The importance of a Ni correction with ion counter in the double spike analysis of Fe isotope compositions using a 57Fe/58Fe double spike

We present a new method capable of measuring iron isotope ratios of igneous materials to high precision by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) using a Fe-57-Fe-58 double spike. After sample purification, near-baseline signal levels of nickel are still present in the sample solution, acting as an isobaric interference on 58 amu. To correct for the interference, the minor Ni-60 isotope is monitored and used to subtract a proportional Ni-58 signal from the total 58 amu beam. The Ni-60 signal is difficult to precisely measure on the Faraday detector due to Johnson noise occurring at similar magnitude. This noise-dominated signal is subtracted from the total 58 amu beam, and its error amplified during the double spike correction. Placing the Ni-60 beam on an ion counter produces a more precise measurement, resulting in a near-threefold improvement in Fe-56 reproducibility, from 0.145 when measured on Faraday to 0.052. Faraday detectors quantify the Ni-60 signal poorly, and fail to discern the transient (NeAr)-Ne-20-Ar-40 interference visible on the ion counter, which is likely responsible for poor reproducibility. Another consideration is instrumental stability (defined herein as drift in peak center mass), which affects high-resolution analyses. Analyses experiencing large drift relative to bracketing standards often yield nonreplicating data. Based on this, we present a quantitative outlier detection method capable of detecting drift-affected data. After outlier rejection, long-term precision on individual runs of our secondary standard improves to 0.046 parts per thousand. Averaging 3-4 analyses further improves precision to 0.019 parts per thousand, allowing distinction between ultramafic minerals.