Abstract: The Electron Capture Indued Decomposition(ECID)spectra of[C₇H₇Cl]²⁺,[C₇H₆Cl]²⁺ and[C₇H₅Cl]²⁺doubly charged ions produced from four kinds of chlorotoluene isomers in the ion source of mass spectrometer under 70 eVelectron impact were studied in this work.The ECID spectra for[C₇H₇Cl]²⁺ from o-chlorotoluene can be distinguished with those from other isomers due to the ortho-effect, which implies the structures of the stable doubly charged molecular ions retain the characteristics of those of neutral molecules to some extent,while the ECIDspectra for[C₇H₆Cl]²⁺and[C₇H₅Cl]²⁺ from the isomers were identical,respectively,which means that the isomerizations occured prior to the decompositions.The dependence of the ECIDreactions on whether the parent ion is a odd or even electron ion was also discussed.

Key words: Doubly charged ion, ECID, [C₇H₇Cl]²⁺, [C₇H₆Cl]²⁺