2011 年 45 巻 3 号 p. 129-145
Precise isotope ratios of most polyisotopic heavy elements can be now determined by ICP (inductively coupled plasma) mass spectrometry with a multiple collector array (MC-ICP-MS), because elements with high ionization potential energy are easily ionized to positive ions using the high ionization efficiency of Ar ICP. Precision typically better than 0.01% reproducibility can be achieved by this technique, though several attentions are needed to spectral and nonspectral interferences and sample preparation. Their isotopic abundance data used in geochemistry are directly linked to atomic weight estimation of elements. Atomic weights of elements are calculated from the absolute isotopic abundance of elements through calibrated mass spectrometers, and related uncertainty of the atomic weight is evaluated from their natural isotopic variation. This recently developed MC-ICP-MS was applied to the determination of absolute Zn isotopic abundance to estimate Zn atomic weight in 2002 first. A double-spike technique was applied to calibrate ICP mass spectrometer, and resultant Zn atomic weight and isotopic abundance were different from the previous IUPAC value reported in 2003. A large isotopic fractionation of 0.12%/u was observed through industrial purification process. This is the case for other elements, and MC-ICP-MS revealed presence of detectable isotopic variation in the current isotopic reference materials. Isotopically homogeneous reference material with SI unit traceability is highly required. Natural isotopic variation of heavy elements will be explained by change in their chemical species in chemical reaction and mass balances between them. Present isotopic studies of heavy elements tend to highlight larger degrees of isotopic fractionation mainly through redox chemical reaction, but the degrees through legand exchange without redox reaction are also important. The variation can be estimated through laboratory equilibrium experiments in some cases. Experimentally determined isotopic fractionation factors will contribute to understand the direction and degrees of the natural variations. In situ determination of chemical species in the isotopic equilibrium chemical reactions by spectroscopic speciation analysis like XAFS, Raman, and IR is an essential approach for a quantitative understanding of the isotopic variations as well as the current empirical computation chemical approach.
