Continuous flow equilibration for mass spectrometric analysis of dinitrogen emissions

The measurement of denitrification in soil by the 15N gas emission method requires determination of the N2 mole masses 28, 29, and 30. Direct determination of 30N2, which is not possible with all isotope mass spectrometers, can be omitted if gas samples are equilibrated prior to isotope analysis. Because a previously published off-line equilibration procedure was inaccurate and laborious, a continuous-flow procedure was developed and evaluated in this study. Analysis of 29N2 and 28N2 was performed with an isotope ratio mass spectrometer coupled to an elemental analyzer. The N2 fraction of the samples was equilibrated in an electrodeless discharge unit placed within the interface connecting both instruments. Measuring sensitivity for the fraction of 15N-pool-derived N2 (d) and 15N atom fraction of the 15N pool (a(p)) was evaluated by analyzing 15N standard gases at d = 5 x 10(-6) to 5 x 10(-4) and a(p) = 0.4903, corresponding to delta15N = 1.3 to 68.3 per thousand. Measured values of d and a(p) satisfactorily coincided with theoretical values. Coefficients of variation of d and a(p) ranged from 0.13 to 19.59% and from 0.66 to 12.86%, respectively. Precision was higher than the off-line equilibration procedure and also higher than methods including direct measurement of 30N2