6 monitors with 1 ml of the mixture composed of dichloromethane: methanol (50:50). They were all subjected to mechanical agitation for 10 min, 60 cycles per min. After 15 min resting, extraction of new samples to be injected in the GC was conducted. Table 3 shows the recovery factors obtained. Both the reference patterns and the solvents used are of chromatographic quality. The 20 mg ml1 standards in methanol were obtained from AccuStandard (Chemicals standard referent, International Edition contents), dichloromethane(J.T. Baker, 99.8%) and methanol (Mallinckrodt, 99.9%).