Versatile inlet system for on‐line compound‐specific δD and δ 13 C gas chromatography–oxidation/reduction–isotope ratio mass spectrometry analysis of gaseous mixtures

Compound‐specific δD and δ 13 C analyses of gas mixtures are useful indicators of geochemical and environmental factors. However, the relative concentrations of individual components in gas mixtures (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 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 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 −20°C, and (iv) a Nafion® 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 µL) in order to measure reproducible δD and δ 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.