On the N2O correction used for mass spectrometric analysis of atmospheric CO2

To obtain accurate values of delta C-13(CO2) and delta O-18(CO2) on environmental CO2 by mass spectrometry, the raw isotope data must be corrected for the isobaric N2O contribution. This is one of the analytical problems limiting inter-laboratory delta C-13(CO2) data consistency. The key parameter, the N2O relative ionisation efficiency (E-N2O), cannot be determined with sufficient accuracy by direct measurements of pure N2O. The determination of E-N2O by analyses on N2O-CO2 mixtures of known isotope composition and mixing proportions has been recently suggested. In this work we propose a new method of N2O correction which uses the m/z 30 signal as a measure of the N2O/CO2 ratio, so that determinations of E-N2O and N2O content are not required. The method uses the fact that fragment-ion spectra of N2O and CO2 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 R-30(N2O) (the key parameter for the new correction) has to be calibrated on known N2O-CO2 mixtures by measuring R-30 signals only. The method accuracy we presently achieved is around 2.5% and any error which appears to come mostly from our N2O-CO2 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 CO2 this means +/- 0.003 parts per thousand and +/- 0.005 parts per thousand for delta C-13(CO2) and delta O-18(CO2), respectively. The proposed method may be valuable for small samples where no separate N2O determinations are available (e.g. ice core samples and CF-IRMS measurements) as well as for determination of E-N2O and testing the 'traditional' N2O correction based on mass balance calculations. Copyright (c) 2006 John Wiley & Sons, Ltd.