How Well Do We Know VPDB-Part 2: Interlaboratory Assessment of Existing δ¹³C_VPDB Reference Materials

In January 2024, the IAEA experts meeting endorsed both δ¹³C scales that are currently used within the scientific community: the Vienna Peedee belemnite (VPDB) δ¹³C scale defined by NBS 19 with a value of +1.95‰ exactly, and the VPDB-LSVEC scale defined by NBS 19 and the lithium carbonate LSVEC with a value of -46.60‰ exactly. Following the discovery of the instability of the LSVEC material, several expert laboratories independently proposed replacement reference materials (RMs). This study compares these calcium carbonate RMs IAEA-610, -611, -612, and USGS44 at the highest level of the metrological traceability chain and recommends values that enable users to realize the VPDB and VPDB-LSVEC scales.

The phosphoric acid reaction that is required to evolve CO₂ from calcium carbonate RMs for isotope analyses is scrutinized by comparing the results of the different apparatuses used in the three participating laboratories. All three laboratories use high-precision dual-inlet isotope ratio mass spectrometers and assess the individual instrument offsets in terms of their effects on interlaboratory comparability of samples.

The reported values for IAEA-610, -611, -612, and USGS44 on the δ¹³C_VPDB scale are -9.114 ± 0.011‰, -30.815 ± 0.011‰, -36.739 ± 0.020‰, and -42.073 ± 0.015‰, respectively. Within their measurement uncertainty they are identical to previously published values. Finally, we provide values on the δ¹³C_VPDB and δ¹³C_VPDB-LSVEC scales for some RMs that are routinely used in elemental analysis-isotope ratio mass spectrometry.