A rapid and precise method for measuring stable carbon isotope ratios of dissolved inorganic carbon

We describe a method for rapid preparation, concentration and stable isotopic analysis of dissolved inorganic carbon (δ13C-DIC). Liberation of CO2 was accomplished by placing 100 μl phosphoric acid and 0.9 ml water in an evacuated 1.7-ml gas chromatograph (GC) injection vial. Following equilibration, the vial headspace gas was injected into a gas chromatograph-combustion-isotope ratio mass spectrometer (GC-C-IRMS) using a standard GC autosampler. A correction factor of −1.0‰ must be added to all isotopic measurements due to the equilibrium isotopic fractionation between the aqueous and gaseous phases in the sample vials. The method allows for the preparation and analysis of 48 samples, in triplicate, in a 40-h period. We report on the analysis of an inter-laboratory CaCO3 standard, as well as two natural water samples. Samples analyzed between 15 and 36 h after preparation exhibited standard deviations of ±0.2‰, while standards analyzed at varying concentrations and injection volumes had standard deviations of ±0.1‰ with a minimum concentration of 0.20 mM DIC. Additionally, chromatographic area vs. concentration of standard carbon yielded an r2=0.996, indicating that DIC concentrations can also be obtained from the GC-C-IRMS data, if desired. Further, the standard deviations observed during the analysis of natural water samples from a seagrass dominated lagoon with diel variations of 1.5‰ were consistent with those observed in laboratory standard analysis.