U.S. Geological Survey, National Center MS431, Reston, VA 20192, USA.
Rapid Commun Mass Spectrom. 2011 Jan 30;25(2):301-6. doi: 10.1002/rcm.4854.
Hair is a keratinous tissue that incorporates hydrogen from material that an animal consumes but it is metabolically inert following synthesis. The stable hydrogen isotope composition of hair has been used in ecological studies to track migrations of mammals as well as for forensic and archaeological purposes to determine the provenance of human remains or the recent geographic life trajectory of living people. Measurement of the total hydrogen isotopic composition of a hair sample yields a composite value comprised of both metabolically informative, non-exchangeable hydrogen and exchangeable hydrogen, with the latter reflecting ambient or sample preparation conditions. Neither of these attributes is directly measurable, and the non-exchangeable hydrogen composition is obtained by estimation using a commonly applied mathematical expression incorporating sample measurements obtained from two distinct equilibration procedures. This commonly used approach treats the fraction of exchangeable hydrogen as a mixing ratio, with a minimal procedural fractionation factor assumed to be close or equal to 1. Instead, we propose to use full molar ratios to derive an expression for the non-exchangeable hydrogen composition explicitly as a function of both the procedural fractionation factor α and the molar hydrogen exchange fraction f(E). We apply these derivations in a longitudinal study of a hair sample and demonstrate that the molar hydrogen exchange fraction f(E) should, like the procedural fractionation factor α, be treated as a process-dependent parameter, i.e. a reaction-specific constant. This is a counter-intuitive notion given that maximum theoretical values for the molar hydrogen exchange fraction f(E) can be calculated that are arguably protein-type specific and, as such, f(E) could be regarded as a compound-specific constant. We also make some additional suggestions for future approaches to determine the non-exchangeable hydrogen composition of hair and the use of standards.
毛发是一种角蛋白组织，它整合了动物所消耗物质中的氢，但在合成后其氢同位素组成是稳定的，且代谢惰性。毛发的稳定氢同位素组成已被用于生态研究，以追踪哺乳动物的迁徙，以及用于法医和考古目的，以确定人类遗骸的来源或活人的近期地理生活轨迹。测量毛发样本的总氢同位素组成会产生一个复合值，其中包含代谢信息丰富的不可交换氢和可交换氢，后者反映环境或样本制备条件。这两个属性都不是直接可测量的，不可交换氢的组成是通过使用一个常见的数学表达式进行估计而获得的，该表达式结合了从两个不同平衡程序获得的样本测量值。这种常用方法将可交换氢的分数视为混合比，并假设最小的程序分馏因子接近或等于 1。相反，我们建议使用全摩尔比来明确地将不可交换氢的组成表示为程序分馏因子α和摩尔氢交换分数 f(E)的函数。我们在一个毛发样本的纵向研究中应用这些推导，并证明摩尔氢交换分数 f(E)应像程序分馏因子α一样，被视为依赖于过程的参数，即反应特异性常数。这是一个违反直觉的概念，因为可以计算出摩尔氢交换分数 f(E)的最大理论值，这些值可以说是蛋白质类型特异性的，因此，f(E)可以被视为化合物特异性常数。我们还对未来确定毛发不可交换氢组成和使用标准的方法提出了一些额外的建议。
