Methods for Reference Materials Calibration

The Biogeochemical Laboratories at Indiana University, originally in collaboration with Woods Hole Oceanographic Institution (J. M. Hayes, former director of NOSAMS) and Caltech (A. L. Sessions), began the development of isotope reference materials in 1998. We first established the purity of n-alkanes and n-alkanoic acid esters by GC-MS, followed by multiple off-line measurements of 2H/1H and 13C/12C ratios for each compound. Multiple analyses for each compound were performed via conventional combustion of milligram-amounts of individual compounds in quartz ampoules and cryogenic purification of combustion gases in a vacuum line. Water was converted to elemental hydrogen in contact with uranium, followed by collection of hydrogen gas using a Toepler pump. Gas yields and atomic H/C ratios were routinely monitored manometrically for quality control. Hydrogen and carbon isotopic ratios were determined using Finnigan MAT 252 and Delta Plus XP mass-spectrometers at Indiana University.

The hydrogen isotopic calibration employed the conventional normalization to VSMOW (zero ‰) and SLAP (-428 ‰) according to Coplen (1996). The same analytical strategy was used in later years to develop additional reference materials, in part in collaboration with Caltech (A.L. Sessions), K. Freeman's group at Pennsylvania State University, and others. Calibration for carbon isotope ratios relied on stable isotope standards NBS 19 and LSVEC (both carbonates were digested in 100% phosphoric acid at controlled temperatures). Carbon isotope ratios are reported relative to the VPDB scale where NBS 19 and LSVEC are defined as exactly +1.95 and -46.6 per mil, respectively (Coplen et al., 2006). Calibration for nitrogen was performed using international nitrogen isotope standards IAEA-N-1 and IAEA-N-2  (combusted and processed in the same way as our reference materials, according to the "principle of identical treatment"). In addition, we used USGS40 and USGS41 to take advantage of their larger isotopic ranges. Starting in 2013, we additionally used VSMOW2 and SLAP2.