Determining the stable isotope ratio of total dissolved inorganic carbon in lake water by GC/C/IIRMS

A simple, precise method for determining the carbon stable isotope ratio of total dissolved inorganic C (ΣCO 2 ) in freshwater samples is described. Water samples are packed in airtight glass bottles of known inner volume (∼70 ml) with no air bubbles. Subsequently, a headspace of 5.0 ml is created inside each bottle with pure helium gas, and each sample is acidified by adding 0.5 ml of a CO 2 ‐free, 6.0 N HCl solution. After the original dissolved CO 2 has equilibrated with the headspace gas, a portion of this headspace gas is subsampled and injected into the GC/C/IRMS (gas chromatograph/combustion furnace/isotope‐ratio mass spectrometer) system to determine the carbon isotope ratio of the CO 2 . The isotope ratio of CO 2 remaining in the liquid phase is calculated by temperature‐dependent isotope discrimination between gas and aqueous phases. The isotope ratio of ΣCO 2 of the original sample is then derived assuming isotope mass balance. The analytical precision of this method is ±0.1‰. The method enables a single operator to determine the isotopic ratio in at least 60 lake‐water samples within 3 d of sampling.