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

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 (epsilon Ca-40) and stable isotope ratios (delta Ca-44/40 and delta Ca-44/42). 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 similar to 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 parts per thousand (2 SD) for delta Ca-44/40 and higher than 0.05 parts per thousand (2 SD) for delta Ca-44/42. The slope of the fractionation line between delta Ca-44/40 and delta Ca-44/42 is 1.9616 +/- 0.0794, within the error of the slope value from the exponential law.