Carbon, hydrogen, and nitrogen isotope fractionation during light-induced transformations of atrazine

The 13C, 2H, and 15N 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 epsilonC, epsilonH, and epsilonN 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 2H fractionations, quantified by EH values of -51.2 +/- 2.5% per hundred and -25.3 +/- 1.7% per hundred, were found for the transformation of atrazine by (3)4-CBBP* and OH radicals, respectively. 13C and 15N enrichment factors were rather small (-0.3% per hundred > epsilon(C, N) > -1.7% per hundred). The combined delta(13)C, delta(2)H, and delta(15)N 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 13C and 15N fractionation (epsilonC = 4.6 +/- 0.3% per hundred, epsilonN = 4.9 +/- 0.2% per hundred) and no detectable 2H 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.