Wet-oxidation and automated colorimetry for simultaneous determination of organic carbon, nitrogen and phosphorus dissolved in seawater

We developed a simple and reliable method which allows simultaneous determination of organic forms of carbon (DOC), nitrogen (DON) and phosphorus (DOP) dissolved in seawater. Conversion of dissolved organic matter (DOM) to inorganic products (carbon dioxide, nitrate+nitrite and soluble reactive phosphate) is performed by a persulfate wet-oxidation in low alkaline condition. After oxidation, the concentration of the inorganic products dissolved in the sample was measured automatically by colorimetry using a 3-channel Technicon AutoAnalyzer system. A number of pure organic compounds were tested in the concentration range encountered in coastal and open ocean, indicating a high efficiency of the digestion procedure. The recovery range is similar to that obtained by other wet-oxidation procedures and by high-temperature catalytic oxidation techniques. Direct comparisons with usual methods used for separate determination of DOC, DON and DOP indicated a high efficiency of the procedure. Reproducibility tests demonstrated a very good precision (around 5%) for lagoonal and coastal waters, while precision was sometimes around 10–25% in oligotrophic oceanic waters, especially for DOP where values approached limits of detection for measuring phosphate. This method is highly suitable for routine analysis and especially appropriate for shipboard work.