Iron isotopic analyses of geological reference materials on MC-ICP-MS with instrumental mass bias corrected by three independent methods

Abstract

Here we report iron (Fe) isotopic data of three pure Fe solution standards (IRMM-014, GSB Fe, and NIST 3126a) and five widely used geological reference materials (RMs) from the United States Geological Survey and Geological Survey of Japan obtained on a Neptune Plus multi-collector–inductively coupled plasma–mass spectrometer (MC-ICP-MS) in our laboratory over the past 3 years. The instrumental mass bias was corrected by three independent methods: sample-standard bracketing (SSB), Ni doping + SSB, and 57Fe–58Fe double spike + SSB. Measurements reveal that both the Ni doping and double spike methods helped calibrate short-term fluctuations in mass bias. Collectively, almost all measurements of RMs yielded δ56Fe within ± 0.05 of recommended values, provided that each sample was measured four times on MC-ICP-MS. For the first time, new recommended values for NIST SRM3126a are reported (δ56Fe = 0.363 ± 0.006, 2SE, 95% CI; and δ57Fe = 0.534 ± 0.010, 2SE).