High-precision measurement of Fe isotopes by double spike CRC (collision/reaction cell)-MC-ICP-MS at the nanogram level

Abstract

Iron (Fe) plays a central role in material cycling throughout the Earth due to its ubiquity and complex chemical behavior. It acts as a limiting nutrient for primary production across the global surface ocean and also serves as a powerful tracer for paleo-redox conditions and igneous differentiation. Consequently, studying the biogeochemical cycling of iron is of great interest in the fields of chemical oceanography, petrology, biology and cosmology. The stable isotopes of iron are fractionated during various natural and anthropogenetic processes, leaving recognizable fingerprints in modern and ancient environments. However, the study of iron isotope variability in iron-depleted samples is plagued by analytical challenges, including its small isotopic variation and the prevalence of argon-based interferents produced directly from argon plasma. Here, we present a new double spike approach to Fe isotopic measurements using a Nu Sapphire multiple collector inductively coupled plasma mass spectrometer equipped with a collision/reaction cell. Our results demonstrate that the He–H₂ gas mixture effectively removes the vast majority of argon-based interferents. Combined with the double-spike technique, this approach achieves superior precision and accuracy over sample-standard bracketing, allowing precise determination of Fe isotopes for IRMM-524A at the 5 ng level (2 SD = 0.02‰, N = 6). Furthermore, our method is less sensitive to acidity and intensity mismatch between samples and standards, significantly increasing the sample throughput for Fe isotopic measurements. The Fe isotope compositions of international reference materials determined by our method are in excellent agreement with published data. Based on repeated analyses of IRMM-524A, the long-term external precision of our method is better than 0.04‰ (2 SD, N = 193). Future applications of this new method can be extended to a variety of low-iron reservoirs, significantly broadening the scope of Fe isotope systems.