A new automated setup for stable isotope analysis of dissolved organic carbon

The measurement of stable isotope ratios on dissolved organic carbon (DOC) has long posed analytical problems and limited the use of this powerful tracer in biogeochemical studies in aquatic systems. Here, we provide a detailed description of a successful coupling of a custom-modified total organic carbon analyzer (Thermo HiPerTOC) to an isotope-ratio mass spectrometer (IRMS). The method is based on the wet oxidation of up to ~20 mL aqueous sample in a closed reactor, whereby complete oxidation is ensured by a combination of sodium persulfate addition, heating, and UV irradiation. The produced CO2 is carried over a water trap and purification column in a stream of He and introduced into the IRMS via a conventional open-split interface, enabling both quantification and δ13C analysis. Typical reproducibility of δ13C analyses on DOC in this setup is in the order of 0.2‰ or better, comparable to that obtained in a more conventional elemental analyzer-IRMS setup. With appropriate blank correction procedures, accurate analyses can be obtained on concentrations as low as 0.5 mg DOC L−1, representing the lower limit typically observed in marine systems. Typical overall system blank values for DOC analysis are in the order of 1 µg C. Analytical conditions (reaction time, reagent concentrations) were optimized for samples from brackish and marine environments, so that a single method can handle all types of environmental DOC samples. Although no certified DOC standards exist for δ13C, we analyzed the δ13C values of a DOC “consensus reference material” from a deep-ocean environment (cfr. Hansell 2005) and found a δ13C value of −19.5 ± 0.4‰ (n = 3), which is consistent with its oceanic origin.