Fig.1-33 Conventional radiometric method and proposed ICP-MS method to determine 90Sr concentration in bone and tooth samples
Table 1-1 Measured 90Sr concentration in 0.1 g of cattle bone and tooth using the proposed ICP-MS and radiometric methods
Strontium-90 (90Sr) is a typical fission product, has a half-life of 28.8 y, and has bone-seeking properties. The radioactivity concentration of 90Sr in hard tissues of mammals (e.g., bones, teeth) is positively correlated with that of plants or soils obtained from the fields where the mammals have grown. Thus, the 90Sr concentration in hard tissues can be used to clarify the 90Sr distribution in the local environment. However, the conventional 90Sr analysis method (i.e., the radiometric method) needs samples of at least 1 g and thus cannot be used to determine 90Sr in small bone, tooth, or fish otolith samples.
Recently, a 90Sr analysis method in soil and plants using inductively coupled plasma mass spectrometry (ICP-MS) has been developed that has a comparable detection limit. Although this method can be used to analyze small samples with low Ca content, such as soils and plants, the high concentration of Ca and isobar (e.g., zirconium-90; 90Zr) in hard tissues interferes with 90Sr measurements. To measure the radioactivity concentration of 90Sr in hard tissues using ICP-MS, therefore, we investigated using a resin column to remove these interfering elements.
After acid digestion of 0.1-g samples of bones and teeth, the interfering elements were removed by chemical separation using a Sr resin to selectively uptake Sr into crown ether sites. ICP-MS was then performed on the solution eluted from the resin to determine the 90Sr concentration. The recovery rate of Sr was calculated using the degree of the decrease in 11 µg of stable strontium-88 (88Sr) in the 0.1 g samples before and after separation.
By performing column separation twice, our developed method recovered more than 80% of the Sr and removed 99.999% of the Ca. Furthermore, the proposed method provided results more rapidly than the conventional radiometric method (11 hours versus 20 days, respectively). Thus, coupling ICP-MS and our proposed separation technique allowed for the rapid analysis of 90Sr in cattle bone and tooth samples, as shown in Fig.1-33. The activity concentrations determined by ICP-MS after separation were in good agreement with those by the conventional radiometric method, as shown in Table 1-1. Further, the 90Sr detection limit using ICP-MS of 0.1 g samples was lower than that of the radioactivity measurement. Therefore, coupling the proposed separation technique with ICP-MS can be used to quantify the 90Sr concentration in small bone and tooth samples of terrestrial animals, thereby allowing researchers to track the distribution of 90Sr in animal habitats.
This work was performed in collaboration with Tohoku University as part of a project entitled, “Investigation of Sr incorporation recorded in teeth of animals related to environmental transfer”.
(Kazuma Koarai)
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