Elsner Martin
Institute of Groundwater Ecology, Helmholtz Zentrum München - German Research Center for Environmental Health, Ingolstädter Landstr. 1, 85764, Neuherberg, Germany.
J Environ Monit. 2010 Nov;12(11):2005-31. doi: 10.1039/c0em00277a. Epub 2010 Oct 29.
Gas chromatography-isotope ratio mass spectrometry (GC-IRMS) has made it possible to analyze natural stable isotope ratios (e.g., (13)C/(12)C, (15)N/(14)N, (2)H/(1)H) of individual organic contaminants in environmental samples. They may be used as fingerprints to infer contamination sources, and may demonstrate, and even quantify, the occurrence of natural contaminant transformation by the enrichment of heavy isotopes that arises from degradation-induced isotope fractionation. This review highlights an additional powerful feature of stable isotope fractionation: the study of environmental transformation mechanisms. Isotope effects reflect the energy difference of isotopologues (i.e., molecules carrying a light versus a heavy isotope in a particular molecular position) when moving from reactant to transition state. Measuring isotope fractionation, therefore, essentially allows a glimpse at transition states! It is shown how such position-specific isotope effects are "diluted out" in the compound average measured by GC-IRMS, and how a careful evaluation in mechanistic scenarios and by dual isotope plots can recover the underlying mechanistic information. The mathematical framework for multistep isotope fractionation in environmental transformations is reviewed. Case studies demonstrate how isotope fractionation changes in the presence of mass transfer, enzymatic commitment to catalysis, multiple chemical reaction steps or limited bioavailability, and how this gives information about the individual process steps. Finally, it is discussed how isotope ratios of individual products evolve in sequential or parallel transformations, and what mechanistic insight they contain. A concluding session gives an outlook on current developments, future research directions and the potential for bridging the gap between laboratory and real world systems.
气相色谱-同位素比率质谱法（GC-IRMS）使分析环境样品中单个有机污染物的天然稳定同位素比率（例如，(13)C/(12)C、(15)N/(14)N、(2)H/(1)H）成为可能。这些比率可用作指纹图谱以推断污染源，并且可以通过降解诱导的同位素分馏导致的重同位素富集来证明甚至量化天然污染物转化的发生情况。本综述强调了稳定同位素分馏的另一个强大特性：环境转化机制的研究。同位素效应反映了同位素异构体（即特定分子位置携带轻同位素与重同位素的分子）从反应物到过渡态时的能量差异。因此，测量同位素分馏本质上可以让人瞥见过渡态！展示了在GC-IRMS测量的化合物平均值中，这种位置特异性同位素效应是如何被“稀释掉”的，以及如何在机理情景中通过双同位素图进行仔细评估来恢复潜在的机理信息。综述了环境转化中多步同位素分馏的数学框架。案例研究表明，在存在传质、酶催化作用、多个化学反应步骤或生物可利用性有限的情况下，同位素分馏如何变化，以及这如何提供有关各个过程步骤的信息。最后，讨论了单个产物的同位素比率在连续或平行转化中如何演变，以及它们包含哪些机理见解。总结部分展望了当前的发展、未来的研究方向以及弥合实验室与实际系统之间差距的潜力。
