Characterisation of new reference materials IAEA-610, IAEA-611 and IAEA-612 aimed at the VPDB δ C scale realisation with small uncertainty.

Rationale: LSVEC, the second anchor Reference Material (RM) for the VPDB δ C scale realisation, was introduced in 2006. In 2015, its δ C value was found to be drifting and, in 2017, its use as an RM for δ C was officially discontinued by IUPAC. New RMs of low uncertainty are needed. This paper describes the preparation and characterisation of IAEA-610, IAEA-611 and IAEA-612 (calcium carbonate, of chemical origin) which shall serve as a set of RMs aimed at anchoring the VPDB scale at negative δ C values.

Methods: The preparation and characterisation of IAEA-610, IAEA-611 and IAEA-612 were performed by addressing the contemporary technical requirements for RM production and characterisation (ISO Guide 35:2017). The three RMs were produced in large quantities, and the first batch was sealed into ampoules (0.5 g) to ensure the integrity of the RM during storage; additional batches were sealed for long-term storage. The most accurate method of CO preparation and stable isotope measurements was used, namely carbonate-H PO reaction under well-controlled conditions combined with well-tested stable isotope ratio mass spectrometry.

Results: The assigned values of δ C and associated uncertainties are based on a large number of analyses (~10 mg aliquots) performed at IAEA and address all the known uncertainty components. For aliquots down to ~100 μg, the δ C uncertainty is increased. The uncertainty components considered are as follows: (i) material homogeneity, (ii) value assignment against IAEA-603, (iii) potential storage effects, (iv) effect of the O correction, and (v) mass spectrometer linearity and cross-contamination memory in the ion source.

Conclusions: The new RMs IAEA-610, IAEA-611 and IAEA-612 have been characterised on the VPDB δ C scale in a mutually consistent way. The use of three RMs will allow a consistent realisation of the VPDB δ C scale with small uncertainty to be established, and to reach metrological compatibility of measurement results over several decades.