A re-evaluation of the carbon isotopic composition of organic reference materials

In 1983, Schoell et al. [Schoell, M., Faber, E., Coleman, M.L., 1983. Carbon and hydrogen isotopic compositions of the NBS 22 and NBS 21 stable isotope reference materials: an inter-laboratory comparison. Organic Geochemistry 5, 3-6] carried out an inter-laboratory comparison of the International Atomic Energy Agency (IAEA) reference materials NBS 21 (a graphite) and NBS 22 (an oil). At that time they found that NBS 22 in particular to be more depleted in ¹³C than the values published by the IAEA at that time (i.e., δ¹³C of -29.81‰ PDB ±0.06 as opposed to δ¹³C of -29.4‰ PDB). Even so, to this day, the IAEA has subsequently used a value of δ¹³C of -29.7‰ ± 0.2 VPDB. Following the advent of the new IAEA VPDB scale, defined by NBS 19 (a carbonate), this laboratory at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Petroleum and Exploration and Mining Divisions in North Ryde, NSW, Australia found that a number of the reference materials depleted in ¹³C (especially "organic" type reference materials) to be more depleted in ¹³C on the VPDB scale than expected. In particular, NBS 22 appears to be 0.25‰ more depleted in ¹³C than generally accepted values compared with a suite of carbonate standards run together. We suggest that the NBS 22 and other "organic" reference materials have probably all been calibrated to NBS 21 or 22 and should now be redefined on the VPDB scale.