Carbon and hydrogen isotopic fractionation of low molecular weight organic compounds during ultraviolet degradation

Ultraviolet (UV) radiation is a ubiquitous energy source for chemical reactions in nature. In particular, UV radiation provides an important pathway for degradation of organic compounds. Although isotopic change in organic compounds exposed to UV radiation may be an indicator for evaluating the extent of degradation, the isotope behavior has not been yet clarified. In this study, stable carbon and hydrogen isotope fractionations were determined for volatile organic compounds including methanol, acetonitrile, benzene and propionic acid during UV degradation in order to reveal the isotopic behavior with respect to different functional groups. All the studied compounds became enriched in ¹³C and D with increase in UV exposure time. The isotopic fractionation factor (α) ranged from 0.9968 to 0.9875 for carbon, and from 0.9890 to 0.9406 for hydrogen. The order of carbon isotopic fractionation was different from that of hydrogen isotopic fractionation, probably due to intra- and inter-molecular reactions with different functional groups. UV degradation is a process potentially able to induce isotope enrichment in organic compounds in natural environments.