On the N 2 O correction used for mass spectrometric analysis of atmospheric CO 2

To obtain accurate values of δ 13 C(CO 2 ) and δ 18 O(CO 2 ) on environmental CO 2 by mass spectrometry, the raw isotope data must be corrected for the isobaric N 2 O contribution. This is one of the analytical problems limiting inter‐laboratory δ 13 C(CO 2 ) data consistency. The key parameter, the N 2 O relative ionisation efficiency ( ${\rm E}_{{\rm N}_2 {\rm O}}$ ), cannot be determined with sufficient accuracy by direct measurements of pure N 2 O. The determination of ${\rm E}_{{\rm N}_2 {\rm O}}$ by analyses on N 2 OCO 2 mixtures of known isotope composition and mixing proportions has been recently suggested. In this work we propose a new method of N 2 O correction which uses the m/z 30 signal as a measure of the N 2 O/CO 2 ratio, so that determinations of ${\rm E}_{{\rm N}_2 {\rm O}}$ and N 2 O content are not required. The method uses the fact that fragment‐ion spectra of N 2 O and CO 2 are very specific. The formalism of the correction is considered. Various tests demonstrate that the new method is robust, stable and easy to implement in practice. The effective value $^{30} {\rm R}_{{\rm N}_2 {\rm O}}$ (the key parameter for the new correction) has to be calibrated on known N 2 OCO 2 mixtures by measuring 30 R signals only. The method accuracy we presently achieved is around 2.5% and any error which appears to come mostly from our N 2 OCO 2 mixture preparation. Based on our tests and error considerations, the error of the proposed method that may be achieved is as low as ±1.5% (relative to the correction magnitude). For tropospheric CO 2 this means ±0.003‰ and ±0.005‰ for δ 13 C(CO 2 ) and δ 18 O(CO 2 ), respectively. The proposed method may be valuable for small samples where no separate N 2 O determinations are available (e.g. ice core samples and CF‐IRMS measurements) as well as for determination of ${\rm E}_{{\rm N}_2 {\rm O}}$ and testing the ‘traditional’ N 2 O correction based on mass balance calculations. Copyright © 2006 John Wiley & Sons, Ltd.