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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Changes in Composition of Nitrogen-15-Labeled Gases during Storage in Septum-Capped Vials

Dep. of Agriculture and Rural Development, Agricultural and Environmental Science Division, Newforge Lane, Belfast BT9 5PX, UK

^* Corresponding author (jim.stevens{at}dardni.gov.uk)

We tested the reliability of 12-mL Exetainers (Labco Ltd., High Wycombe, UK) fitted with bromobutyl rubber septa for the storage of a gas sample containing ¹⁵N-labeled N₂O and N₂. The gas mixture was analyzed monthly over a 1-yr storage period by continuous-flow isotope-ratio mass spectrometry (IRMS). After a year the N₂O concentration had decreased by 30% but the change in ¹⁵N enrichment of N₂O was not detectable. Accurate determination of the N₂O concentration was possible up to 1 yr if a calibration gas was stored and analyzed along with the test mixture. This technique has wide applicability because diffusive loss of N₂O as a fraction of starting concentration was predicted by Fick's Law to be almost independent of concentration above 3 µL L^-1. The rate of decrease in the enrichment of N₂ as measured by the ratio differences for 29/28 and 30/28 was slow. Decreases were not significantly different from time zero values after storage for 8 wk. After 1 yr of storage the losses of ²⁹N₂ and ³⁰N₂ significantly lowered the calculated value for the fraction of the N₂ from the labeled pool (d) by about 2% of the initial value, but had no effect on the calculated value for the enrichment of the labeled pool (¹⁵X_N). The diffusivity of N₂O, calculated from loss rate and concentration gradient between Exetainer and atmosphere using Fick's Law, was 40 times higher than that of ²⁹N₂ or ³⁰N₂. Nitrous oxide may have been lost from the Exetainer because of adsorption by the septum as well as diffusion through the septum.

Abbreviations: a_D, the enrichment of the pool from which the labeled N₂O is derived • d, the fraction of the N₂ derived from the labeled source • I, ion current • IRMS, isotope-ratio mass spectrometry • LSD, least significant difference • R, molecular ratio • ¹⁵X_N, the enrichment of the pool from which the labeled N₂ is derived • ²⁹R, difference between the molecular ratios for ²⁹N₂/²⁸N₂ in the enriched test sample and normal atmosphere • ³⁰R, difference between the molecular ratios for ³⁰N₂/²⁸N₂ in the enriched test sample and normal atmosphere

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