Principles and mechanisms of isotope fractionation

The fate of organic compounds in the environment is controlled by a number of processes. These include transport processes in the aqueous and gas phase, phase transfer processes, and transformation processes. During the first two types of processes, the molecules remain intact. The third process—chemical or biological transformation—involves breaking of bonds. The presence of a heavy isotope in molecules can have a particularly large effect on such transformation processes. Therefore, the origin and magnitude of isotope fractionation during transformation processes will be discussed in detail below. However, transport processes in particular diffusion in the gas phase can also lead to isotope fractionation. Before the effect of different types of processes on isotope ratios is discussed, some general definitions and concepts related to isotope fractionation are introduced.