Intercalibration of Mg Isotope Delta Scales and Realisation of SI Traceability for Mg Isotope Amount Ratios and Isotope Delta Values

Jochen Vogl,Olaf Rienitz,Savelas Rabb,Jan A Schuessler,Rebecca Kraft,Robert D Vocke,Simone A Kasemann,Martin Rosner,Michael Tatzel,Anette Meixner,Janine Noordmann

doi:10.1111/ggr.12327

Abstract

The continuous improvement of analytical procedures using multi‐collector technologies in ICP‐mass spectrometry has led to an increased demand for isotope standards with improved homogeneity and reduced measurement uncertainty. For magnesium, this has led to a variety of available standards with different quality levels ranging from artefact standards to isotope reference materials certified for absolute isotope ratios. This required an intercalibration of all standards and reference materials, which we present in this interlaboratory comparison study. The materials Cambridge1, DSM3, ERM‐AE143, ERM‐AE144, ERM‐AE145, IRMM‐009 and NIST SRM 980 were cross‐calibrated with expanded measurement uncertainties (95% confidence level) of less than 0.030‰ for the δ25/24Mg values and less than 0.037‰ for the δ26/24Mg values. Thus, comparability of all magnesium isotope delta (δ) measurements based on these standards and reference materials is established. Further, ERM‐AE143 anchors all magnesium δ‐scales to absolute isotope ratios and therefore establishes SI traceability, here traceability to the SI base unit mole. This applies especially to the DSM3 scale, which is proposed to be maintained. With ERM‐AE144 and ERM‐AE145, which are product and educt of a sublimation–condensation process, for the first time a set of isotope reference materials is available with a published value for the apparent triple isotope fractionation exponent θapp, the fractionation relationship ln α(25/24Mg)/ln α(26/24Mg).

Highlights

Et al 2015), and the detection of small but measurable isotopic differences in different chips of National Institute for Standards and Technology (NIST) SRM 980 became apparent (Galy et al 2003)
The traceability problem has been solved in a recent project (Brandt et al 2016, Vogl et al 2016) within which a series of three magnesium iRMs (European Reference Materials ERM-AE143, ERM-AE144 and ERM-AE145) has been produced, which are certified for their magnesium isotope amount ratios being traceable to the SI in the most direct way
We realised this in the present interlaboratory comparison study for Mg isotope ratios, within which all currently available Mg iRMs and artefact standards with natural-like Mg isotopic composition were analysed in order to establish comparability of Mg d-scales obtained by different d-materials

Summary

Introduction

Et al 2015), and the detection of small but measurable isotopic differences in different chips of NIST SRM 980 became apparent (Galy et al 2003). Still covered by the expanded uncertainty of NIST SRM 980, these differences of 0.69‰ (29 standard deviation of the mean, 2sm) were considered to be too large for current magnesium isotope research and a replacement for NIST SRM 980 was proposed (Galy et al 2003, Vogl et al 2004) To avoid such heterogeneity issues with solid iRMs, the community introduced two new artefact standards for Mg d-scale measurements. As the study includes SI-traceable Mg iRMs with sufficiently low measurement uncertainties, the calculation of Mg isotope amount ratios with associated measurement uncertainties for DSM3 and Cambridge was possible for the first time based on a fully calibrated approach

Methods

Results

Conclusion