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

An analytical oxidation system employing supercritical oxidation has been developed. It is designed to measure concentration and the natural carbon isotope ratios (C-13, C-14) Of dissolved organic carbon (DOC) and is especially suited for marine samples. The oxidation takes place in a ceramic tube at 650 degrees 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 parts per thousand in C-13 abundance, 2% in C-14 abundance and 3% in DOC concentration. The blank is 9 mu M of carbon and has a C-14 content of 80 pmC, measured by oxidizing traces of natural and synthetic material in distilled water. The C-14 is measured by accelerator mass spectrometry, the C-13 isotope ratio by isotope ratio mass spectrometry. This is, to our knowledge, the first analytical application of supercritical oxidation. (C) 1998 Elsevier Science B.V. All rights reserved.