Nitrogen isotopes determination in natural gas: analytical method and first results on magmatic, hydrothermal and soil gas samples

Abstract

A continuous-flow GC/IRMS technique has been developed to analyse δ15N values for molecular nitrogen in gas samples. This method provides reliable results with accuracy better than 0.15‰and reproducibility (1σ) within ±0.1‰ for volumes of N2 between 1.35 (about 56 nmol) and 48.9μL (about 2μmol). The method was tested on magmatic and hydrothermal gases as well as on natural gas samples collected from various sites. Since the analysis of nitrogen isotope composition may be prone to atmospheric contamination mainly in samples with low N2 concentration, we set the instrument to determine also N2 and 36Ar contents in a single run. In fact, based on the simultaneously determined N2/36Ar ratios and assuming that 36Ar content in crustal and mantle-derived fluids is negligible with respect to 36Ar concentration in the atmosphere, for each sample, the degree of atmospheric contamination can be accurately evaluated. Therefore, the measured δ15N values can be properly corrected for air contamination.