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., ⁴⁰Ar¹⁶O⁺ on ⁵⁶Fe⁺) 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 N₂-based plasma, for direct measurement of ⁵⁶Fe/⁵⁴Fe isotopic composition in biological and environmental samples, combined with a novel fully automated purification procedure.

The N₂ plasma eliminates Ar-based interferences, enabling direct measurement of ⁵⁶Fe/⁵⁴Fe under low resolution conditions with intermediate precision 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 ⁵⁶Fe/⁵⁴Fe 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 δ⁵⁶Fe/⁵⁴Fe_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 and environmental samples, making it more accessible to a broader range of users.