Reference materials selection for the stable carbon isotope analysis of dissolved carbon using a wet oxidation system

Wet chemical oxidation combined with isotope ratio mass spectrometry has become a routine technique for analyzing the stable carbon isotope composition of dissolved organic (DOC) and inorganic (DIC) carbon. However, methodological inconsistencies between laboratories in using different reference materials lead to a discrepancy in results. We experimentally tested the precision and accuracy of the analysis of commonly available international reference materials and other chemicals potentially suitable for laboratory standards.

The solid international reference materials and other simple chemicals were used to prepare water solutions. A range of carbon concentrations was chosen to optimize tests for (1) precision and accuracy, (2) linearity, (3) detection limits, (4) memory effects, and (5) efficiency of DIC removal from a DOC/DIC mixtures. Samples were analyzed using an LC-IsoLink coupled with a Delta V Plus isotope ratio mass spectrometer (Thermo Fisher Scientific).

The analytical setup had a negligible memory effect, good reproducibility (<0.21‰) and accuracy (maximum difference from the true values <0.35‰) for the analyzed organic compounds if approximately ≥9 × 10^-09 moles of dissolved carbon was injected into the system (~11 mg C L^-1 if a 10-μL loop was used). Analyses of sodium bicarbonate or calcium carbonate solutions had a two-fold lower accuracy despite maintaining a high precision.

Aqueous solutions of international reference materials such as L-glutamic acids (USGS40, USGS41), benzoic acid (IAEA-601) and sucrose (IAEA-CH-6) can be successfully used for direct normalization of results to the VPDB scale. By contrast, analyses of caffeine and urea returned very reproducible but highly inaccurate results and these materials are not recommended for standards.