sodium perchlorate changes solubility greatly with temperature, and the liquid in the cell frequently turns to a thick slurry. It is likely that this type of cell shows larger overvoltage because of dehydration of the cadmium chloride hydrate by the large excess of sodium perchlorate. Type A cells without hydrated cadmium chloride showed considerably larger overvoltage than cells containing the hydrated salt. Sufficient vapor pressure data have not been found t o determine whether NaC104will dehydrate CdClr. H 2 0 . The best reference electrodes appear to be those with just NaCI, CdCI2, and CdC12.H20 in contact with the cadmium amalgam. If chloride ion is incompatible with the system under investigation, a saturated sodium perchlorate salt
bridge can be interposed without difficulty. The Cd/CdCls electrode would seem to be directly applicable to polarographic use. Consideration of the low current level used in polarographic measurements, and the fact that the overvoltage is so low a t such a level, leads one to conclude that this electrode system would function as a nonpolarizable electrode in D M F solvent. The fact that no extra purification of D M F is generally necessary for the electrode to function properly speaks for the practicality of the system. RECEIVED for review December 16, 1966. Accepted March 15, 1967. Research supported by National Science Foundation Grant G P 5044.
Determination of Strontium-90 in Human Bones by Tributyl Phosphate Edmond J. Baratta and Esther S. Ferri Northeastern Radiological Health Laboratory, National Center for Radiological Health, Public Health Service, U . S . Department of Health, Education, and Welfare, Winchester, Mass.
THEPROCEDURE "Simplified Determination of Strontium-90" ( I ) has been adapted for the determination of strontium-90 in human bone. The yttrium-90, which is in equilibrium with the strontium-90 at time of analysis, is directly extracted from bone ash in approximately 14N H N 0 3 with 100% n-tributyl phosphate (TBP), and is stripped from the TBP into 3N "03. The Y is then precipitated as the oxalate for counting. Mercer (2) has shown that adequate extraction is obtained even in the presence of phosphate. Stripping with 3 N H N 0 3 assures that any naturally occurring radioisotopes of Th and U will be separated from the Y (3). Velten and Goldin ( I ) have stated that Zr-Nb and Pm would constitute a serious interference in the TBP extract. However, gamma analysis of human bones in this laboratory have shown no detectable amounts of activity due to Zr-Nb. Precipitation of the Y oxalate for counting further discriminates against carrying through of Zr-Nb. Pm activity has been reported to be about 1% of the strontium activity in steer bone (4). One would not expect this nuclide to be more concentrated in human bone. Because strontium-90 activity in human bone is one fifth or less than that of steer bone, any Pm contamination would be negligible. The method, which is being routinely applied to the determination of Human Bone Network samples of the USPHS (3, permits the determination of strontium-89, if this nuclide is present, by conventional purification treatment of the separated strontium fraction. It also permits, if necessary, re-
(1) R. J . Velten and A. S. Goldin, ANAL.CHEM., 33, 128 (1961).
(2) E. R. Mercer, J . D. Burton, K . B. Gunn, and A. Black, Healfh Phys., 11, 37 (1965). ( 3 ) V. R. Hunt, E. P. Radford, and A. J. Segall, Intern. J . Radiation Biol., l , 217-87 (1963). (4) M. Ekenbud and H. G . Petrow, U. S . A t . Energy Comm. Repi. .4T-(30-1), 2896 (1964). (5) Rad. Health Data arid Reports, U. S. Depdrttnent of Health, Education, and Welfare, July 1965, April 1966, June 1966, S2ptember 1966, October 1966, and December 1966.
846
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ANALYTICAL CHEMISTRY
milking of Y from the strontium fraction after suitable ingrowth. The procedure is rapid requiring 2.5 man-hours for analysis of four samples. EXPERIMENTAL
Apparatus. A low-level background beta counter with a background of less than 0.50 count per minute was used. Procedure. A portion of bone ash (5-10 grams) is dissolved in hot 12N H N 0 3 and diluted t o 100 ml with 12N HNOI. A 50-1111 aliquot of this is transferred t o a 250-ml centrifuge bottle and 90 mg of S r + 2 and 20 mg of Y+3carriers are added. The strontium is precipitated by the addition of fuming (90%) "03, cooled, centrifuged, and the supernate transferred to a 250-1111 separatory funnel. The time is recorded at this point as the beginning of the decay of the separated yttrium-90. Then the solution is made 14N in H N 0 3 . The solution is extracted with 50 ml of 100% TBP, freshly equilibrated with 16N "03 for 3-4 minutes, and the aqueous phase transferred to a second separatory funnel. The aqueous phase is again extracted with TBP, the organic phases are combined and washed 3 times with 40 ml of 14N H N 0 3 , discarding the washings each time. The yttrium is back-extracted with one 50-ml HzO wash and four 25-1111 washes of 3N H N 0 3 , collecting the washes in a 250-ml centrifuge bottle. To the centrifuge bottle is added 10 ml of 2 N H2C204,and the pH is adjusted to 1.0-1.5 with concentrated N H 4 0 H . The precipitate is collected by centrifuging, transferred to a 40-ml centrifuge tube, and washed with warm H20. The precipitate is taken u p in warm HzO, transferred to a pre-weighed filter paper, washed 3 times with a water miscible solvent (acetone, methyl alcohol, ethyl alcohol), and dried in an oven a t 125" C for 20 minutes. The sample is cooled and reweighed to determine the chemical yield. The sample is mounted on a nylon support, covered with Mylar, and the beta activity of yttrium-90 is measured in a low-background beta counter. The strontium-90 activity is computed using the following equation:
A = detector background; B = reagent blank; C = chemical yield; D = detector geometry; E = decay factor for yttrium between separation and counting; W = weight ash in grams. RESULTS AND DISCUSSION The purity of the yttrium90 has been checked by decay measurements of several half-lives. Overall chemical yields of Y carrier have averi3ged 85% based on multiple analyses of 5- to 10-gram samples and tracer studies using yttrium-88. Samples of simulated bone Ca3(P04)2spiked with known amounts of 90Sr-90Y in equilibrium a t three activity levels were analyzed in quadrupliceie by the above procedure. Replicate analyses on human and animal bone pool samples were also performed. Results are presented in Table I. Samples of bone ash obtained from a n outside laboratory were analyzed in quadriiplicate. A comparison of results are shown in Table 11. The two-sigma ana1y::ical error based on the mean range of 43 sets of duplicate network samples averaging 1 .OO pCi gOSr/ gram ash was calculated to be 0.21 pCi goSr/gram ash using the described method. Decontamination fac,tors for various naturally occurring nuclides and fission products are presented in Table 111. Thorium and promethium both show relatively low factors, but for the activity levels of these nuclides that have been reported in bone (4, 6) .these factors are adequate t o cause no interference. Reagent blanks were determined and did not show any appreciable activity above counter background. Backgrounds for the counters were obtained by placing a nylon ring and disk with glass fiber filter paper covered with Mylar in the counting chamber. Reagent blanks averaged 0.49 cpm compared to counter plus ring backgrounds of 0.44 cprn. With this background iind 40-45x geometry as little as 0.5 pCi of strontium-90 can be determined per sample. RECEIVED for review July 16, 1965. Resubmitted January 25, 1967. Accepted March 28,1967. (6) D. F. Peppard, J. P. Faris, P. R. Gray, and G . W. Mason, J . Phys. Chem., 57, 294 111953).
Table 1. Replicate Analysis of Simulated Spiked Bone and Human and Animal Bone Pool Ash Type Simulated bone spike 0.99 pCi/gm ash 3.08 pCi/gm ash 10.36 pCi/gm ash Animal pool Human pool
pCi Yk/gm ash.--Determined Mean f 2 S.D. 1.00 0.90 2.77 2.79 10.29
0.88
0.86 f 0.28
0.67 2.18 2.57 8.66
2.57 f 0.56
8.43 8.80 11.32 10.99 11.19 10.30 10.15 0.91 0.87 0.89 0.80
9.04 f 1.68 10.79 f 1.06 0.87 f 0.10
Table 11. Interlaboratory Comparison of Bone Results Sample 1 2 3
NERHL dpm '%r/gm ash 0.98 f 0.22 - 2 S.D. 0.55 f 0.24 - 2 S.D. 1 . 0 4 f O . 1 8 - 2S.D.
HASL dpm *Sr/gm ash 1.05 0.51 1.09
Table 111. Decontamination Factors Cerium-144 >2 x IO'
Iron-55 Zinc-65 Zirconiun~niobium-95 Manganese55 Strontium-90 Ruthenium-106 Cobalt-60 Thorium-230 Uranium-238 Promethium-147 Radium-226 Lead-210 Actini um-227
3 >2 >5
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30 >lo' 20 >5
x 10'
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