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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 delta(13)C 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 delta(18)O 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 delta(18)O 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. Copyright (C) 2004 John Wiley Sons, Ltd. [References: 12]