In situ calcium isotopic ratio determination in calcium carbonate materials and calcium phosphate materials using laser ablation-multiple collector-inductively coupled plasma mass spectrometry

Abstract

This study presents a highly precise and accurate Ca isotope analysis method using laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) in calcium carbonates and calcium phosphates. High-sensitivity cones and nitrogen gas were used to improve the performance of the LA-MC-ICP-MS. The experiment results show that at the N2 mode, the signal intensity of 44Ca+ in the high-sensitivity cones (Jet + X cones) was 5-fold higher than that in the normal cones (S + H cones), while the polyatomic interferences of 40Ar1H1H+ and 40Ar16O+ in the high-sensitivity cones were suppressed by factors of 6.1 and 10.6, respectively, after the addition of 8 ml min−1 N2. The combination of high sensitivity cones and N2 addition did not increase the production of double charged ions. Additionally, the interference correction of doubly charged Sr ions was investigated in detail. The experiment results demonstrate that the concentration ratio of Sr to Ca (represented as I87Sr++/I44Ca+), the 87Sr/86Sr ratio and the mass fractionation factor of Sr (fSr) during the measurement process were key parameters for in situ Ca isotope ratios analysis. The threshold values of these key parameters were evaluated and are recommended to ensure an accurate correction of doubly charged Sr ions. Using the optimized instrumental conditions and correction strategies, an excellent within-run precision (<0.06‰, 2SE) and external reproducibility (<0.15‰, 2SD) for the microanalysis of δ44/42Ca915b were obtained in natural calcium carbonates and calcium phosphates. These precisions are better than the values previously reported in the literature regarding using LA-MC-ICP-MS. Matrix effects were observed during the in situ Ca isotope analysis among calcium carbonates, hydroxy calcium phosphates and igneous apatites. Using the matrix-matching standards, accurate δ44/42Ca915b values in natural materials with various matrices were achieved, confirming the availability of the proposed method to resolve the spatial variation of δ44/42Ca in natural materials.