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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, Ar(Zn, IRMM‐3702) = 65.3604 ± 0.0023 (95% confidence interval) and Ar(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. [ABSTRACT FROM AUTHOR]

Azaspiracids (AZAs) are a group of polyether marine algal toxins known to accumulate in shellfish, posing a risk to human health and the seafood industry. Analysis of AZAs is typically performed using LC–MS, which can suffer from matrix effects that significantly impact the accuracy of measurement results. While the use of isotopic internal standards is an effective approach to correct for these effects, isotopically labelled standards for AZAs are not currently available. In this study, 18O-labelled AZA1, AZA2, and AZA3 were prepared by reaction with H218O under acidic conditions, and the reaction kinetics and sites of incorporation were studied using LC–HRMS/MS aided by mathematical analysis of their isotope patterns. Analysis of the isotopic incorporation in AZA1 and AZA3 indicated the presence of four exchangeable oxygen atoms. Excessive isomerization occurred during preparation of 18O-labelled AZA2, suggesting a role for the 8-methyl group in the thermodynamic stability of AZAs. Neutralized mixtures of 18O-labelled AZA1 and AZA3 were found to maintain their isotopic and isomeric integrities when stored at −20 °C and were used to develop an isotope-dilution LC–MS method which was applied to reference materials of shellfish matrices containing AZAs, demonstrating high accuracy and excellent reproducibility. Preparation of isotopically labelled compounds using the isotopic exchange method, combined with the kinetic analysis, offers a feasible way to obtain isotopically labelled internal standards for a wide variety of biomolecules to support reliable quantitation. [ABSTRACT FROM AUTHOR]

Isotopic analysis by mass spectrometry relies on certified reference materials (CRMs) in order to correct instrumental isotopic fractionation (aka mass bias). Currently, SRM976 copper isotopic standard from the National Institute of Standards and Technology (NIST) remains the sole internationally recognized reference material for copper isotope ratio. However, the SRM976 material is no longer available and new copper isotopic CRMs are needed to serve as reference points for decades to come. In this study, we report an independent and fully calibrated isotope ratio measurement of a high-purity copper certified reference material, HICU-1, at the National Research Council Canada (NRC). MC-ICP-MS was employed for the isotope ratio measurements which were calibrated using the gravimetric isotope mixture approach involving two near-pure enriched copper isotopes. Copper isotope ratio n(63Cu)/n(65Cu) = 2.2413 ± 0.0009 (95% CI, k = 2) was established in the NRC HICU-1 and its isotopic composition has been shown to be nearly identical to that of NIST SRM976. [ABSTRACT FROM AUTHOR]