West Australian Biogeochemistry Centre, School of Biological Sciences, The University of Western Australia, 35 Stirling Highway, Perth, Western Australia, 6000, Australia.
National Measurement Laboratory, LGC, Queens Road, Teddington, TW11 0LY, UK.
Rapid Commun Mass Spectrom. 2020 Nov 15;34(21):e8892. doi: 10.1002/rcm.8892.
The stable HCNOS isotope compositions can be reported as (a) the isotope ratio of two stable isotopes (R); (b) the isotope delta value (δ); and (c) the atom fraction of the isotopes (x). Recalculations between these different expressions are needed frequently and require the use of the absolute isotope ratio for the zero points of the stable isotope delta scales (R). The inconsistent use of R values may lead to a discrepancy in recalculated results.
We summarised the recalculation procedures between different expressions of the stable isotope compositions and introduced a user-friendly EasyIsoCalculator that allows the recalculation between the main expressions of isotope compositions. We mathematically and empirically evaluated the possible inconsistencies in reporting of the stable isotope data due to the use of different R and different normalisation methods.
The recalculation between δ-values and other expressions of the stable isotope compositions always involves the use of R. The choice of R will have a significant influence on the recalculated values. The use of different R values has a significant influence also on the normalisation of raw values but only when the normalisation is conducted versus the working standard gas value, causing discrepancy, e.g. for δ(C/C) up to ~ 0.3 ‰.
Differences in the selection of R value may lead to significant differences among different laboratories. The uncertainty in the calculations originates primarily from the uncertainty in the R determination; however, it is lower than the discrepancy arising from the inconsistent use of R. Consistent use of the same R values is required to eliminate the unnecessary discrepancy if different data sets are recalculated from delta value to other expressions.
稳定的 HCNOS 同位素组成可以表示为 (a) 两种稳定同位素的同位素比 (R)；(b) 同位素差值 (δ)；和 (c) 同位素的原子分数 (x)。这些不同表示之间的换算经常需要，并且需要使用绝对同位素比作为稳定同位素差值标度的零点 (R)。R 值的不一致使用可能导致换算结果的差异。
我们总结了不同稳定同位素组成表示之间的换算程序，并介绍了一个用户友好的 EasyIsoCalculator，它允许在同位素组成的主要表示之间进行换算。我们从数学和经验上评估了由于使用不同的 R 和不同的归一化方法而导致稳定同位素数据报告可能存在的不一致性。
δ 值与其他稳定同位素组成表示之间的换算总是涉及到 R 的使用。R 的选择将对换算值产生重大影响。不同 R 值的使用对原始值的归一化也有重大影响，但仅当对工作标准气体值进行归一化时，才会导致差异，例如 δ(C/C) 高达约 0.3‰。
R 值选择的差异可能导致不同实验室之间存在显著差异。计算的不确定性主要源于 R 值测定的不确定性；然而，它低于由于不一致使用 R 值而产生的差异。如果要从 δ 值换算到其他表示，重新计算不同数据集，则需要一致使用相同的 R 值，以消除不必要的差异。
