Nitrogen plasma meets automated purification: Direct and reliable Fe isotopic analysis with MC-MICAP-MS

In conventional MC-ICP-MS, Fe isotopic analysis is hindered by spectral interferences arising from both the Ar plasma ( e.g ., 40 Ar 16 O + on 56 Fe + ) and the sample matrix ( e.g. , Cr, Ni, and matrix effects). To overcome these limitations, we evaluated the recently developed MC-MICAP-MS, which uses an N 2 -based plasma, for direct measurement of 56 Fe/ 54 Fe isotopic composition in biological samples, combined with a novel fully automated purification procedure. The N 2 plasma eliminates Ar-based interferences, enabling direct measurement of 56 Fe/ 54 Fe under low resolution conditions with intermediate precisions of 0.07 ‰. CaN + and KN + interferences were negligible at Ca/Fe ≤ 0.01 and K/Fe < 0.1, while Na and Mg caused no significant matrix effects up to a matrix/Fe ratio of 1. Compared with other plasma-based techniques, MC-MICAP-MS provides a more robust approach for direct 56 Fe/ 54 Fe analysis. The automated purification method enabled unattended sample processing and yielded low, reproducible blanks (<5 ng), no carryover, high Fe recovery (98 % ± 8 %, 2 SD), and no significant on-column fractionation across bone, liver, hair, and river water matrices. Accuracy was confirmed using four biological and environmental reference materials. We propose δ 56 Fe/ 54 Fe IRMM014 values for NIST SRM 1400 of 0.06 ± 0.07 ‰, for GBW07601 of -0.14 ± 0.07 ‰, for GBW09101 of 0.06 ± 0.07 ‰, and for SLRS-6 of 0.44 ± 0.14 ‰. Automated purification combined with MC-MICAP-MS provides a robust, reliable, and high-throughput approach for precise Fe isotopic analysis of biological samples, making it more accessible to a broader range of users. • N 2 plasma enables direct measurement of 56 Fe/ 54 Fe and 57 Fe/ 54 Fe isotope ratios • Assessment of spectral interferences and tolerance of matrix effect • Fully automated dual-column purification procedure for Fe • Reporting δ 56 Fe/ 54 Fe and δ 57 Fe/ 54 Fe values of biological reference materials