A natural calcite reference material for microbeam oxygen and carbon isotope analysis

High-quality carbon and oxygen isotope data critically depend on the quality of reference materials used for instrumental calibration and data monitoring during in situ analysis. We present a new natural calcite reference material MNP for in situ carbon and oxygen isotope analysis by LA-MC-ICP-MS and SIMS, respectively. Extensive characterization by IRMS, LA-MC-ICP-MS and SIMS analyses demonstrated that MNP calcite is homogeneous in carbon and oxygen isotopes. Bulk isotope analyses yielded a mean δ¹³C_VPDB of −6.81 ± 0.33‰ (2SD, n = 15) and a mean δ¹⁸O_VPDB of −22.38 ± 0.41‰ (2SD, n = 15) across two independent laboratories. A total of 469 LA-MC-ICP-MS carbon isotope analyses of randomly selected calcite grains show homogeneous isotopic compositions with an average δ¹³C_VPDB of −6.82 ± 0.40‰ (2SD, n = 469), consistent with the bulk analytical data. Carbon isotopic compositions determined from five LA-MC-ICP-MS sessions agree with those obtained by IRMS analysis, suggesting that MNP calcite is suitable as a matrix-matched calibration reference material. Moreover, a total of 80 SIMS oxygen isotope analyses of randomly selected calcite grains show consistent oxygen isotopic compositions with an average δ¹⁸O_VPDB of −22.41 ± 0.32‰ (2SD, n = 80), consistent with bulk analyses. These findings suggest that MNP calcite is a homogeneous, matrix-matched reference material suitable for in situ carbon and oxygen isotope determinations.