THE ANALYSIS OF THE C1-C5 COMPONENTS OF NATURAL-GAS SAMPLES USING GAS-CHROMATOGRAPHY COMBUSTION ISOTOPE RATIO MASS-SPECTROMETRY

Recent developments in porous polymer fused silica capillary columns c ombined with high sensitivity GC-C-IRMS have enabled the routine analy sis of the C1 to C5 compounds of natural gases without any prior sampl e preparation. The analysis time for the carbon isotope ratios of the individual components is reduced to that of routine GC-FID analysis. S election of column type is critical to analytical performance. Analyti cal conditions have been determined which allow the separation and iso topic analysis of C1, C2, C3, iC4, nC4, iC5 and nC5 in an aliquot of < 250 mul of natural gas. Good separation of components was achieved usi ng a Poraplot Q GC column. Analytical precision of between 0.03 and 0. 21 parts per thousand (+/- 1sigma) has been determined using a standar d reference natural gas. Isotopic fractionation during sample injectio n and chromatographic analysis has been identified. Using this method we have been able to analyse samples on a VG Isochrom II system from n atural gas samples previously too small for analysis (e.g. seeps, head space). A study of headspace ps data, which would not have been possib le previously, demonstrates the effects of sample leakage and biodegra dation on deltaC-13 of samples from an actively leaking reservoir. Bio degradation appears to have caused an increase in deltaC-13 of methane and ethane with a lesser increase in deltaC-13 of propane. The carbon dioxide in the samples appears to be derived largely from oxidation o f oil with a contribution from oxidation of gas.