Abstract: The unique sex pheromone components were employed by insects in order to achieve their intraspecific communication and interspecific reproductive isolation. However, the trace differences in the isomers of sex pheromone components can affect their biological activity extensively. The monounsaturated pheromone components of major insect pests were methylthiolated with dimethyldisulfide (DMDS) and analyzed by capillary gas chromatography (GC)-electron impact (EI) mass spectrometry (MS) in order to determine the double bond position in the aliphatic monounsaturated chain with alcohol, aldehyde or acetate functional groups. GC-EI-MS analyses of the resulting DMDS adducts of pheromone components with alcohol or acetate functional groups showed simple mass spectra with recognizable molecular ions and two high intensity diagnostic ions at m/z 61+14m ［H-(CH₂)_m-CH=S⁺ CH₃］ and 77+14n［CH₃S⁺ =CH(CH₂)_nOH］(or 119+14n，［CH₃S⁺ =CH(CH₂)_nOOCCH₃］)indicating the original double bond position in the aliphatic chain. Although the DMDS adducts of monounsaturated aldehydes also produced the molecular ions of the derivatives and a series of key fragment ions, this can not allow an immediate determination of the position of the double bond in the original unsaturated compounds by low-resolution EI-MS since the nominal mass of a carbonyl group is the same as that of two methylene groups. The location of the double-bond position using the DMDS adducts and GC-EI-MS methods is simple, time efficient, and highly sensitive, and this procedure shows a good prospect in sex-pheromone identification.

Key words: Sex pheromone, dimethyl disulfide, double bond position, GC-EI-MS, DMDS adducts