A practical method for measuring high precision calcium isotope ratios without chemical purification for calcium carbonate samples by multiple collector inductively coupled plasma mass spectrometry

A procedure for the measurement of high precision Ca isotope ratios for calcium carbonate samples by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) is reported. Under instrumental low mass resolution mode, Ca-42, Ca-43 and Ca-44 were measured directly without column chemistry and the measurement uncertainty was 0.08 parts per thousand (2sd, n = 32). The background interference on mass 42 u was investigated and minimized to an acceptable level by tuning the sweep gas and ICP torch position. In order to ensure the reliability of the method and quantify the tolerance of samples with incorporated Sr, the effect of Sr as an interference was evaluated for different concentrations, results showing no statistical difference with Sr/Ca < 0.1, implying that most natural carbonate materials are accessible by this method. To validate the accuracy of this method, several stalagmite samples were analyzed in a comparative study in the Isotope Geochemistry Laboratory, Department of Earth Sciences, Oxford University. Results from both laboratories are in good statistical agreement which supports the use of this method as a simple, practical approach for Ca isotope measurement of calcium carbonate samples.