Freshwater DOC measurements by high-temperature combustion: Comparison of differential (DTC - DIC) and DIC purging methods

Two methods were compared for the measurement of dissolved organic carbon (DOC) in waters collected from lakes, rivers and several pollution sources. Both the differential method (D-DOC method), derived from dissolved total carbon (DTC) minus DIC (dissolved inorganic carbon), and the purging method (P-DOC method), which purges DIC, were evaluated with a Shimadzu TOC-5000 with a high temperature catalytic oxidation system. Particularly in cases of gray water and/or water which had seeped through soil-layers, considerably lower concentrations were noted for P-DOC purged at around pH 2 than those measured by the D-DOC method or the P-DOC method at pH 4. DOC measurements for both field samples and specific compounds decreased due either to precipitation or flotation of colloidal organic particles insolubilized during sparging at pH 2 or 3 or to their incomplete combustion. The D-DOC method, exhibiting lower precision than the P-DOC method particularly for samples with high DIC vs DOC ratios, is, however, recommendable because of its accuracy.