High-precision Fe isotope analysis for low contents using a Nu Sapphire instrument†

Abstract

Iron (Fe) isotopes serve as a powerful tracer for studying planetary evolution, magmatic processes, redox conditions, biological activities, and other key geological processes. However, the application of stable Fe isotopes in depleted-Fe samples has been significantly constrained by pervasive argon-related isobaric interferences inherent to conventional multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). This study purified rock samples with varying Fe concentrations and precisely measured Fe isotope ratios using the collision cell pathway in the low-resolution mode of the Nu Sapphire instrument. We systematically evaluated the effects of total Fe concentration in solution, Fe signal intensity mismatch between samples and standards, and HNO₃ molarity differences on measurement precision and accuracy. For precise Fe isotope ratio measurements using sapphire, strict analytical conditions must be met: (1) matched nitric acid concentrations between samples and bracketing standards (1% deviation induces 0.2‰ Fe isotope offset); (2) consistent Fe signal intensities (5% concentration mismatch introduces 0.05‰ bias); and (3) suppression of matrix interferences to minimize isotopic fractionation. The results demonstrate that the Nu Sapphire can achieve precise measurements with as little as 1 μg of Fe, representing a tenfold improvement relative to conventional instruments. The Fe isotopic data obtained for 13 geological references show good agreement with previous studies. Therefore, the exceptional sensitivity of Nu Sapphire facilitates high-precision Fe isotope ratio measurements for iron-depleted samples, offering broad application potential.