NMR Study of the Influence of pH on the Persistence of Some Neonicotinoids in Water

The influence of the pH on the persistence of four neonicotinoid insecticides: imidacloprid, thiamethoxam, acetamiprid and thiacloprid, was studied by H NMR spectrometry at different pH values of Britton-Robinson buffers (4.0, 7.0 and 9.0) at a constant temeprature. Thiamethoxam and imidacloprid, having an identical nitroguanidine functional group, exhibited low persistence in alkaline media, as their degradation started already after three and five days, respectively. On the other hand, at pH 4.0 and 7.0 they remained unaffected for several months. In contrast to them, thiacloprid and acetamiprid, the compounds with a cyanoimine functional group, were less persistent in acidic solution. In alkaline media, they were stable for about 30 days. The proportion of hydrolyzed pesticide was calculated from the H NMR spectra by comparing the integrals of compound signals with the signal of acetate standard.