Organic Reference Materials for Hydrogen, Carbon, and Nitrogen Stable Isotope-Ratio Measurements: Caffeines, <i>n</i>‑Alkanes, Fatty Acid Methyl Esters, Glycines, l‑Valines, Polyethylenes, and Oils

An international project developed, quality-tested, and determined isotope−δ values of 19 new organic reference materials (RMs) for hydrogen, carbon, and nitrogen stable isotope-ratio measurements, in addition to analyzing pre-existing RMs NBS 22 (oil), IAEA-CH-7 (polyethylene foil), and IAEA-600 (caffeine). These new RMs enable users to normalize measurements of samples to isotope−δ scales. The RMs span a range of δ²H_VSMOW‑SLAP values from −210.8 to +397.0 mUr or ‰, for δ¹³C_VPDB‑LSVEC from −40.81 to +0.49 mUr and for δ¹⁵N_Air from −5.21 to +61.53 mUr. Many of the new RMs are amenable to gas and liquid chromatography. The RMs include triads of isotopically contrasting caffeines, C₁₆ <i>n</i>-alkanes, <i>n</i>-C₂₀-fatty acid methyl esters (FAMEs), glycines, and l-valines, together with polyethylene powder and string, one <i>n</i>-C₁₇-FAME, a vacuum oil (NBS 22a) to replace NBS 22 oil, and a ²H-enriched vacuum oil. A total of 11 laboratories from 7 countries used multiple analytical approaches and instrumentation for 2-point isotopic normalization against international primary measurement standards. The use of reference waters in silver tubes allowed direct normalization of δ²H values of organic materials against isotopic reference waters following the principle of identical treatment. Bayesian statistical analysis yielded the mean values reported here. New RMs are numbered from USGS61 through USGS78, in addition to NBS 22a. Because of exchangeable hydrogen, amino acid RMs currently are recommended only for carbon- and nitrogen-isotope measurements. Some amino acids contain ¹³C and carbon-bound organic ²H-enrichments at different molecular sites to provide RMs for potential site-specific isotopic analysis in future studies