Ghosh Prosenjit, Brand Willi A
Max-Planck-Institute for Biogeochemistry, P.O. Box 100164, 07701 Jena, Germany.
Rapid Commun Mass Spectrom. 2004;18(16):1830-8. doi: 10.1002/rcm.1560.
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.
通过将纯净的一氧化二氮（N₂O）和二氧化碳（CO₂）引入同位素比值质谱仪的电子轰击离子源，详细测量了N₂O与CO₂的电离效率及其比值。发现不同电子能量设置下电离效率比（IER）会发生变化，并将其与文献中纯N₂O和CO₂的电离截面值之比进行了比较。为了确定N₂O和CO₂混合物在质谱仪中的影响，通过将来自校准良好的加标气瓶中不同量的N₂O和CO₂与不含CO₂的空气混合，制备了人工空气混合物。N₂O的混合比从8 - 512 ppb变化，CO₂的混合比从328 - 744 ppm变化。利用这些混合物，确定了不同N₂O浓度对空气样品中表观CO₂同位素比值的影响。应用质量平衡校正后，δ¹³C结果在小误差范围内保持一致。在校正算法中，数据似乎几乎与N₂O与CO₂的IER的特定选择无关。对于δ¹⁸O，发现N₂O与CO₂的电离效率比对其有较小影响。进行了几组计算，将IER在0.88至0.62之间变化。在空气中CO₂同位素分析的质量平衡校正方程中，采用0.68 - 0.72的IER时，δ¹⁸O的依赖性最小。对于空气样品中CO₂稳定同位素比值的高精度测量，有必要仔细评估质谱仪的性能。不同的离子源，甚至不同的离子源设置，都会改变用于N₂O校正算法的N₂O与CO₂的IER。最好使用纯净的N₂O/CO₂混合物或覆盖更大N₂O浓度范围的空气混合物来确定特定的质谱行为。
