Versatile inlet system for on-line compound-specific dD and d 13 C gas chromatography–oxidation/ reduction–isotope ratio mass spectrometry analysis of gaseous mixtures Mark Henning 1 , Dariusz Stra ˛poc ´ 2 * , Grzegorz P. Lis 3 , Peter Sauer 2 , Jon Fong 2 , Arndt Schimmelmann 2 and Lisa M. Pratt 2 1 Inertia Oil & Gas, LLC, Evansville, IN 47708, USA 2 Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA 3 Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada Received 9 April 2007; Revised 7 May 2007; Accepted 7 May 2007 Compound-specific dD and d 13 C analyses of gas mixtures are useful indicators of geochemical and environmental factors. However, the relative concentrations of individual components in gas mix- tures (e.g., H 2 , CO 2 , 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 Chromatog- raphy – 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 CO 2 and a reductive graphitized Al 2 O 3 reactor at 1420-C for production of H 2 . In addition, the system is equipped with (iii) a liquid nitrogen spray-cooling unit for cryo-GC-focusing at S20-C, and (iv) a Nafion 1 dryer for removal of water vapor from product CO 2 . The three injection loops of the GASIS inlet allow flexibility in the volume of injected analyte gas (e.g., from 0.06 to 500 mL) in order to measure reproducible dD and d 13 C 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. Copyright # 2007 John Wiley & Sons, Ltd. Stable carbon and hydrogen isotopic compositions of hydro- carbon gases (C 1 through C 5 ) vary widely due to strong isotopic effects associated with biological and abiotic physicochemical processes. Patterns of isotopic variability have shed light on the classification of natural gas mixtures, 1 sources and sinks of atmospheric methane, 2,3 environmental contaminants, 4 and bacterial metabolism. 5,6 Early work on the isotopic composition of hydrocarbon gases 1,7 was labor intensive, requiring oxidation or reduction in sealed tubes and vacuum-line purification of large samples, followed by dual-inlet isotope ratio mass spectrometry (IRMS). A sig- nificant improvement in sample size requirements and automation was achieved with the development of gas chromatography (GC)-IRMS, in which GC effluents are either combusted (oxidized) to CO 2 for 13 C/ 12 C analysis (GC-ox-IRMS 8 ) or thermally converted (reduced) into H 2 for D/H analysis (GC-red-IRMS 9–11 ). Two basic designs of gas-handling inlets have been developed for analysis by GC-ox-IRMS and GC-red-IRMS. Commercially available GC injection loops (typically 0.06 to 1.0 mL; Valco Instruments Co., Inc., Houston, TX, USA) conveniently inject samples in which the sample is dominated by a single gas component. For more dilute components such as atmospheric methane (1.7 ppm) or CO 2 (380 ppm), analytes are extracted from larger samples (20–200 mL) and preconcentrated and cryofocused prior to injection onto the GC column. 3,12,13 Each type of inlet has been optimized for relatively narrow mixing ratios and delivers appropriate amounts of analyte to the IRMS RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 2007; 21: 2269–2272 Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rcm.3083 *Correspondence to: D. Stra ˛poc ´, Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA. E-mail: dstrapoc@indiana.edu Copyright # 2007 John Wiley & Sons, Ltd.