Table 11. Comparison of Results with Neutron Activation Analyses Neutron activation Present method 3 . 8 ppm i= 10% 3.9, 4.0, 3.9 2 . 0 ppm & 10% 2.0, 2.1, 2.0 1.25 ppm 10% 1 . 3 , 1 . 3 , 1.3
Sample A
B
*
133
Table 111. Recovery of Chlorine by Proposed Distillation Procedure
C1 added, PPm
C1 found, PPm
Deviation, PPm
Recovery, %
60.0 60.0 62.5 62.5
59.8 60.5 61.5 62.5
0.2 0.5 1 .o 0.0
99.7 100.8 98.4 100.0
Distillation from concentrated nitric-selenious acids or from nitric-sulfuric-selenious acids results in low (approximately 85 %) recovery of chlorine. This was assumed to be caused by partial oxidation of hydrogen chloride to higher valence states. Complete recovery can be attained, however, by diluting the nitric acid to 3 :l. Errors and Interferences. Protection of the precipitated silver chloride from direct ultraviolet light is necessary. When no such precaution was taken, results 7 4 % higher than expected were obtained. When selenium pellets were exposed to the laboratory atmosphere for several days, a n increase of about 1 ppm of chloride was found. The absorbed Contamination can be
removed by rinsing the sample with chloride-free distilled water. Bromine and iodine interfere if present in the original sample. Bromine as hydrobromic acid and iodine as free iodine distil with hydrochloric acid and precipitate as silver halide. Freshly precipitated silver bromide is almost completely soluble in ammoniacal solutions, while silver iodide is only sparingly soluble. Table I shows the results in presence of bromide and iodide. Multiple portions of 5 grams of chlorine-doped selenium were used as matrix, to which increments of bromide (as KBr solution) and iodide (as K I solution) were added. Sensitivity, Accuracy, and Precision. For 20-gram sample weight under the conditions prescribed, the limit of detection is 0.2 ppm chlorine. Accuracy of the procedure was evaluated by comparison with results of neutron activation analyses (Table 11) and by recovery experiments on synthetic samples (Table 111). Recovery was established by adding chloride as NaCl solution to 5-gram portions of high purity selenium and treating samples as described under “Procedure.” Precision was evaluated on a chlorine-doped selenium sample with an average chlorine concentration of 46.49 ppm. Standard deviation for a set of 56 results obtained in 19 separate runs during a 5-month period was 1.243 ppm (coefficient of variation :2.67
x).
ACKNOWLEDGMENT
The authors are indebted to A. Szell and G. Desabrais, who undertook most of the experimental work.
RECEIVED for review June 6, 1969. Accepted August 25, 1969.
Activation Analysis for Molybdenum in Samples Containing Large Amounts of Tungsten Barbara A. Thompson and Philip D. LaFleur Analytical Chemistry Division, National Bureau of Standards, Washington, D. C . 20234
THEDETERMINATION of molybdenum by neutron activation analysis has been carried out with excellent sensitivity by a number of workers (Z-12). The 0.142 MeV y-ray of 99mTc is usually used as a measure of the molybdenum concentration. Because of this low energy, chemical separations are often (1) J. F. Cosgrove and G. H. Morrison, ANAL.CHEM., 29, 1017 (1957). (2) B. A. Thompson, B. M. Strause, and M. B. Leboeuf, ibid., 30, 1023 (1958). (3) H. J. M. Bowen, Int. J . Appl. Radiat. Isotopes., 5, 227 (1959). (4) B. Van Zanten, D. Decat, and G. Leliaert, Talanta, 9, 213 (1962). ( 5 ) K. Samsahl, D. Brune, and P. 0. Wester, Int. J . Appl. Radiat. Isotopes, 16, 273 (1965). (6) H. Grosse-Ruyden and H. G. Doge, Talanfa, 12,73 (1965). (7) W. B. Healy and L. C. Bate, Anal. Chim. Acta, 33,443 (1965). (8) H. J. M. Bowen, Analyst (London), 92, 124 (1967). (9) H. D. Livingston and H. Smith, ANAL.CHEM., 39,538 (1967). (10) B. A. Thompson and P. D. LaFleur, ibid., 41,852 (1969). (11) L. D. LaFleur, Radiochem. Radioanal. Lett., 1(3), 225 (1969). (12) R. Dams and J. Hoste, Anal. Chim. Acta, 41, 197 (1968). 1888
necessary to avoid interferences from higher energy y-rays which may be present. A problem arises in cases where molybdenum must be determined in samples containing large amounts of tungsten. The chemistry of the two elements is similar and most methods which separate molybdenum also separate tungsten. Tungsten-187 interferes with the measurement of 99mTc not only because of the Compton radiation from its higher energy y-rays, but also because of the 0.136 MeV y-ray present in its decay. If the relative amount of lS7Wis not too high, the tungsten activity can be allowed to decay away before the 99mTc is measured. However, considerations of time required for the results of the analysis and simultaneous decay of 9 9 M ~ often make this approach undesirable. Other techniques incorporating spectrum stripping or resolution of composite decay curves do not, in general, give very high precision for this type of measurement. Cosgrove and Morrison ( I ) have described a chemical separation of traces of molybdenum from a tungsten matrix, but this is quite time consuming and requires a determination
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
of the chemical yield. Others (6,9,12)have reported methods based on the separation of 99mTcfrom the irradiated sample after allowing time for equilibration with the g9Mo parent. These methods are also fairly time consuming and the method of Livingston and Smith (9) requires chemical yield determinations for both molybdenum and technetium. In this paper we describe a method involving the separation of ggmTc which is very rapid and does not require any chemical yield determinations.
