Isotope analysis method for the herbicide bromoxynil and its application to study photo-degradation processes

Bromoxynil is an increasingly applied nitrile herbicide used for post-emergent control of annual broadleaved weeds. Compound-specific isotope analysis (CSIA) of the compound is of interest for studying its environmental fate, yet is challenging following its polar nature. We present a CSIA method for bromoxynil that includes offline thin-layer chromatography purification followed by an elemental analyzer isotope ratio mass spectrometer (EA-IRMS). This method was shown to be accurate and precise for δ¹³C and δ¹⁵N analysis of the compound (standard deviation of replicate standards <0.5‰). The method was applied to photodegraded samples, either radiated under laboratory condition with a UV lamp, or exposed to sunlight under environmental conditions. Dominating degradation products were similar in both cases. Nevertheless, isotope effects differed, presenting a strong inverse carbon isotope effect (εC = 4.74 ± 0.82‰) and a weak inverse nitrogen isotope effect (εN = 0.76 ± 0.12‰) for the laboratory experiment, and an insignificant carbon isotope effect (εC = 0.34 ± 0.44‰) and a normal nitrogen isotope effect (εN = −3.70 ± 0.30‰) for the natural conditions experiment. The differences in δ¹³C vs. δ¹⁵N enrichment trends suggest different mechanism for the two processes. Finally, the obtained dual isotope trend for natural conditions provide the basis for studying the dominance of photodegradation as a degradation route in the environment.