Mass spectrometric method for the determination of the stable isotopic content of nitrous oxide by the technique of direct injection

A method for the direct isotopic analysis of N2O has been developed for VG-Prism H dynamic dual inlet stable isotope mass spectrometer. Tests have shown extreme sensitivity to contamination by carbon dioxide with a dependence of 6.8 per mille per % CO2 for δ15N and 12.4 per mille per % CO2 for δ18O. Trace amounts of CO2 have proven difficult to eliminate, particularly for small sample sizes. In order to ensure accurate analyses an indicator of CO2 contamination must be monitored and corrections applied when necessary. We have found that ions with mass to charge ratios of 12 and 22 are excellent proxies for CO2 contamination. Empirical relationships between these indicator species and CO2 contamination are documented which allow for corrections to be applied to raw data. Additional corrections required due to the presence of isobaric molecules containing 17O are described. Issues of standardization are discussed and sample handling procedures described. Finally, these methods and corrections are applied to a suite of stratospheric air samples. (author)