Improved nickel-corrected isotopic analysis of iron using high-resolution multi-collector inductively coupled plasma mass spectrometry

Abstract

Herein, we describe the use of a Nu Plasma 1700 multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) in true high-resolution mode to determine Fe isotopic ratios at the center of Fe peaks instead of their shoulders. A standard-sample bracketing (SSB) technique and a Ni doping method were used for instrumental mass bias calibration. The effects of Fe/Ni ratio and plasma mode were investigated in order to establish a high-precision Fe isotope analysis method. The results showed that the Ni-doping method provided more precision isotopic ratios than that of the SSB technique. Moreover, this work also presents Fe isotopic compositions of some geological reference materials (BCR-2, BHVO-2, AGV-2, GSR-1, GSR-2, GSR-3, GSP-2, and DSN-1), which were highly accurate with respect to the international Fe isotope standard IRMM-014. The iron isotopic compositions of most geological reference materials measured in this study agreed with the previously published data within quoted analytical uncertainties. Long-term reproducibility analyses of all samples indicate that the obtained isotopic ratios are highly reproducible, with precisions of better than ±0.03‰ for δ56Fe (2 sd) and ±0.06‰ (2 sd) for δ57Fe. Our results demonstrated the usefulness of internal element doping for improving the stability and precision of isotope analysis by MC-ICP-MS.