On-Line Analysis of Stable Isotopes of Nitrogen in NH3, NO, and NO2 at Natural Abundance Levels

Abstract

Methods were developed for the on-line analysis of stable isotopes of nitrogen (at natural abundance levels) in NH(3), NO and NO(2) in order to study the contribution of these trace gases to nitrogen cycling in ecosystems. Standard methods for the combustion of organic substances to N(2) (for the determination of the (15)N/(14)N ratios by mass spectrometry) failed to satisfactorily oxidize or reduce the respective trace gases. The following oxidation or reduction techniques ensured quantitative conversion of the trace gases to N(2) and analytical precision close to the internal precision of the instrument (precision ±0.15‰): (1) oxidation of NH(3) to N(2) at 1150 °C on a NiO surface (which needs a reoxidation before each NH(3) analysis; measuring range 11.7-58.8 nmol of NH(3), precision ±0.30‰); (2) reduction of NO to N(2) at 1150 °C on a Ni surface (measuring range 33.3-133.3 nmol of NO, precision ±0.28‰); (3) reduction of NO(2) to form NO at 420 °C on a Mo surface followed by further treatment as for NO (measuring range 26.0-129.9 nmol of NO(2), precision ±0.90‰). This last technique was developed due to the poor chromatographic properties of NO(2).