Paul Debajyoti, Skrzypek Grzegorz, Fórizs István
Department of Earth and Environmental Science, The University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA.
Rapid Commun Mass Spectrom. 2007;21(18):3006-14. doi: 10.1002/rcm.3185.
In stable isotope ratio mass spectrometry (IRMS), the stable isotopic composition of samples is measured relative to the isotopic composition of a working gas. This measured isotopic composition must be converted and reported on the respective international stable isotope reference scale for the accurate interlaboratory comparison of results. This data conversion procedure, commonly called normalization, is the first set of calculations done by the users. In this paper, we present a discussion and mathematical formulation of several existing routinely used normalization procedures. These conversion procedures include: single-point anchoring (versus working gas and certified reference standard), modified single-point normalization, linear shift between the measured and the true isotopic composition of two certified reference standards, two-point and multi-point linear normalization methods. Mathematically, the modified single-point, two-point, and multi-point normalization methods are essentially the same. By utilizing laboratory analytical data, the accuracy of the various normalization methods (given by the difference between the true and the normalized isotopic composition) has been compared. Our computations suggest that single-point anchoring produces normalization errors that exceed the maximum total uncertainties (e.g. 0.1 per thousand for delta(13)C) often reported in the literature, and, therefore, that it must not be used for routinely anchoring stable isotope measurement results to the appropriate international scales. However, any normalization method using two or more certified reference standards produces a smaller normalization error provided that the isotopic composition of the standards brackets the isotopic composition of unknown samples.
在稳定同位素比率质谱分析法（IRMS）中，样品的稳定同位素组成是相对于工作气体的同位素组成进行测量的。为了在实验室间准确比较结果，必须将这种测量得到的同位素组成转换并报告到相应的国际稳定同位素参考标度上。这种数据转换程序，通常称为归一化，是用户进行的第一组计算。在本文中，我们对几种现有的常规使用的归一化程序进行了讨论并给出了数学公式。这些转换程序包括：单点锚定（相对于工作气体和有证参考标准物质）、改进的单点归一化、两种有证参考标准物质的测量同位素组成与真实同位素组成之间的线性转换、两点和多点线性归一化方法。从数学角度来看，改进的单点、两点和多点归一化方法本质上是相同的。通过利用实验室分析数据，比较了各种归一化方法的准确性（由真实同位素组成与归一化同位素组成之间的差异给出）。我们的计算表明，单点锚定会产生超出文献中经常报道的最大总不确定度（例如，δ(13)C为每千分之0.1）的归一化误差，因此，不能将其用于将稳定同位素测量结果常规锚定到适当的国际标度上。然而，只要标准物质的同位素组成涵盖未知样品的同位素组成，任何使用两种或更多有证参考标准物质的归一化方法产生的归一化误差都会更小。
