A supercritical oxidation system for the determination of carbon isotope ratios in marine dissolved organic carbon Original Research Article

An analytical oxidation system employing supercritical oxidation has been developed. It is designed to measure the concentration and the natural carbon isotope ratios (13C, 14C) of dissolved organic carbon (DOC) and is especially suited for marine samples. The oxidation takes place in a ceramic tube at 650°C and 350 bar, bringing the seawater in the supercritical state. Oxygen is used as oxidant and copper as a catalyst to ensure a complete oxidation. The system is tested by oxidizing urea, phthalic acid, tannic acid and other materials. The precisions are estimated to be 1‰ in 13C abundance, 2% in 14C abundance and 3% in DOC concentration. The blank is 9 μM of carbon and has a 14C content of 80 pmC, measured by oxidizing traces of natural and synthetic material in distilled water. The 14C is measured by accelerator mass spectrometry, the 13C isotope ratio by isotope ratio mass spectrometry. This is, to our knowledge, the first analytical application of supercritical oxidation.