Methods for the Production and Calibration of Reference Materials
The development of organic stable isotope reference materials began historically in ca. 1995 in 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). After first establishing the purity of n-alkanes and n-alkanoic acid esters by GC-MS, multiple off-line measurements of 2H/1H and 13C/12C ratios were performed for each compound via conventional combustion of milligram-amounts 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, Delta Plus XP, and later model mass-spectrometers.
Subsequent collaborations with many other laboratories, especially with the U.S. Geological Survey (USGS) in Reston, Virginia, resulted in a much wider range of organic reference materials, many of which were calibrated in international ring-tests. Details of the methods employed in recent collaborative efforts were published by (1) Schimmelmann et al., 2016, New organic reference materials for hydrogen, carbon, and nitrogen stable isotope-ratio measurements: caffeines, n-alkanes, fatty acid methyl esters, glycines, L-valines, polyethylenes, and oils. Analytical Chemistry 88 (8), 4294-4302. https://doi.org/10.1021/acs.analchem.5b04392 and its supporting information (further details available upon request), and (2) Schimmelmann et al., 2020, Food matrix reference materials for hydrogen, carbon, nitrogen, oxygen, and sulfur stable isotope-ratio measurements: collagens, flours, honeys, and vegetable oils. Journal of Agricultural and Food Chemistry 68 (39), 10852-10864. https://doi.org/10.1021/acs.jafc.0c02610. The hydrogen isotopic calibration employed the conventional normalization to VSMOW (zero ‰) (or VSMOW2) and SLAP (-428 ‰) (or SLAP2) according to principles published by Coplen et al. in 1996, New guidelines for reporting stable hydrogen, carbon, and oxygen isotope-ratio data. Geochimica et Cosmochimica Acta 60 (17), 3359-3360. https://doi.org/10.1016/0016-7037(96)00263-3. Calibration for carbon isotope ratios relied on stable isotope standards NBS 19 and fresh, uncontaminated LSVEC. Carbon isotope ratios are reported relative to the VPDB scale where NBS 19 and LSVEC are defined as +1.95 and -46.6 per mil, respectively (Coplen et al. in 2006, New Guidelines for δ13C Measurements. Analytical Chemistry 78 (7), 2439−2441. https://doi.org/10.1021/ac052027c). 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.