Quantification of isotope fractionation in experiments with deuterium-labeled substrate.

Isotope analysis is a potentially sensitive method to trace in situ degradation of organic contaminants. In a recent paper, Morasch et al. (3) investigated the mechanism of isotope fractionation during toluene biodegradation using deuteriumlabeled toluene. The authors overlooked that the Rayleigh equation that is normally used to evaluate isotope fractionation at natural abundance level (2) is not applicable to studies with labeled substrate, particularly if large isotope fractionation occurs. For several of their experiments they obtained negative hydrogen isotope fractionation factors (see Table 1 in reference 3), which contradict the definition of the fractionation factor (see below). Since labeled compound will likely be used in further investigations to study isotope fractionation, it is important to demonstrate why the commonly used Rayleigh equation is usually not applicable in such studies and to provide an alternative method to quantify isotope fractionation. The magnitude of isotope fractionation is normally characterized by the fractionation factor, which is defined as follows for kinetic isotope fractionation: