Address: 112 Research West
Methane Concentration and d13CCH4 in Gas/Water Samples
We can measure d13CCH4 in gas samples that have a CH4 concentration greater than ambient air (1.8 ppm). We routinely measure d13CH4 in select headspace samples collected from streams to identify the methane source using a slightly modified technique used to measure d13CH4 in air extracted from ice core samples (Sowers et al., 2005). Using a gas tight syringe, we sample ~ 5nmoles of CH4 from a sample bottle headspace. The sample is then injected into the helium (He) carrier stream on the PreCon instrument. The CH4 is then cryogenically and chromatographically separated from the other headspace constituents before being converted to CO2 for Continuous Flow Isotope Ratio Mass Spectrometry (CF-IRMS) detection utilizing a Thermo Delta V analyzer with a Gas Bench interface. d13CCH4 results are reported on the internationally accepted VPDB scale. Air standards are run at the start of each day to correct for slight (<0.20/00) day-to-day instrument drift. The measured air standard value is always within 0.20/00 of the assigned value. Analytical uncertainty associated with d13CCH4 analyses based on replicate analyses of a 1% CH4 in N2 flask standard is better than 0.30/00. We are able to measure d13CCH4 in water samples containing CH4 above 0.1µg/L.
Ethane Concentration and d13CC2H6 in Gas/Water Samples
Using similar techniques we can also measure ethane concentrations and d13CC2H6 in water or gas samples. The detection limit for ethane concentration and d13CC2H6 measurements is similar (0.1µg/L) but the analytical uncertainty associated with the d13CC2H6 measurements is slightly higher (~0.50/00) compared to d13CCH4 (~0.30/00).