Compound-Specific Isotope Analysis: Perspectives to Study Reaction Mechanisms in Complex Systems

Martin Elsner, Technical University of Munich, Munich, Germany

Investigating organic transformation mechanisms in complex environments (ground and surface water, living organisms, heterogeneous catalysis) is of fundamental importance in chemical sciences, yet challenged by the uncertainty whether lab-based studies adequately mirror real-world processes. To bridge this gap between nature and reaction flask, and to enable mechanistic studies directly in complex systems, my group has advanced compound-specific stable isotope analysis (CSIA) of organic substances at their natural isotopic abundance. Through gas chromatography (GC) or liquid chromatography (LC) coupled to isotope ratio mass spectrometry (IRMS) we have accomplished the measurement of ¹³C-, ¹⁵N-, ²H- and ³⁷Cl- isotope effects in pesticides, pharmaceuticals, chlorinated hydrocarbons and petroleum hydrocarbons at trace (microgram per liter) concentrations. This information does not only allow us to detect degradation of chemicals in the environment in complex situations and over time scales otherwise not accessible (months to years). Isotope effect analysis of multiple elements also enables us to elucidate transformation mechanisms (i.e. the manner and order of bond breaking) where conventional analysis provides complementary information about the identity of products (i.e. the net outcome of a reaction). My presentation summarizes analytical developments, presents latest results on the ability of CSIA to pinpoint micropollutant degradation in soil and groundwater and illustrates the capability of CSIA to elucidate underlying reaction mechanisms such as in Vitamin B₁₂-catalyzed dehalogenation of chlorinated ethenes. This dual advantage of CSIA – the ability to tackle complex systems, yet to retrieve mechanistic information on the molecular level – offers prospects for studying transformation mechanisms of chemical compounds in environmental and engineered systems.