Reaffirming the Consistency of the Original Stock of NBS 19 Limestone and Its Byproduct With Finer Grain Size for δ 13 C and δ
Haiping Qi, Tyler B. Coplen, Lauren T. Reid, Stanley J. MroczkowskiABSTRACT
Rationale
Approximately 3–4 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 δ 13 C and −2.2‰ for δ 18 O 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 the 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 δ 13 C and δ 18 O values across these storage locations is essential.
Method
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 the IAEA and the National Institute of Standards and Technology (NIST) were included. During evaluation, two distinct grain sizes were identified within these units. Grain size was assessed using sieves on five selected samples. The δ 13 C and δ 18 O values of finer and coarser fractions (both labeled NBS 19) were measured at USGS using continuous‐flow isotope‐ratio mass spectrometry on 100‐ and 200‐μg samples.
Results
Expanded 95% measurement uncertainties ( k = 2) for δ 13 C and δ 18 O determinations are 0.027‰ and 0.038‰ or better, respectively, for quarantined NBS 19 stocks and finer byproduct calcite, which we term “NBS‐19 byproduct,” regardless of storage location. Between the 1990s and circa 2011, NBS‐19 byproduct labeled “NBS 19,” distributed by NIST and the IAEA, shows these findings.
Conclusions
NBS 19 primary isotopic reference material should be used for calibration of new isotopic reference materials rather than NBS‐19 byproduct secondary isotopic reference material.