N 2 O influence on isotopic measurements of atmospheric CO 2

In spite of extensive efforts, even the most experienced laboratories dealing with isotopic measurements of atmospheric CO 2 still suffer from poor inter‐laboratory consistency. One of the complicating factors of these isotope measurements is the presence of N 2 O, giving rise to mass overlap in the isotope ratio mass spectrometer (IRMS). The aim of the experiment reported here has been twofold: first, the re‐establishment of the correction for ‘mechanical’ interference of N 2 O in the IRMS, along with its variability and drift, and the best way to quantitatively determine the correction factors. Second, an investigation into secondary effects, i.e. the influence of N 2 O admitted with the CO 2 sample on the “cross contamination”1 between sample and (pure CO 2 ) working gas. To make the suspected effects better detectable, isotopically enriched CO 2 gas with different concentrations of N 2 O has been measured for the first time. No evidence of secondary effects was observed, from which we conclude that N 2 O is not a major player in the inter‐laboratory consistency problems. Still, we also found that the determination of the ‘mechanical’ N 2 O correction needs to be very carefully determined for each individual IRMS, and should be periodically re‐determined. We show that the determination of the correction should be performed using CO 2 /N 2 O mixtures with concentration ratios around that of the atmosphere, as the extrapolation from pure gas end member behaviour will give erroneous results due to non‐linearities. For our IRMS, a VG SIRA series II, we find a correction of 0.23‰ for δ 45 CO 2 and 0.30‰ for δ 46 CO 2 of atmospheric samples, (with 0.85‰ mixing ratio). This implies that the relative ionisation efficiency (E) value associated with this machine is 0.75. Copyright © 2004 John Wiley & Sons, Ltd.