High precision determination of 48Ca/42Ca ratio by TIMS for Ca isotope fractionation studies

For studies of mass-dependent fractionation of calcium isotopes in natural materials, the Ca-48/Ca-42 ratio is a superior choice to the conventionally measured Ca-44/Ca-40 ratio for two important reasons. These are (1) mass-dependent fractionation can be determined free from the effects of inherited or ingrown radiogenic Ca-40 and (2) this ratio increases the spread of measured isotopic masses by 50%, resulting in statistically better resolution of fractionation, assuming similar precision. A third, though strictly technical, advantage is the inherent ability of a mass spectrometer to measure ratios close to unity (Ca-48/Ca-42) more precisely than very small or large ratios (Ca-44/Ca-40). However, because of the very low natural abundance of both Ca-48 and Ca-42, their ratio has been very difficult to measure, the only attempt so far being on a high mass resolution MC-ICP-MS with a precision of 0.33%. We report here determination of the Ca-48/Ca-42 ratio by the more commonly available and user-friendly multi-collector TIMS using a Ca-43-Ca-46 double-spike, with a significantly better precision of 0.18% (2s). The Ca-48/Ca-40 or Ca-44/Ca-40 ratio can also be measured in the same mass spectrometer run to provide complementary information on any radiogenic component.