL.CHEM., 30, 1759 (1958). (2) R. A. Penneman and T. K. Keenan, U. S. At. Energy Comm., Rept. NAS-NS-3006 (1960). (3) F. P. Roberts and F. P. Brauer, U. S. At. Energy Comm., Rept. HW-60552 (1959). (4) F. L. Moore and J. E. Hudgens, Jr., ANAL.CHEM.,29, 1767 ( 1957).
An extraction method which seemed suitable to the present problem involves the use of trioctylphosphine oxide (TOPO). Extraction coefficients between TOPO in an organic solvent and 6 M nitric acid have been reported to be above 600 for plutonium(1V) and about 100 for uranium(VI), while americium(II1) shows no extractability (5). The possibility of utilizing such a system whereby americium could be determined in the aqueous phase after extraction was therefore investigated. Recently the determination of americium by extraction into di(2-ethylhexyl) orthophosphoric acid in toluene from 0.05-0.1M nitric acid followed by stripping into 3 M nitric acid has been described (6). Decontamination from plutonium and uranium were each greater than l o 4 and the americium was measured by counting its alpha activity. Although this method seemed applicable to the present problem, it was not evaluated. EXPERIMENTAL Reagents and Equipment. Trioctylphosphine oxide (TOPO) was obtained from Eastman Kodak Co., Rochester, N. Y. Solutions for extraction contained 50 grams of this reagent per liter of octane. Gamma activity measurements were made using a welltype sodium iodide (TI) crystal. The high voltage on the multiplier phototube was set for the detection of gamma rays with energy above 30 keV. Procedure. Pipet an aliquot of sample into a 3-ml extraction tube, rinsing the pipet with 3 drops of 1M nitric acid, Add 2 drops of 1 M hydroxylamine hydrochloride, mix and allow the solution to stand for 20-30 minutes. Add carefully 6 drops of 1 M sodium nitrite, then 10-12 drops of concentrated nitric acid. Add about one rnl of 5 TOPO in octane to the tube and mix the phases thoroughly for 5 minutes. Discard the organic phase containing plu( 5 ) J. C . White and W. J. Ross, U. S. At. Energy Comm., Rept. NAS-NS-3102 (1961).
(6) M. H. Campbell, ANAL.CHEM., 36,2065 (1964). VOL. 39, NO. 10, AUGUST 1967
1167
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Figure 2. Measured growth of americium-241 from purified plutonium sample by two methods RESULTS AND DISCUSSION TIME, DAYS
Figure 1. Calculated growth of americium-241 and uranium-237 from plutonium
tonium and uranium. Repeat the extraction with 1 ml of TOPO-octane, again discarding the organic phase. Rinse down the inside of the tube with octane and discard the rinse. Transfer the aqueous phase quantitatively to a test tube and measure the americium-241 gamma activity.
Table I. Extractability of Americium-241 by TOPO (Original aliquots contained 44409 gamma counts/minute of z41Am) Aqueous phase Organic phase cim Per cent clm Per cent 43932 98.9 165 0.4 43688 98.4 439 1 .o 43592 98.2 192 0.4 44344 99.9 186 0.4 43933 98.9 173 0.4 44011 99.1 164 0.4 Av. 98.9 0.5 Rel. std. dev. 0 . 6 z Table 11. Effect of Aliquot Size on Determination of Americium in Plutonium by the TOPO Method Pu activity Pu Am in aqueous phase, Aliquot, pl present, bg found, ppm equiv. Am ppma 25 248 12.0 0, 0 50 495 11.9 090 100 990 12.0
