Adapting an elemental analyser to perform high-temperature catalytic oxidation for dissolved organic carbon measurements in water

RationaleMany laboratories employ elemental analyzers (EAs) coupled to isotope ratio mass spectrometers (IRMSs) to measure carbon stable isotope ratios (delta C-13) in solid samples. Dissolved organic carbon (DOC) in most natural water samples cannot be analyzed using this approach unless time-consuming preconcentration is employed. MethodsAn EA-IRMS can be used to measure DOC delta C-13 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 degrees 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. ResultsMeasurements were accurate (deviation <0.3 parts per thousand compared to solid sample measurements) and precise (error of 0.3 parts per thousand for concentrations >= 46 mu M). 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 delta C-13 in a river/estuary varied between -25.7 and -23.2 parts per thousand, with higher values for waters with higher salinity, as expected. Deep-sea water reference material had a value of -22.9 +/- 0.5 parts per thousand, very similar to those found in recent reports employing similar techniques. ConclusionAdapting an EA is a feasible approach for the measurement of DOC delta C-13 in natural waters. The low cost and simplicity of the system allow its use in any laboratory already equipped with EA-IRMS.