ONLINE ANALYSIS OF STABLE ISOTOPES OF NITROGEN IN NH3, NO, AND NO2 ATNATURAL-ABUNDANCE LEVELS

Methods were developed for the on-line analysis of stable isotopes of nitrogen (at natural abundance levels) in NH3, NO and NO2 in order to study the contribution of these trace gases to nitrogen cycling in eco systems. Standard methods for the combustion of organic substances to N-2 (for the determination of the N-15/N-14 ratios by mass spectrometr y) failed to satisfactorily oxidize or reduce the respective trace gas es. The following oxidation or reduction techniques ensured quantitati ve conversion of the trace gases to Na and analytical precision close to the internal precision of the instrument (precision +/-0.15 parts p er thousand): (1) oxidation of NH3 to N-2 at 1150 degrees C on a NiO s urface (which needs a reoxidation before each NH3 analysis; measuring range 11.7-58.8 nmol of NH3, precision +/-0.3D parts per thousand); (2 ) reduction of NO to N-2 at 1150 degrees C on a Ni surface (measuring range 33.3-133.3 nmol of NO, precision +/-0.28 parts per thousand); (3 ) reduction of NO2 to form NO at 420 degrees C on a Mo surface followe d by further treatment as for NO (measuring range 26.0-129.9 nmol of N O2, precision +/-0.90 parts per thousand). This last technique was dev eloped due to the poor chromatographic properties of NO2.