MEMBRANE EXTRACTED CARBON ISOTOPE ANALYSIS OF DISSOLVED METHANE

Dissolved methane and the carbon isotopic ratio of methane are strong indicators of a variety of contamination, attenuation, redox, and biological processes in natural waters. Typical concentrations in natural waters are quite low, (low ppm to ppb). The low concentrations in natural waters have hampered the widespread use of carbon stable isotopes in these systems as complex inlet systems and large sample volumes have typically been required. A novel method for the analysis of dissolved methane and methane carbon isotopic ratios for dissolved methane was developed. The analysis uses a degassing membrane interfaced to a cavity ring down spectrometer (CRDS). Water is forced across the membrane by displacement of the sample water with methane-free headspace by a push gas. A second methane-free gas flows across the membrane and is introduced to the CRDS. The CRDS measures the methane concentration and the carbon isotope ratio. Water samples were collected in 125 mL open-top glass bottles. Standards were mixed using methane saturated water in 120 mL crimp-top vials. Sample analysis volume is dependent of the flow regime for the extraction unit, but varied with gas flows between 50 and 120 mL. Isotopic precision was dependent on the sweep-gas methane concentration and was better than 2.0 permil for low water concentrations (ppb) and increased to approximately 0.5 permil for ppm concentrations. Increasing the salinity had no effect on the extracted gas concentrations or the isotopic ratio of the extracted gas. Optimization of the gas-flow regime was required to maximize the extraction efficiency while ensuring sufficient instrument read time. The system is capable of rapid, cost effective measurements which require no headspace measurements, no large volume samples and no cryo-trapping units.