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 48Ca/42Ca ratio is a superior choice to the conventionally measured 44Ca/40Ca ratio for two important reasons. These are (1) mass‐dependent fractionation can be determined free from the effects of inherited or ingrown radiogenic 40Ca 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 (48Ca/42Ca) more precisely than very small or large ratios (44Ca/40Ca). However, because of the very low natural abundance of both 48Ca and 42Ca, 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 48Ca/42Ca ratio by the more commonly available and user‐friendly multi‐collector TIMS using a 43Ca‐46Ca double‐spike, with a significantly better precision of 0.18% (2s). The 48Ca/40Ca or 44Ca/40Ca ratio can also be measured in the same mass spectrometer run to provide complementary information on any radiogenic component.