Carbon isotope fractionation of the neonicotinoid insecticide imidacloprid during photolytic degradation

Compound specific isotope analysis (CSIA) can be used to improve our understanding of how degradation mechanisms of environmental pollutants change under varying photolysis conditions. The influence of factors like pH, light wavelength and dissolved oxygen content can be investigated in laboratory batch experiments. However, polar and thermally labile materials like imidacloprid present a particular challenge in CSIA. Using GC-IRMS is not straightforward for such compounds as they are not volatile and existing derivatization protocols are not suitable for this application. LC-IRMS offers a way to separate non-volatile substances like imidacloprid from its transformation products and convert them to CO₂ for carbon isotope analysis by wet chemical persulfate oxidation.

In this study a simple aqueos LC-IRMS method to separate imidacloprid from its transformation products is presented. This method can be used to study various types of abiotic imidacloprid degradation regarding their carbon isotope fractionation. Degradation by hydrolysis, photolysis or different radical species can be compared. Changes in degradation mechanisms dependend on factors like pH, oxygen content or lamp type are investigated. Photolysis as a major pathway has been tested by means of a filtered xenon-lamp as a sunlight simulator (λ>280nm). The results were contrasted with a UVC-lamp (λ=254nm). This data shall improve our knowledge about the degradation process.