DETERMINATION AND DEGRADATION OF METHOMYL IN TOMATOES AND GREEN BEANSGROWN IN GREENHOUSES

A liquid chromatographic (LC) method using UV detection at 233 nm was used to study the degradation of methomyl in tomatoes and green beans grown in greenhouses, A liquid-liquid extraction with CH2Cl2-methanol (90 + 10, v/v) and a cleanup step with Florisil were combined with LC to isolate, recover, and quantitate the pesticide, Average recoveries obtained at spike levels of 0.03 and 0.40 mg/kg were 83.2-84.7% for to matoes and 83.3-87.5% for green beans, Determination limits were 0.03 mg/kg for tomatoes and 0.01 mg/kg for green beans, Levels of methomyl residues were studied in tomatoes and green beans grown in an experime ntal greenhouse to establish the effect of the kind of greenhouse, app lication dose, species grown, and climatic conditions on the degradati on of this pesticide, Analysis of variance showed that doses did not a ffect the response, The half-life, however, is greater in a flat-roof greenhouse than in an asymmetric-roof greenhouse and is significantly longer for green beans than for tomatoes and longer in winter than in spring. A preharvest time of about 5 days may be suitable for green be ans sprayed with methomyl, Tomatoes show residue levels at the time of application lower than Spanish minimum residue levels.