Inductively coupled plasma mass spectrometry for stable isotope tracer investigations

Inductively coupled plasma mass spectrometry (ICP-MS) has now been developed for application to stable isotope tracer investigations of several minerals/trace elements. Use of this method for such purposes requires an understanding of a number of fundamental issues: analytical chemistry performance of the method of isotopic analysis, relationship of the level of enriched isotope administered to the subject with background level of the isotope already present, the issues of cost, and finally the specific details of the biological issues to be explored.In this paper, a brief discussion of these issues is presented. As an example, the discussion is presented in relation to selected aspects of metabolism of selenium, employing the three stable isotopes⁷⁴Se,⁷⁷Se, and⁸²Se in the rat as the biological model.Analytical performance of hydride generation/ICP-MS is discussed for the required analyses of selenium isotopes. It is shown that for solutions containing 10 ng/ml Se of natural isotopic composition, optimized signal/background ratios greater than 40/1 can be obtained, resulting in worst-case detection limits (ng Se) of 2 (⁷⁴Se), and 0.6 (^77,82Se). The precision and accuracy of isotope ratio measurements for the method used routinely in biological studies is 1%. The accuracy of the method for quantitative isotopic analysis is compared with hydride generation/atomic absorption spectrophotometry (HG/AAS). The following results are given (g Se/g or ml; mean + 1 SD,n = 3–5; first HG/ICP-MS, second HG/AAS): SRM 1577a bovine liver] 0.697 ± 0.002 versus 0.69 ± 0.01; human blood plasma 0.098 ± 0.001 versus 0.135 ± 0.008; human red cells 0.211 ± 0.002 versus 0.216 ± 0.012; and human urine 0.0473 ± 0.0003 versus 0.0489 ± 0.0003.An experiment is described with the rat to show the feasibility of the method for studies of selenium metabolism. Rats were placed on Se-free diet for eight weeks, given their Se requirements in the drinking water in the form of⁷⁶SeO ₃ ^2– and a single-day (day 3) replacement of their water with that containing highly enriched⁷⁴SeO ₃ ^2– . Isotopic analysis of carcass and selected organs revealed a high degree of isotopic enrichment with respect to⁷⁴Se during the entire eight weeks of the experiment, indicating the feasibility of this approach for detailed investigations of selenium metabolism in the rat.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria