High-precision isotopic analysis of Mg and Ca in biological samples using multi-collector ICP-mass spectrometry after their sequential chromatographic isolation - Application to the characterization of the body distribution of Mg and Ca isotopes in mice.

A sequential chromatographic separation procedure for subsequent high-precision isotopic analysis of Mg and Ca via multi-collector ICP-mass spectrometry (MC-ICP-MS) from a single aliquot of sample was developed and used for a variety of animal/human biofluids and tissues. The procedure consists of a one-stage Mg isolation protocol (for most of the sample types) and a three-stage isolation protocol for Ca. AG50W-X8 strong cation exchange resin was used for the isolation of Mg and Ca, while Sr-resin was used to additionally purify the Ca fraction from Sr. Potential effects on the Mg isotope ratio measurement results caused by the possible presence of concomitant matrix elements (Cu, Fe, Zn, Ca) were systematically evaluated. δ26Mg values were biased for a Fe/Mg ratio > 0.13 and a Ca/Mg ratio > 1.5, resulting in a shift towards a lighter Mg isotopic composition. It was shown that the Mg isotope ratio data for Mg standards, the isotopic reference materials ERM-AE143 and IRMM 009 and the biological samples investigated are located on a mass-dependent fractionation line. Biological reference materials and commercially available serum samples were analyzed for both their Mg and Ca isotope ratios. For some of the biomaterials analyzed, the Ca isotope ratio data as obtained using MC-ICP-MS were further validated via their determination using double-spike thermal ionization mass spectrometry (DS-TIMS). The expanded uncertainty for δ26Mg was ≤ 0.12‰ and for δ44/42Ca ≤ 0.29‰. Biological fluids and tissues of mice were analyzed to characterize the body distribution of the stable isotopes of Mg and Ca. The isotopic variability among the body compartments was about 1.5‰ for Mg and 1.0‰ for Ca. Among the tissues explored, muscle tissue shows the lightest Mg and Ca isotopic compositions and liver the heaviest Mg and Ca isotopic compositions, respectively.