Table I. Molybdenum Concentrations found by HDEHP Double Extraction % Mo found in NBS Sample this work certified value Steel, NBS lOle 0.428 3Z 0.005a 0.426 Steel, NBS 178 0.00260 f O.ooOo9" 0.003 Steel, NBS 37b 1.08 3~ 0.02" 0.996* 23.90 =t0.06, W-Mo alloy (-70% W) 23.96 =!= 0.23c 23.75 i: 0.15*
EXPERIMENTAL
* Limits quoted are ts/d\/n for the 95 % confidence level; n = 24.
Limits quoted are ts/.\/fI for the 9 5 z confidence level; n
In this laboratory activation analysis for molybdenum has been carried out using solvent extraction to separate 99M0 from other interfering activities. Extraction of the a-benzoinoxime complex into CHCl, from 3N HCl has been used for analysis of steels (IO), and extraction with bis (2-ethylhexyl) orthophosphoric acid (HDEHP) from 1N-11N "03 has been used with a variety of matrices (11). Both methods give better than 99 % chemical yield of molybdenum, but both are subject to the interferences noted above whenever appreciable amounts of tungsten are present. In the case of steel samples the 9gmTc is also subject to interferences from the 0.143 MeV y-ray of 59Fe produced from the iron matrix. In both of the above solvent extraction procedures less than 1 % of the technetium present is extracted. Thus, in principle, it should be a simple matter to separate both tungsten and molybdenum by either procedure and then, after allowing time for 9gmTcto equilibrate with 99Mo,to separate technetium quantitatively by washing the organic solution with acid of the same strength as used for the original extraction. In practice, the a-benzoinoxime complexes are apparently too unstable over the time period required for equilibration to give satisfactory results with this procedure and so all work was carried out using the HDEHP system. The detailed results of extensive tracer studies carried out with the HDEHP system have been described elsewhere (13). The procedure outlined above has been applied to the determination of molybdenum in several NBS Standard Reference Material (SRM) steels and in a tungsten-molybdenum alloy containing about 7 0 x tungsten. The steel samples (-100 mg each) were irradiated for 1 hour in the glory tube facility of the Naval Research Laboratory Reactor (-8 X 1012 n . cm-2 . sec-1). The samples were contained in polyethylene snap-cap vials with a copper-flux monitor taped to the outside of each vial. After irradiation the samples were dissolved by warming with 1 : l HN03-HClOa. When the metal particles had dissolved, about 0.5 ml of H F was added and the mixture was evaporated to fumes of HClOa to volatilize SiF4. Fuming was continued for 30 minutes or longer to oxidize all carbon. The residue was taken up in 10 ml of 6N HCl and warmed with 2-3 drops of H F to redissolve any WOa. The solution was made up to 100-ml volume and 5-ml aliquots were taken for analysis as described below. (In the case of SRM 178, which has a low molybdenum concentration, the entire sample was used.) Previous experience (IO, I I ) has shown that no loss of either molybdenum or tungsten occurs in the course of this dissolution procedure. The tungsten-molybdenum alloy samples weighed about 5 mg each, After irradiation under the same conditions as the steel samples, they were dissolved in a small amount of HF-HN03 and made up to 100 ml volume. Six 5-ml aliquots were taken for analysis as follows. Each aliquot was combined with 22 ml of conc. HNOa and 3 ml of saturated H 3 B 0 3 (to complex any remaining
(13) I. H. Qureshi, L. T. McClendon, and P. D. LaFleur, Radiochim.Acta, in press.
c
= 5.
Provisional results.
fluoride) making the aqueous phase 11N in HNOI. For samples with low molybdenum concentrations -50 pg of molybdenum carrier was added. This solution was extracted with 25 ml of 0.75M HDEHP in either n-heptane or petroleum ether and the organic phase set aside for at least 24 hours to allow 9 9 m Tto~grow in. Because of simultaneous , is sufficient time for 9 g m Tto~ reach its decay of g g M ~this maximum level. The organic phase was then washed with This wash solution was placed in a 30 ml of 11N "03. polyethylene bottle and counted directly with a 3 in. x 3 in. NaI(T1) detector and a multichannel analyzer. The 0.142 MeV peak was used as a measure of the molybdenum present. In all cases comparisons were made to molybdenum standards irradiated simultaneously with the samples, carried through the same chemical procedure, and counted at the same geometry. Corrections were made for decay of 9gmTc after separation and for decay of 99M0 up to the time of separation. No corrections for chemical yield were considered necessary. RESULTS AND DISCUSSION
The results obtained are given in Table I. NBS certified values for the steels are shown for comparison. In the case of the tungsten-molybdenum alloy, comparison values were obtained by Christopher and Menis (14) using a differential spectrophotometric method. The results show that the method discussed has good precision and accuracy. Excellent decontamination is obtained from virtually all other activities because the first step removes all elements having extraction properties similar to technetium and the second step removes those with properties different from technetium. The method can thus be applied to nearly any type of material and is particularly suited to samples containing large amounts of tungsten. Under the conditions used for these experiments, the lower limit of detection is below 1 pg of molybdenum. ACKNOWLEDGMENT
The authors thank the staff of the NRL Reactor for assistance with the irradiations. RECEIVED for review July 29, 1969. Accepted September 8, 1969. Presented in part at the 156th National ACS Meeting, Atlantic City, N. J., September 1968.
(14) D. H. Christopher and 0. Menis, presented in part at the 4th Middle Atlantic Regional ACS Meeting Washington, D. C., February 1969.
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