Determination of the CO Contribution to the 15N/14N Ratio Measured by Mass Spectrometry

The isobaric effect of CO on the determination of δ 15 N has been calibrated for CO/N 2 mixtures. An effect of 6.9‰ on δ 15 N by percent of CO has been observed; the linearity of the isobaric effect for CO concentrations in the range 11-0.02% (measured manometrically) has been established. Consequently, a method to determine accurately the CO/N 2 ration (especially for CO amounts lower than 1%) in natural samples, along with the δ 15 N determination in a single mass spectrometric manipulation has bee developed