Accurate determination of iron, copper and zinc in human serum by isotope dilution analysis using double focusing ICP-MS

The development of an ICP-MS method for the determination of three essential trace elements, Fe, Cu and Zn, in human serum by isotope dilution analysis is described. A double focusing ICP mass spectrometer (ELEMENT) working at a nominal resolution of 3000 allowed the complete resolution of polyatomic ions overlapping the main isotopes of these elements. The isotope ratios measured for the determination of Fe, Cu and Zn were Fe-57/Fe-56, Cu-65/Cu-63 and Zn-67/Zn-64, respectively. Instrumental parameters of the mass spectrometer were optimised for optimum precision, while keeping the total time of analysis under 6 min (2 min per element). The RSD values obtained for the isotope ratios of Fe (57/56), Cu (65/63) and Zn (67/64) were 0.7, 0.2 and 0.8%, respectively, for a natural standard aqueous solution of 50 mu g l(-1). The measured isotope ratios were corrected for dead time on the detector and mass discrimination errors. The sample to spike ratio was optimised to minimise the relative error on the isotope ratio measurement. Serum samples were diluted 1+19 with ultrapure water after the addition of the Fe-57, Cu-65 and Zn-67 spikes. The proposed method was validated by the analysis of NIST SRM 1598 Inorganic Constituents in Bovine Serum, the levels found being in agreement with the certified values. Finally, the method was applied to the analysis of 21 serum samples from healthy blood donors.