Determination of Radiostrontium in Food and Other Environmental Samples C . R. Porter,' B. Kahn,2 M. W. Carter,' G . L. Rehnberg,l and E. W. Pepper' National Center for Radiological Health, Public Health Service, U.S. Department of Health, Education, and Welfare, Rockville, Md.
Radiostrontium in environmental samples is separated from the other alkaline earths in an EDTA medium. Earlier methods using the ion-exchange technique were modified to permit analysis of large samples, to provide quantitative separation from calcium in the wide range of concentrations normally encountered, to recover strontium in high yield, and to accommodate multiple analyses. The sensitivity of the procedure is 0.1 pc. per gram of ash when samples are counted for 50 minutes in a low-background beta counter; the average strontium yield is 7 0 x . Decontamination factors are 5 X l o 4 from calcium and a t least 1 X l o 4 from major radionuclides in fallout other than barium. Barium decontamination is 130. A procedure for the analysis of the barium plus radium fraction is also given.
T
he classical separation of strontium from calcium by means of the greater solubility of calcium nitrate in strong nitric acid and in certain organic solvents (Hillebrand, Lundell, et al., 1953; Willard and Goodspeed, 1936) has commonly been used to determine radiostrontium in environmental and biological samples (Porter, Augustine, et al., 1965). Complete removal of calcium simplifies yield measurements and self-absorption calculations in determining the concentrations of the pure beta emitters, strontium-89, and strontium-90. The procedure, however, requires numerous steps, and the use of fuming nitric acid holds a degree of hazard. Alternate methods for separation have therefore been introduced (Beccu, 1964; Butler, 1965; Campbell, 1965; Sunderman and Townley, 1960). Especially effective is the complexing of calcium with ethylenediaminetetraacetic acid (EDTA) at a pH at which strontium is mostly cationic. Strontium is then sorbed on cation-exchange resin (Davis, 1959; Elfers, Hallbach, c r a/., 1964; Farabee, 1958; Ibbett, 1967; Tsubota, 1965; Wade and Seim, 1961) or precipitated (Berak and Munick, 1961; Eakins and Gomm, 1966; Hunter and Mitchell, 1964), while the calcium remains in solution. Present address, Southeastern Radiological Health Laboratory, P. 0. Box 61, Montgomery, Ala. Present address, Nuclear Engineering Laboratory, 4676 Columbia Parkway, Cincinnati, Ohio
I n one of the simpler procedures, strontium, barium, and radium are retained o n a resin column and the calcium-EDTA complex and other contaminants pass through. Strontium is then selectively eluted to separate it from barium and radium. The technique has been demonstrated in tracer studies (Bouquiaux and Gillard, 1964; Davis, 1959; Duyckaerts and Lejeune, 1960) and applied to the analysis of urine (Farabee, 1958), bone (Wade and Seim, 1961), and a variety of environmental samples (Davis, 1961; Elfers, Hallbach, et al. 1964). The separation of strontium from calcium, however, occurs only within narrow p H ranges that depend critically on EDTA concentrations at equilibrium, and these, in turn, are affected by the amount of calcium that must be complexed. Groups of samples that contain a constant amount of calcium are easily analyzed, but variable amounts of calcium lead to difficulties. The problem has been minimized or eliminated in the past either by removing most of the calcium during pretreatment (Elfers, Hallbach, er al., 1964) or by matching the EDTA concentration to the predetermined calcium content (Davis, 1961). I n the method described here, sufficient excess EDTA is provided to analyze samples with a wide range of calcium content without prior treatment or individual adjustment of EDTA concentration. Moreover, calcium is removed so effectively that strontium may be determined gravimetrically as the carbonate, even though the calcium-strontium ratio in the sample may exceed 1000 t o 1. The strontium yield is consistently near 70 Z, and other radionuclides occurring in fallout do not contaminate the purified sample. Barium and radium can be determined as part of the same procedure. The procedure was tested with a variety of individual foods, mixed diets, bone, soil, vegetation, and biological specimens and has been used routinely for analyzing hundreds of mixeddiet samples. The procedure begins with the alkaline carbonate fusion of sample ash plus carriers. After the insoluble carbonates and hydroxides are washed and dissolved, EDTA is added, and magnesium is precipitated as H,Mg E D T A . 6 H 2 0(Brunisholz, 1957). The ion-exchange separation is performed, and the eluted strontium is precipitated as the carbonate. Strontium yield is measured gravimetrically, and radiostrontium is measured with a low-background beta counter. Strontium-90 is distinguished from strontium-89 either by recounting the sample and computing strontium-89 decay and yttrium-90 Volume 1, Number 9, September 1967 745
Table I. Effect of Hydrochloric Acid Concentration on Strontium and Barium Recovery Fraction 0 to 60 ml. acid 61 to 460 ml. acid 600 ml. 4 M NaCl Remaining on resin
3 68 25 < O , 01
