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

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 15 N‐labeled N 2 O and N 2 The gas mixture was analyzed monthly over a 1‐yr storage period by continuous‐flow isotope‐ratio mass spectrometry (IRMS). After a year the N 2 O concentration had decreased by 30% but the change in 15 N enrichment of N 2 O was not detectable. Accurate determination of the N 2 O 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 N 2 O 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 N 2 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 29 N 2 and 30 N 2 significantly lowered the calculated value for the fraction of the N 2 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 ( 15 X N ). The diffusivity of N 2 O, calculated from loss rate and concentration gradient between Exetainer and atmosphere using Fick's Law, was 40 times higher than that of 29 N 2 or 30 N 2 Nitrous oxide may have been lost from the Exetainer because of adsorption by the septum as well as diffusion through the septum.