A mechanistic study of the high-temperature oxidation of organic matter in a carbon analyzer

This study examines the mechanism of oxidation of dissolved organic matter (DOM) in a high-temperature combustion (HTC) carbon analyzer. The HTC analyzer, which is commonly used to measure the concentration of dissolved organic carbon (DOC) in seawater, is still largely empirical in its operation, and little information is yet available on the chemical and physical mechanisms responsible for oxidation of DOM. To examine the role of water, which has been hypothesized to be a source of reactive oxidants in HTC analyzers, several experiments were conducted using gaseous hexane samples and several wet/dry carrier gases. Because the quantity of O2(g) needed to oxidize an injected sample of DOM is quite small, a substantial effort has been made to exclude O2(g) from the combustion furnace of the HTC instrument. In this modified, “air-tight” instrument, the efficiency of conversion of hexane into CO2 was greatest in dry O2 and slightly lower in moist O2. Hexane was only slightly oxidized in dry N2, but it was largely converted into CO2 when moist N2 was used as the carrier gas. These experiments confirm that water provides reactive oxidizing species (perhaps hydroxyl radical) that rapidly convert hexane into CO2 in the combustion tube of a carbon analyzer. Additional experiments with aqueous solutions of potassium hydrogen phthalate and a variety of carrier gases support this basic hypothesis.