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Continuous Flow Equilibration for Mass Spectrometric Analysis of Dinitrogen Emissions

Institut für Bodenwissenschaft, Universität Göttingen, Von-Siebold-Str. 4, 37075 Göttingen, Germany

R. Langel and A. Reineking

Isotopenlaboratorium für biologische und medizinische Forschung, Universität Göttingen, Burkhardtweg 2, 37077 Göttingen, Germany

*Corresponding author (rwell{at}gwdg.de).

ABSTRACT

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 ¹⁵N = 1.3 to 68.3. 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.

Received for publication September 18, 1995.

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