Development of automated preparation system for isotopocule analysis of N₂O in various air samples

Abstract. Nitrous oxide (N₂O), an increasingly abundant greenhouse gas in the atmosphere, is the most important stratospheric ozone-depleting gas of this century. Natural abundance ratios of isotopocules of N₂O, NNO molecules substituted with stable isotopes of nitrogen and oxygen, are a promising index of various sources or production pathways of N₂O and of its sink or decomposition pathways. Several automated methods have been reported to improve the analytical precision for the isotopocule ratio of atmospheric N₂O and to reduce the labor necessary for complicated sample preparation procedures related to mass spectrometric analysis. However, no method accommodates flask samples with limited volume or pressure. Here we present an automated preconcentration system which offers flexibility with respect to the available gas volume, pressure, and N₂O concentration. The shortest processing time for a single analysis of typical atmospheric sample is 40 min. Precision values of isotopocule ratio analysis are < 0.1 ‰ for δ¹⁵N^bulk (average abundances of ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O relative to ¹⁴N¹⁴N¹⁶O), < 0.2 ‰ for δ¹⁸O (relative abundance of ¹⁴N¹⁴N¹⁸O), and < 0.5 ‰ for site preference (SP; difference between relative abundance of ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O). This precision is comparable to that of other automated systems, but better than that of our previously reported manual measurement system.