Continuous-flow analyses of calcium isotopes using the Neoma MC-ICPMS/MS with H2 or SF6

Abstract

This paper reports Ca isotope ratio measurements (n ~ 1,000) from May 2024 to October 2025 using the Neoma MC-ICPMS/MS equipped with a continuous flow sample introduction system (microFAST Isotope 2) and dry plasma. Using H 2 in the collision/reaction cell, the sensitivity ranges from ~500 to 1,000 V/ppm in the low-resolution mode for 40 Ca with typical noise-to-signal ratios of ca. 0.2%. The associated reproducibility is 120, 130, 80, and 70 ppm (± 2 SD, n = 716) on the 44Ca/40Ca, 43Ca/40Ca, 42Ca/40Ca, and 44Ca/42Ca ratios, respectively. Effects of concentration or matrix mismatches between sample and standard were scrutinised over six sessions. We find that mismatch offsets, despite being expectedly linear, can have a positive or negative influence on the Ca isotope ratios, confirming that these are session-dependent rather than instrument-dependent. In all cases, the concentration mismatch is corrected using the adjustable injection flow rate of the sample introduction system. The overall accuracy of the method is acknowledged using a suite of ten certified reference materials with various biological matrices. While the Sr2+ double charge interferences are generally corrected up to a Sr/Ca ratio of 1-2% by monitoring 87Sr2+, we however explore the potential of using SF6 as a reaction gas to measure Ca isotope ratios free of these interferences. Using SF6 in the collision/reaction cell, the sensitivity drastically drops to ca. 100 V/ppm in the low-resolution mode with representative blank/signal ratios of ca. 0.2%. In these conditions, the typical reproducibility is 140 and 90 ppm (± 2 SD, n = 265) on the 44CaF/40CaF and 42CaF/40CaF ratios, respectively, the 43Ca19F+ signal being too low to be measured. We find that the 42CaF/40CaF ratio is remarkably exempt from concentration mismatch up to 30% and from Sr2+ double charge interferences up to a Sr/Ca ratio of 20%. However, the 44Ca19F+ mass does not react in a mass-dependent manner, such that the d44/40CaF and d44/42CaF values are correlated with a slope of ca. 1 instead of ca. 2. The immunity of the 42CaF/40CaF ratio from analytical mass-biases suggests strong potential for measuring Ca isotopes with high throughput in solution or with laser ablation.