Dissipation rates of fenitrothion in greenhouse grown lettuce and under cold storage conditions

Summary The dissipation of fenitrothion ( O , O ‐dimethyl O ‐ p ‐nitro‐ m ‐tolyl phosphothioate) was evaluated by gas chromatography (GC) with nitrogen‐phosphorus detection (NPD) in lettuce samples grown in an experimental greenhouse and harvested over a 4 weeks period following treatment with this insecticide. Dissipation followed a pseudo‐first‐order kinetics ( r was 0.996). The recommended preharvest time (PT) of application is 15 days, and when the residual value of fenitrothion was evaluated at harvest, it was found to be above the maximum residue limit (MRL) established by Spanish law in the field experiment for this compound. During the experiment, the evolution of the two main fenitrothion metabolites (3‐methyl‐4‐nitrophenol and fenitrothion‐oxon) was also evaluated. When the dissipation of fenitrothion was evaluated in cold‐stored lettuce the half‐life was 9–10 times greater than in greenhouse conditions.