Carbon, Hydrogen, and Nitrogen Isotope Fractionation During Light-Induced Transformations of Atrazine

The C-13, H-2, and N-15 fractionation associated with light-induced transformations of N-containing pesticides in surface waters was investigated using atrazine as a model compound. In laboratory model systems, bulk isotope enrichment factors cc, EH, and EN were determined during the photooxidation of atrazine by excited triplet states of 4-carboxybenzophenone ((3)4-CBBP*), by OH radicals, and during direct photolysis at 254 nm. Moderately large H-2 fractionations, quantified by CH values of -51.2 +/- 2.5 parts per thousand and -25.3 +/- 1.7 parts per thousand, were found for the transformation of atrazine by 34-CBBP* and OH radicals, respectively. 13C and 15N enrichment factors were rather small (-0.3 parts per thousand > epsilon(C), (N) > -1.7 parts per thousand). The combined delta C-13, delta H-2, and delta N-15 analysis suggests that isotope effects are most likely due to H abstraction at the N-H and C-H bonds of the N-alkyl side chains. Direct photolysis of atrazine yielding hydroxyatrazine as main product was characterized by inverse C-13 and N-15 fractionation (epsilon(c)=4.6 +/- 0.3 parts per thousand, epsilon(N) =4.9 +/- 0.2 parts per thousand) and no detectable H-2 fractionation. We hypothesize that isotope effects from photophysical processes involving the excited states of atrazine as well as magnetic isotope effect originating from the magnetic interactions of spin-carrying C and N nuclei have contributed to the observed inverse fractionation. Our study illustrates how compound-specific isotope analysis can be used to differentiate between important direct and indirect phototransformation pathways of agrochemicals in the environment.