Effect of the Storage Conditions (Light and Temperature) on the Detection of Thiamethoxam and Clothianidin Content in Rapeseeds by LC-DAD

A new method has been developed and validated to determine potential differences in thiamethoxam and its metabolite (clothianidin) contents in treated rapeseed samples, which were stored under different conditions of light exposure and temperature (protected at 20 and 30 °C; unprotected at 30 °C), using liquid chromatography coupled to a diode array detector. An efficient extraction procedure has also been proposed (average analytes recoveries were between 82 and 104%); this involved a solvent extraction using a mixture of acetonitrile and sodium chloride (60:40, v/v), centrifugation, and a concentration step in a rotary evaporator. The chromatographic analysis of the compounds was achieved using a core-shell technology-based column (Kinetex C18, 150 × 4.6 mm, 2.6 μm, 100 A). The mobile phase consisted of 0.1% formic acid in water and 0.1% of formic acid in acetonitrile (25:75, v/v), with a flow rate of 0.5 mL/min in isocratic elution mode. The method was fully validated in terms of selectivity, limits of detection, and quantification, as well as matrix effect, linearity, precision, and trueness. Finally, the developed methodology was applied to determine thiamethoxam and clothianidin content in rapeseed samples, which were stored under different conditions of light exposure and temperature during 100 days. The results showed that rapeseeds should be stored at 20 °C and protected from light exposure, and that the loss of thiamethoxam was directly related to the formation of clothianidin.