The effect of N₂O on the isotopic composition of air‐CO₂ samples

Abstract

The ionisation efficiencies of N2O vs. CO2 as well as their ratios were measured in detail introducing clean N2O and CO2 into the electron impact ion source of an isotope ratio mass spectrometer. Changes in the ionisation efficiency ratio (IER) were found for different electron energy settings and compared with the ratios of literature ionisation cross-section values for pure N2O and CO2. To establish the influence of mixtures of N2O and CO2 in a mass spectrometer, artificial air mixtures were prepared by mixing different amounts of N2O and CO2 from well-calibrated spike cylinders with CO2-free air. The mixing ratios varied from 8-512 ppb for N2O and from 328-744 ppm for CO2. With these mixtures the effects of varying N2O concentrations on apparent CO2 isotope ratios in air samples were determined. After applying a mass balance correction the delta13C results were consistent within small error margins. The data seemed almost independent from a particular choice for the IER of N2O vs. CO2 in the correction algorithm. For delta18O a small effect of the ionisation efficiency ratio of N2O vs. CO2 was found. Several sets of calculations were made varying the IER between 0.88 and 0.62. The dependence of delta18O was the smallest with an adopted IER of 0.68-0.72 in the mass balance correction equation for isotopic analysis of CO2 in air. For high-precision measurements of the CO2 stable isotope ratios in air samples a careful assessment of the mass spectrometer performance is necessary. Different ion sources, even different ion source settings, alter the IER of N2O vs. CO2 which is used in the N2O correction algorithm. Preferably, the specific mass spectrometric behaviour should be established with clean N2O/CO2 mixtures or with air mixtures covering a larger range of N2O concentrations.

CO2 isotope analyses using large air samples collected on intercontinental flights by the CARIBIC Boeing 767

“…The value of E N2O determined on our MAT 252 mass spectrometer on pure N 2 O compared with CO 2 (filled in the same bellow to the same pressure) was slightly variable around 0.71 over a period of several years. It has been reported that E N2O determined on pure gases can deviate from the effective value characterizing the N 2 O behaviour in CO 2 þN 2 O air mixtures, [29][30][31] but for our MAT 252 the deviation appeared to be not significant, as shown below. In a dedicated study, 31 31 ).…”

Jena Reference Air Set (JRAS): a multi-point scale anchor for isotope measurements of CO<sub>2</sub> in air

“…5). Factors contributing to the uncertainty are the correction for N 2 O present in the analyzed gas (the correction for the m/z 46 ion current is larger than that for m/z 45; see Sirignano et al, 2004;Ghosh and Brand, 2004), the δ 18 O value used for digesting the calcite during standardization (Wendeberg et al, 2011) and the 17 O correction . The latter have been the focus of the WMO recommendations for several years, where the need for a unified use of the Assonov correction has been emphasized (Assonov et al, 2003).…”

“…Further inconsistencies were found in the correction procedures for the 17 O contribution used when translating ion currents into delta values and in the algorithms applied for removing the inevitable presence of N 2 O in the CO 2 gas extracted from air cryogenically (Sirignano et al, 2004;Ghosh and Brand, 2004). …”

Reformulated 17O correction of mass spectrometric stable isotope measurements in carbon dioxide and a critical appraisal of historic ‘absolute’ carbon and oxygen isotope ratios

“…The ensuing discussion assumes that the measured 45 d and 46 d values have been, if necessary, corrected for cross-contamination between sample and working reference gas Rama Rao and Bindal, 2001) and, in the case of atmospheric CO 2 samples, for isobaric N 2 O interference at m/z 44, 45 and 46 Craig and Keeling, 1963;Mook and van der Hoek, 1983;Mook and Jongsma, 1987;Ghosh and Brand, 2004;Sirignano et al, 2004;Assonov and Brenninkmeijer, 2006).…”