Coupling a high-temperature catalytic oxidation total organic carbon analyzer to an isotope ratio mass spectrometer to measure natural-abundance delta13C-dissolved organic carbon in marine and freshwater samples.

The stable isotope composition of dissolved organic carbon (delta(13)C-DOC) provides powerful information toward understanding carbon sources and cycling, but analytical limitations have precluded its routine measurement in natural samples. Recent interfacing of wet oxidation-based dissolved organic carbon analyzers and isotope ratio mass spectrometers has simplified the measurement of delta(13)C-DOC in freshwaters, but the analysis of salty estuarine/marine samples still proves difficult. Here we describe the coupling of the more widespread high-temperature catalytic oxidation-based total organic carbon analyzer to an isotope ratio mass spectrometer (HTC-IRMS) through cryogenic trapping of analyte gases exiting the HTC analyzer for routine analysis of delta(13)C-DOC in aquatic and marine samples. Targeted elimination of major sources of background CO2 originating from the HTC analyzer allows for the routine measurement of samples over the natural range of DOC concentrations (from 40 microM to over 2000 microM), and salinities (<0.1-36 g/kg). Because consensus reference natural samples for delta(13)C-DOC do not exist, method validation was carried out with water-soluble stable isotope standards as well as previously measured natural samples (IAEA sucrose, Suwannee River Fulvic Acids, Deep Sargasso Sea consensus reference material, and St. Lawrence River water) and result in excellent delta(13)C-DOC accuracy (+/-0.2 per thousand) and precision (+/-0.3 per thousand).