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

摘要

A continuous-flow GC/IRMS technique has been developed to analyse δ~(15)N 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 N_2 between 1.35 (about 56 nmol) and 48.9 μL (about 2 μmol).rnThe 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 N_2 concentration, we set the instrument to determine also N_2 and ~(36)Ar contents in a single run. In fact, based on the simultaneously determined N_2/~(36)Ar ratios and assuming that ~(36)Ar content in crustal and mantle-derived fluids is negligible with respect to ~(36)Ar concentration in the atmosphere, for each sample, the degree of atmospheric contamination can be accurately evaluated. Therefore, the measured δ~(15)N values can be properly corrected for air contamination.