Insight into\nImidacloprid Degradation through Compound\nSpecific Carbon Isotope Analysis and High-Resolution Mass Spectrometry

The insecticide imidacloprid\n(IMI) is an ubiquitous environmental\ncontaminant. This study presents an approach to investigate its degradation\nthrough the combined use of compound-specific stable carbon isotope\nanalysis and high-resolution mass spectrometry (HRMS). It demonstrates\nhow isotope ratio and HRMS product analysis can complement each other.\nThe photolysis of imidacloprid by simulated sunlight, UV–C\n(λ = 254 nm), and alkaline hydrolysis all exhibited distinguishable\ncarbon isotope enrichment factors (ε<sub>C</sub>). The largest\nfractionation is observed during alkaline hydrolysis at pH 12 and\n30 °C (ε<sub>C</sub> = −3.9 ± 0.4‰).\nHRMS product analysis confirms that only one carbon atom is in a reactive\nposition, allowing an AKIE calculation (ε<sub>C AKIE</sub> = −34.1 ± 3.7‰). Carbon isotope enrichment factors\nof photolysis were not significantly affected by dissolved oxygen\ncontent or pH. Irradiation by UV–C did not result in significant\nisotope enrichment, whereas experiments with simulated sunlight yielded\nε<sub>C</sub> values between −1.1 ± 0.1 and −1.4\n± 0.1‰. It was hypothesized that an increased contribution\nof indirect photolysis led to isotopic enrichment. Finally, we demonstrate\nhow the developed method can be used to determine the carbon isotope\nfingerprint of imidacloprid in veterinary flea control products.