Detection of Inconsistencies in Isotope Ratios Certified in the Isotopic Primary Standards for Copper, Zinc and Nickel

Abstract

Calibrated isotope ratio measurements underpin numerous areas of scientific enquiry. Isotope measurement results rely on reference materials in order to correct instrumental isotopic fractionation, a bias induced by all mass spectrometers. The reference values assigned to these materials are typically obtained using gravimetric mixtures of separated isotopes but such experiments are costly and thus rarely cross-examined. Here we advance the concept of linking measurements of various isotope systems to provide a cost-effective way to assess the reliability of the values assigned to isotopic primary standards. Using MC-ICP-MS, we apply this workflow to examine the consistency of a couple of long-standing isotopic primary standards of copper, nickel and zinc. Our measurement results of the reference materials that define the isotope ratios and consequently the atomic weights of zinc and nickel, A_r(Zn, IRMM-3702) = 65.3604 ± 0.0023 (95% confidence interval) and A_r(Ni, NIST SRM 986) = 58.6979 ± 0.0010 (95% confidence interval), show zinc to be 15-sigma lighter and nickel 16-sigma heavier than their certified values. These discrepancies are suggestive of a wider reproducibility crisis surrounding isotopic standards and a further cross-examination of isotope ratios associated with primary reference materials is likely to identify yet-unrecognised biases for many other elements of the Periodic Table leading to improvements of the overall reliability of research conclusions that relies on them.