Kaiser Jan, Röckmann Thomas
School of Environmental Sciences, University of East Anglia, Norwich, UK.
Rapid Commun Mass Spectrom. 2008 Dec;22(24):3997-4008. doi: 10.1002/rcm.3821.
Gas isotope ratio mass spectrometers usually measure ion current ratios of molecules, not atoms. Often several isotopologues contribute to an ion current at a particular mass-to-charge ratio (m/z). Therefore, corrections have to be applied to derive the desired isotope ratios. These corrections are usually formulated in terms of isotope ratios (R), but this does not reflect the practice of measuring the ion current ratios of the sample relative to those of a reference material. Correspondingly, the relative ion current ratio differences (expressed as delta values) are first converted into isotopologue ratios, then into isotope ratios and finally back into elemental delta values. Here, we present a reformulation of this data reduction procedure entirely in terms of delta values and the 'absolute' isotope ratios of the reference material. This also shows that not the absolute isotope ratios of the reference material themselves, but only product and ratio combinations of them, are required for the data reduction. These combinations can be and, for carbon and oxygen have been, measured by conventional isotope ratio mass spectrometers. The frequently implied use of absolute isotope ratios measured by specially calibrated instruments is actually unnecessary. Following related work on CO2, we here derive data reduction equations for the species O2, CO, N2O and SO2. We also suggest experiments to measure the required absolute ratio combinations for N2O, SO2 and O2. As a prelude, we summarise historic and recent measurements of absolute isotope ratios in international isotope reference materials.
气体同位素比率质谱仪通常测量的是分子而非原子的离子流比率。通常，几种同位素分子会对特定质荷比（m/z）下的离子流产生贡献。因此，必须进行校正以得出所需的同位素比率。这些校正通常以同位素比率（R）来表示，但这并未反映出测量样品相对于参考物质的离子流比率的实际做法。相应地，相对离子流比率差异（以δ值表示）首先被转换为同位素分子比率，然后转换为同位素比率，最后再转换回元素δ值。在此，我们提出一种完全基于δ值和参考物质的“绝对”同位素比率对该数据简化程序进行的重新表述。这也表明，数据简化所需的并非参考物质本身的绝对同位素比率，而仅仅是它们的乘积和比率组合。这些组合可以通过传统同位素比率质谱仪进行测量，对于碳和氧而言，已经进行了测量。实际上，频繁暗示使用经过特殊校准仪器测量的绝对同位素比率是不必要的。继关于二氧化碳的相关工作之后，我们在此推导出了氧气、一氧化碳、一氧化二氮和二氧化硫的物种数据简化方程。我们还建议进行实验来测量一氧化二氮、二氧化硫和氧气所需的绝对比率组合。作为前奏，我们总结了国际同位素参考物质中绝对同位素比率的历史和近期测量情况。
