Carbon and oxygen isotopic compositions of NBS 19 limestone isotopic reference material and its byproduct calcite having finer grain size, measured at the U.S. Geological Survey Reston Stable Isotope Laboratory (RSIL), 2025

Approximately 3 to 4 kg of NBS 19 limestone, prepared in 1982 by the U.S. Geological Survey (USGS), serves as a primary reference for the carbon isotope scales VPDB and VPDB-LSVEC and the oxygen isotope scale VPDB. Its consensus values are +1.95 ‰ for δ¹³C and –2.2 ‰ for δ¹⁸O measurements. After more than three decades of distribution, the number of units labeled “NBS 19” shrunk substantially and the remaining material was quarantined, with a small fraction retained at USGS and a larger fraction entrusted to the International Atomic Energy Agency (IAEA) for secure storage. Given the critical role of NBS 19, verifying the integrity of its δ¹³C and δ¹⁸O values across these storage locations is essential. Samples from IAEA and USGS stock materials were selected to ensure traceability of carbon and oxygen isotopes to NBS 19 distribution units from various storage locations. To achieve this, distribution units labeled as NBS 19 from both IAEA and NIST (National Institute of Standards and Technology) were included. During evaluation, two distinct grain sizes were identified within these units. Grain size was assessed using sieves on five selected samples. The δ¹³C and δ¹⁸O values of finer and coarser calcite fractions (both labeled NBS 19) were measured at USGS using continuous-flow isotope-ratio mass spectrometry (CF-IRMS) on 100-µg and 200-µg samples.

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