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

As the calcium isotope fractionation may occur during its diffusion, condensation, evaporation and some chemical reactions, it was widely used in geochemical investigation such as source tracing, paleoclimate, paleoenvironment studies and biology. Also, there are still increasing demands for measurements of Ca in carbonate samples to better understand paleoclimate change and help to unravel global matter cycling. Ca isotopes are commonly analyzed by TIMS and alternatively measured by MC-ICP-MS. But the large ⁴⁰Ar and unignorably polyatomic interference generally precludes precise Ca isotope measurements by MC-ICP-MS. What's more, matrix elements which may affect Ca isotope measurement should also be removed before measurement. But for some samples with simple chemical matrices it is not cost-efficient to expend considerable effort in chemical purification when equivalent precision and accuracy could be achieved by a simpler one-step chemical purification process or for some high purity carbonate samples no chemical purification whatsoever. We developed a practical method to measure Ca isotope ratios without column chemistry separation by MC-ICP-MS for calcium carbonate samples. The interference of ArH₂ and ion/electron scattering and sputtering from ⁴⁰Ar can be reduced by simultaneously tuning the ICP torch position, sweep gas flow rate of membrane desolvation and adjusting the collector mask slit position. The Sr²⁺ interference can be corrected when the Sr/Ca ratio in the sample is less than 0.1, which allows for the measurement of most calcium carbonate samples. By this method we obtained the δ^44/42Ca of an internal laboratory standard which made by stalagmite 0.29 ± 0.08 ‰ (2sd, n = 32). In addition, several random selected samples were analyzed in the Oxford University for comparison. Our measurement results agreed well with the data obtained in Oxford University. These results demonstrate that our proposed method could be used for high precision Ca isotope analysis for carbonate samples without the need for column chemical separation procedures.