Adapting an elemental analyser to perform high-temperature catalytic oxidation for dissolved organic carbon...

Many laboratories employ elemental analyzers (EAs) coupled to isotope ratio mass spectrometers (IRMSs) to measure carbon stable isotope ratios (δ¹³ C) in solid samples. Dissolved organic carbon (DOC) in most natural water samples cannot be analyzed using this approach unless time-consuming preconcentration is employed.

An EA-IRMS can be used to measure DOC δ¹³ C in natural waters without the need for sample preconcentration by employing high-temperature catalytic oxidation. An autosampler injects water into the EA reactor at 680°C filled with platinum catalyst beads, where all carbon is converted to CO₂ . Remaining water and halides are removed, while CO₂ is trapped in a cryotrap and later released to the IRMS.

Measurements were accurate (deviation <0.3‰ compared to solid sample measurements) and precise (error of 0.3‰ for concentrations ≥46 μM). Blanks were present and accounted for. Salinity up to seawater level did not affect accuracy or precision, but limited the number of samples that could be run before cleaning of the reactor was needed. DOC δ¹³ C in a river/estuary varied between -25.7 and -23.2‰, with higher values for waters with higher salinity, as expected. Deep-sea water reference material had a value of -22.9 ± 0.5‰, very similar to those found in recent reports employing similar techniques.

Adapting an EA is a feasible approach for the measurement of DOC δ¹³ C in natural waters. The low cost and simplicity of the system allow its use in any laboratory already equipped with EA-IRMS.