Advancing Ca isotopic analysis: Direct measurement of 44Ca/40Ca isotopic composition by MC-MICAP-MS with nitrogen plasma

Background: In conventional multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), major isobaric interference from 40Ar+ ions generated in the plasma prevents the direct measurement of 40Ca+ ions. To address this limitation, we investigated the performance of a microwave inductively coupled atmospheric-pressure plasma (MICAP) ion source, operating with N2 as plasma gas, recently integrated with multi-collector mass spectrometry (MC-MICAP-MS), for the measurement of 44Ca/40Ca and 44Ca/42Ca isotope abundance ratios. Results: The use of a N2 plasma effectively eliminates Ar-related interferences, enabling direct measurement of 44Ca/40Ca and 44Ca/42Ca isotopic composition under low-resolution conditions with an intermediate precision of 0.10 ‰ (2 SD). The method exhibits high tolerance to K+ interference (Ca/K ratio  30), MgO+ interferences (Ca/Mg ratio 20), and Sr++ interferences (Ca/Sr ratio 350), which makes MC-MICAP-MS a more robust approach for the direct measurement of 44Ca/40Ca isotopic composition than other plasma-based methods such as cold plasma MC-ICP-MS and CRC-MC-ICP-MS. The accuracy was validated by comparison with data from established techniques obtained for four biological reference materials (bone, hair, and liver), showing consistent results. For the bovine liver material NIST SRM 1577c, we propose a δ44Ca/40CaSRM915a value of 0.50 ‰  0.09 ‰ (U, k = 2) and a δ44Ca/42CaSRM915a value of 0.21 ‰  0.07 ‰ (U, k = 2). Significance: The MC-MICAP-MS approach offers a simple, robust, and reliable alternative for high-precision Ca isotope abundance measurements that has the potential to significantly advance the applications of stable Ca isotope research in various fields.