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

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 parts per thousand for the delta Mg-25/24 values and less than 0.037 parts per thousand for the delta Mg-26/24 values. Thus, comparability of all magnesium isotope delta (delta) measurements based on these standards and reference materials is established. Further, ERM-AE143 anchors all magnesium delta-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 theta(app), the fractionation relationship ln alpha(Mg-25/24)/ln alpha(Mg-26/24).