How Well Do We Know VPDB—Part 2: Interlaboratory Assessment of Existing <i>δ</i> ¹³ C _VPDB Reference Materials

ABSTRACT Rationale In January 2024, the IAEA experts meeting endorsed both δ 13 C scales that are currently used within the scientific community: the Vienna Peedee belemnite (VPDB) δ 13 C 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. Methods The phosphoric acid reaction that is required to evolve CO 2 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. Results The reported values for IAEA‐610, ‐611, ‐612, and USGS44 on the δ 13 C 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 δ 13 C VPDB and δ 13 C VPDB‐LSVEC scales for some RMs that are routinely used in elemental analysis–isotope ratio mass spectrometry.