Changes in Composition of Nitrogen-15-Labeled Gases during Storage in Septum-Capped Vials
- Ronald J. Laughlin and
- R. James Stevens *
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 (15XN). 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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