Carbon isotope analysis of dissolved organic carbon in fresh and saline (NaCl) water via continuous flow cavity ring-down spectroscopy following wet chemical oxidation

摘要

This work examines the performance and limitations of a wet chemical oxidation carbon analyser interfaced with a cavity ring-down spectrometer (WCO-CRDS) in a continuous flow (CF) configuration for measuring delta C-13 of dissolved organic carbon (delta C-13-DOC) in natural water samples. Low-chloride matrix (<5 g Cl/L) DOC solutions were analysed with as little as 2.5 mg C/L in a 9 mL aliquot with a precision of 0.5 parts per thousand. In high-chloride matrix (10-100 g Cl/L) DOC solutions, bias towards lighter delta C-13-DOC was observed because of incomplete oxidation despite using high-concentration oxidant, extended reaction time, or post-wet chemical oxidation gas-phase combustion. However, through a combination of dilution, chloride removal, and increasing the oxidant:sample ratio, high-salinity samples with sufficient DOC (>22.5 mu g C/aliquot) may be analysed. The WCO-CRDS approach requires more total carbon (mu g C/aliquot) than conventional CF-isotope ratio mass spectrometer, but is nonetheless applicable to a wide range of DOC concentration and water types, including brackish water, produced water, and basinal brines.