USGS24 was prepared by T. B. Coplen, U.S. Geological Survey, from Baker® technical grade graphite (96 percent smaller than 44 µm)[1]. Prior to splitting with a sample splitter, six spatially separated ~ 1 mg samples were analyzed to verify the isotopic homogeneity of the material. The range of variation of values was 0.11‰. The carbon stable isotopic composition was re-evaluated in an interlaboratory calibration[2].
| Analyte | Reference Value | Unit | Combined standard uncertainty with k=1 |
| δ(13C) | -16.05 | ‰VPDB-LSVEC | 0.04 |
Note: For δ(13C), currently two scales exist and are in use: VPDB and VPDB-LSVEC. δ(13C) values reported on the VPDB and VPDB-LSVEC scale can differ, with maximum reported differences (VPDB minus VPDB-LSVEC) of +0.3 ‰ for materials with carbon isotope delta values around –45 ‰ and negligible difference (below current analytical combined uncertainty) at carbon isotope delta values close to zero [https://doi.org/10.1002/rcm.10018)].
End users should state the values of reference materials used to normalize isotope data following the minimum requirements established by IUPAC [https://doi.org/10.1515/pac-2021-1108] and in the case of carbon, whether they are publishing δ(13C) values on the VPDB scale or the VPDB-LSVEC scale. Users should ensure that in one calibration they only use values for reference materials assigned on one scale.
References:
GONFIANTINI, R., STICHLER, W., ROZANSKI, K., Standards and intercomparison materials distributed by the International Atomic Energy Agency for stable isotope measurements. IAEA-TECDOC-825. International Atomic Energy Agency, Vienna, Austria (1995) 13-29.
COPLEN, T.B., BRAND, W.A., GEHRE, M., GRÖNING, M., MEIJER, H.A.J., TOMAN, B., VERKOUTEREN, R.M., New guidelines for δ13C measurements, Anal. Chem. 78 (2006) 2439-2441.
