Insight into Imidacloprid Degradation through Compound Specific Carbon Isotope Analysis and High-Resolution Mass Spectrometry

The insecticide imidacloprid (IMI) is an ubiquitous environmental contaminant. This study presents an approach to investigate its degradation through the combined use of compound-specific stable carbon isotope analysis and high-resolution mass spectrometry (HRMS). It demonstrates how isotope ratio and HRMS product analysis can complement each other. The photolysis of imidacloprid by simulated sunlight, UV–C (λ = 254 nm), and alkaline hydrolysis all exhibited distinguishable carbon isotope enrichment factors (εC). The largest fractionation is observed during alkaline hydrolysis at pH 12 and 30 °C (εC = −3.9 ± 0.4‰). HRMS product analysis confirms that only one carbon atom is in a reactive position, allowing an AKIE calculation (εC AKIE = −34.1 ± 3.7‰). Carbon isotope enrichment factors of photolysis were not significantly affected by dissolved oxygen content or pH. Irradiation by UV–C did not result in significant isotope enrichment, whereas experiments with simulated sunlight yielded εC values between −1.1 ± 0.1 and −1.4 ± 0.1‰. It was hypothesized that an increased contribution of indirect photolysis led to isotopic enrichment. Finally, we demonstrate how the developed method can be used to determine the carbon isotope fingerprint of imidacloprid in veterinary flea control products.