IMPROVED STABLE NITROGEN ISOTOPE RATIO MEASUREMENTS OF NATURAL GASES

A system has been developed at BGR for rapid and precise nitrogen isotopic ratio measurements on natural gases in northern Germany using a gas chromatograph coupled with an isotope ratio mass spectrometer (GC/IRMS). The main advantage of the GC/IRMS system relative to conventional techniques is the analysis of gases with varying nitrogen concentrations down to 0.1% in routine operation. Small sample size down to 10 nmol of nitrogen is sufficient for precise measurement within the 1 sigma range of +/-0.3 to +/-0.5 parts per thousand. Care was taken to avoid sample contamination by atmospheric nitrogen during measurement, which in other preparation systems occasionally yields systematic errors of up to several per mill. The system was designed for user friendly, fast operation with computer-aided data acquisition and calculations. A comparison of reanalyzed with already published delta(15)N values for nitrogen gas from several natural gas fields in northwest and southern Germany reveals significant differences which depend on the nitrogen concentration. A careful handling of the data is recommended.