Customized Injection Techniques for Compound-specific Isotope Analysis of Natural Gas Samples

The aim of this study was to develop an easy-to-use, reliable method for measuring the isotopic composition of individual alkanes present in natural gases at concentrations below 1000 ppmv without the need of expensive or complex pre-concentration devices. The proposed method combines pre-concentration of the gaseous compounds (C1 to C5) in a cooled, programmable temperature injector equipped with a Carbosieve-packed liner followed by compound-specific isotope analysis (CSIA) by gas chromatography – isotope ratio mass spectrometry (GC-IRMS). This method requires only small sample volumes of up to 10 mL and allows reliable isotopic measurements at low concentrations (< 100 ppmv) within the typical precision of GC-IRMS: ≤ 0.5‰ for carbon and ≤ 5‰ and hydrogen isotope analysis, respectively. The proposed injection techniques have been developed and validated using gaseous hydrocarbon samples of different origins.