Traceability for magnesium isotope ratios

The new mathematical methods which were developed by PTB to determine the isotope ratios in silicon allow an analytical solution to be used for the first time for the evaluation of the measurements that are based on mass spectrometry – and thus ensure traceability to the SI. They have already contributed to considerably improving the measurement uncertainty within the scope of the Avogadro project. Now they are used to improve the measurement uncertainty also for the determination of the isotope ratios of magnesium by a factor of 10. Moreover, the magnesium isotope ratios will be comparable worldwide on an absolute scale (rather than one based on consensus) for the first time.

Geology, archeology, climate research, environmental protection, food safety and origin/authenticity tests are all areas in which isotope ratio measurements play an important role. For this purpose, an isotopic reference material (IRM) with certified isotope ratios is often necessary. For magnesium, with a natural variation of its molar mass of 1.2 × 10^–3 g/mol, NIST SRM 980 has been the most widely used reference material so far; its measurement uncertainty, however, already exploits 30 % of this variation. Publications suggest that the material is so inhomogeneous that the measurement uncertainty covers nearly 50 % of the range; under such circumstances, sensible measurements seem impossible. Within the scope of EMRP project SIB09, three isotopic reference materials were developed under the leadership of BAM, together with LGC and based on PTB's methods. These exhibit a drastically reduced measurement uncertainty which, thus, lives up to practical applications for the first time. Even taking the homogeneity into account, the isotope ratio n(²⁶Mg)/n(²⁴Mg) has an expanded measurement uncertainty of only 0.010 %.

This also prevents the progressive increase of the measurement uncertainties, which seemed inevitable when replacing one consensus material with another. The three magnesium IRMs developed will be available for purchase in the near future under the designations ERM^®-AE143, -AE144 and -AE145 from BAM.