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

Dechlorinated fragmental ions of organochlorines can be commonly found on electron ionization-mass spectrometry (EI-MS). Yet it is still unclear whether multiple dechlorination reactions taking place in EI-MS are concerted or stepwise. This study investigated the concertedness of the multiple dechlorination reactions of perchlorethylene (PCE) in EI-MS in light of the observed chlorine isotope effects during different dechlorination reactions along with the detected MS signal intensities. The fluctuant isotope ratios among different ions revealed that the observed multiple dechlorinated reactions in the EI-DFS-HRMS were stepwise. And the changing trends of isotope ratios of the ions detected by EI-qMS might suggest that the observed multiple dechlorination reactions in this MS were concerted. Mechanisms for these concerted/stepwise multiple dechlorination reactions are tentatively interpreted. Besides the evidence of isotope effects, according to the quasi-equilibrium theory, the MS signal intensities also suggested that the multiple dechlorination reactions in the EI-HRMS were likely stepwise, while those in the EI-qMS might be concerted. The distance from the ionization region to the mass analyzer entrance in an MS may be a critical factor affecting the dechlorination reactions of ions flying over this distance. This distance in the EI-qMS is significantly shorter than that in the EI-HRMS. Thus, the main detected ions in the EI-qMS might be generated during the ionization and in-source fragmentation processes, while the main ions detected in the EI-HRMS might be produced during the fragmentation of precursor ions flying over the distance. We deduce that the main dechlorination reactions in the ionization region were concerted, while those taking place during the flight of precursor ions over the distance from the ionization region to the mass analyzer entrance were stepwise.