High-precision analysis of calcium isotopes using a Nu Sapphire collision cell (CC)-MC-ICP-MS

Abstract

This study presents high-precision analyses of calcium (Ca) isotope ratios using a collision-cell multi-collector inductively coupled plasma mass spectrometer (CC-MC-ICP-MS, Nu Sapphire). This instrument allows Ca isotope measurement in the low resolution mode, requiring much less sample compared with TIMS and conventional MC-ICP-MS, and has additional measurement capability of both radiogenic (ε⁴⁰Ca) and stable isotope ratios (δ^44/40Ca and δ^44/42Ca). The effects of several factors on the precision and accuracy of Ca isotopic data were evaluated, including the total Ca concentration, Ca intensity mismatch between the sample and standard, HNO₃ molarity mismatch between the sample and standard, and the presence of matrix elements. High precision was obtained on 100 ng g⁻¹ Ca samples (i.e., for 6 replicates ∼600 ng of Ca). Accurate measurements require the following conditions: (1) Ca intensities matched within 3% between the sample and standard, (2) low contents of Sr, Mg and Cr (Sr/Ca, Mg/Ca and Cr/Ca ratios need to be lower than 0.01%, and K/Ca, Al/Ca, Ti/Ca and Cr/Ca ratios <1%). Eleven standard reference materials analyzed using the established method yield highly consistent Ca isotope results with literature values. The long-term external precisions are higher than 0.07‰ (2 SD) for δ^44/40Ca and higher than 0.05‰ (2 SD) for δ^44/42Ca. The slope of the fractionation line between δ^44/40Ca and δ^44/42Ca is 1.9616 ± 0.0794, within the error of the slope value from the exponential law.