Compound-specific D and 13C analyses of gas mixtures are useful indicators of geochemical and environmental factors. However, the relative concentrations of individual components in gas mixtures (e.g., H2, CO2, methane, ethane, propane, i-butane, n-butane) may vary over several orders of magnitude. The determination of hydrogen and carbon compound-specific stable isotope ratios requires that the hydrogen and carbon dioxide produced from each separated component has a concentration adjusted to match the dynamic range of the stable isotope mass spectrometer. We present a custom-built gas sampling and injection system (GASIS) linked with a Delta Plus XP mass spectrometer that provides flexibility, ease of operation, and economical use of small gas samples with wide ranges of analyte concentrations. The overall on-line GC-ox/red-IRMS (Gas Chromatography - oxidation/reduction - Isotope Ratio Mass Spectrometry) system consists of (i) a customized GASIS inlet system and (ii) two alternative reactors, namely an oxidative Cu-Ni-Pt reactor at 950°C for production of CO2 and a reductive graphitized Al2O3 reactor at 1420°C for production of H2. In addition, the system is equipped with (iii) a liquid nitrogen spray-cooling unit for cryo-GC-focusing at -20°C, and (iv) a Nafion® dryer for removal of water vapor from product CO2. The three injection loops of the GASIS inlet allow flexibility in the volume of injected analyte gas (e.g., from 0.06 to 500 µL) in order to measure reproducible D and 13C values for gases at concentrations ranging from 100% down to 10 ppm. We calibrate our GC-ox/red-IRMS system with two isotopically distinct methane references gases that are combusted off-line and characterized using dual-inlet IRMS.
|Translated title of the contribution||Versatile inlet system for on-line compound-specific dD and d13C gas chromatography–oxidation/ reduction–isotope ratio mass spectrometry analysis of gaseous mixtures|
|Pages (from-to)||2269 - 2272|
|Journal||Rapid Communications in Mass Spectrometry|
|Publication status||Published - 2007|