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Dechlorinated fragmental ions of organochlorines can be commonly found onelectron ionization-mass spectrometry (EI-MS). Yet it is still unclear whethermultiple dechlorination reactions taking place in EI-MS are concerted orstepwise. This study investigated the concertedness of the multipledechlorination reactions of perchlorethylene (PCE) in EI-MS in light of theobserved chlorine isotope effects during different dechlorination reactionsalong with the detected MS signal intensities. The fluctuant isotope ratiosamong different ions revealed that the observed multiple dechlorinatedreactions in the EI-DFS-HRMS were stepwise. And the changing trends of isotoperatios of the ions detected by EI-qMS might suggest that the observed multipledechlorination reactions in this MS were concerted. Mechanisms for theseconcerted/stepwise multiple dechlorination reactions are tentativelyinterpreted. Besides the evidence of isotope effects, according to thequasi-equilibrium theory, the MS signal intensities also suggested that themultiple dechlorination reactions in the EI-HRMS were likely stepwise, whilethose in the EI-qMS might be concerted. The distance from the ionization regionto the mass analyzer entrance in an MS may be a critical factor affecting thedechlorination reactions of ions flying over this distance. This distance inthe EI-qMS is significantly shorter than that in the EI-HRMS. Thus, the maindetected ions in the EI-qMS might be generated during the ionization andin-source fragmentation processes, while the main ions detected in the EI-HRMSmight be produced during the fragmentation of precursor ions flying over thedistance. We deduce that the main dechlorination reactions in the ionizationregion were concerted, while those taking place during the flight of precursorions over the distance from the ionization region to the mass analyzer entrancewere stepwise.

Observation of Concerted and Stepwise Multiple Dechlorination Reactions  of Perchlorethylene in Electron Ionization Mass Spectrometry According to  Measured Chlorine Isotope Effects