Santrock J, Studley S A, Hayes J M
Department of Chemistry, Indiana University, Bloomington 47405, USA.
Anal Chem. 1985 Jun;57(7):1444-8. doi: 10.1021/ac00284a060.
Measurements of carbon and oxygen isotopic abundances are commonly based on the mass spectrum of carbon dioxide, but analysis of that spectrum is not trivial because three isotope ratios (17O/16O, 18O/16O, and 13C/12C) must be determined from only two readily observable ion-current ratios (45/44 and 46/44). Here, approaches to the problem are reassessed in the light of new information regarding the distribution of oxygen isotopes in natural samples. It is shown that methods of calculation conventionally employed can lead to systematic errors in the computed abundance of 13C and that these errors may be related to incorrect assessment of the absolute abundance of 17O. Further, problems arising during the analysis of samples enriched by admixture of 18O-labeled materials are discussed, and it is shown (i) that serious inaccuracies arise in the computed abundance of 17O and 13C if methods of calculation conventionally employed in the analysis of natural materials are applied to material labeled with 18O but (ii) that computed fractional abundances of 18O are always within 0.4% of the correct result. Methods for exact calculation of two isotope ratios when the third is known are presented and discussed, and a more exact approach to the computation of all three isotope ratios in natural materials is given.
碳和氧同位素丰度的测量通常基于二氧化碳的质谱，但对该质谱的分析并非易事，因为必须仅从两个易于观测的离子流比率（45/44和46/44）确定三个同位素比率（17O/16O、18O/16O和13C/12C）。在此，根据有关天然样品中氧同位素分布的新信息，重新评估了解决该问题的方法。结果表明，传统采用的计算方法可能会导致计算出的13C丰度出现系统误差，并且这些误差可能与对17O绝对丰度的错误评估有关。此外，还讨论了在分析通过添加18O标记材料而富集的样品过程中出现的问题，结果表明：（i）如果将天然材料分析中传统采用的计算方法应用于用18O标记的材料，则计算出的17O和13C丰度会出现严重不准确的情况；但（ii）计算出的18O丰度分数始终在正确结果的0.4%以内。本文介绍并讨论了在已知第三个同位素比率时精确计算两个同位素比率的方法，并给出了一种更精确的计算天然材料中所有三个同位素比率的方法。
