Chemical and isotopic analysis of hydrocarbon gas at trace levels: Methodology and results
Résumé
Isotopic mass spectrometry coupled online with gas chromatography (GC-C-IRMS) permits measurement of relative proportions of gaseous hydrocarbon (CH4 to C4H10) and CO2, and determination of carbon isotope ratio of hydrocarbon molecules. Access to these parameters provides valuable information about the source and the genesis of naturally-occurring gas, as well as on post-formation physico-chemical processes which might have taken place in the geological environment. In particular, it is possible to distinguish hydrocarbon gas of bacterial origin from that of thermogenic origin based on proportion and carbon isotope ratio of methane as measured by GC-C-IRMS. However, in samples containing very low amounts of hydrocarbons (from 1 ppm to 1000 ppm), accurate measurement of isotope ratios is often impossible due to the limitations of conventional GC-C-IRMS techniques using direct sample introduction. A technique was developed to overcome this limitation. It is based on a novel approach allowing pre-concentration of hydrocarbons prior to GC-C-IRMS analysis. The pre-concentration step consists in selective trapping of hydrocarbon molecules on a cold adsorbent phase, and removal of non-adsorbed gases (N2, O2, Ar,...). In a second step, pre-concentrated alkanes are desorbed, and released in an inert carrier gas, focused through a capillary and introduced into the GC-C-IRMS for chromatographic separation and measurement of concentration and carbon isotope composition of each individual carbon molecule. In order to achieve sufficient accuracy, several operating conditions are of prime importance, including sufficient signal intensity, well defined peak shape and low signal/noise ratio. Accurate measurements can be performed on samples as small as 10 cm3 of bulk gas in standard conditions, with concentrations as low as 1 ppm of methane, 0.5 ppm of ethane and 0.3 ppm of propane and butane. Total analytical uncertainty on δ13C measurements ranges from ± 0.2‰ to ± 1.5‰, depending on the hydrocarbon molecule.