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

Abstract

A simple, precise method for determining the carbon stable isotope ratio of total dissolved inorganic C (ΣCO2) 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 CO2-free, 6.0 N HCl solution. After the original dissolved CO2 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 CO2. The isotope ratio of CO2 remaining in the liquid phase is calculated by temperature-dependent isotope discrimination between gas and aqueous phases. The isotope ratio of ΣCO2 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.