Sirignano Carmina, Neubert Rolf E M, Meijer Harro A J
Centre for Isotope Research, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG, The Netherlands.
Rapid Commun Mass Spectrom. 2004;18(16):1839-46. doi: 10.1002/rcm.1559.
In spite of extensive efforts, even the most experienced laboratories dealing with isotopic measurements of atmospheric CO2 still suffer from poor inter-laboratory consistency. One of the complicating factors of these isotope measurements is the presence of N2O, 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 N2O 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 N2O admitted with the CO2 sample on the "cross contamination" between sample and (pure CO2) working gas. To make the suspected effects better detectable, isotopically enriched CO2 gas with different concentrations of N2O has been measured for the first time. No evidence of secondary effects was observed, from which we conclude that N2O is not a major player in the inter-laboratory consistency problems. Still, we also found that the determination of the 'mechanical' N2O 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 CO2/N2O 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 per thousand for delta45CO2 and 0.30 per thousand for delta46CO2 of atmospheric samples, (with 0.85 per thousand mixing ratio). This implies that the relative ionisation efficiency (E) value associated with this machine is 0.75.
尽管付出了巨大努力，但即使是处理大气二氧化碳同位素测量的最有经验的实验室，实验室间的一致性仍然很差。这些同位素测量的复杂因素之一是一氧化二氮的存在，这在同位素比率质谱仪（IRMS）中导致了质量重叠。本文报道的实验目的有两个：第一，重新建立IRMS中一氧化二氮“机械”干扰的校正，以及其变异性和漂移，以及定量确定校正因子的最佳方法。第二，研究次要效应，即与二氧化碳样品一起引入的一氧化二氮对样品和（纯二氧化碳）工作气体之间“交叉污染”的影响。为了使可疑效应更易于检测，首次测量了含有不同浓度一氧化二氮的同位素富集二氧化碳气体。未观察到次要效应的证据，由此我们得出结论，一氧化二氮不是实验室间一致性问题的主要因素。尽管如此，我们还发现，对于每台单独的IRMS，“机械”一氧化二氮校正的确定都需要非常谨慎，并且应该定期重新确定。我们表明，校正的确定应该使用浓度比接近大气浓度比的二氧化碳/一氧化二氮混合物来进行，因为由于非线性，从纯气体端元行为进行外推会得出错误的结果。对于我们的IRMS，一台VG SIRA系列II，我们发现大气样品的δ45CO2校正为千分之0.23，δ46CO2校正为千分之0.30，（混合比为千分之0.85）。这意味着与这台仪器相关的相对电离效率（E）值为0.75。
