Observation of varied characteristics of chlorine isotope effects of organochlorines in dechlorination reactions on different types of electron ionization mass spectrometers

Abstract

Dechlorination reactions of organochlorines can commonly take place in fragmentation on electron ionization mass spectrometry (EI-MS) and collision-induced dissociation (CID) on electron ionization tandem mass spectrometry (EI-MS/MS), possibly accompanied with chlorine isotope effects. However, it remains unclear whether characteristics of chlorine isotope effects of individual organochlorines occurring on different types of mass spectrometers are similar or different. Revelation of characteristics of chlorine isotope effects on different mass spectrometers may help to elucidate mechanisms of dechlorination reactions and fragmental pathways of organochlorines. This study investigated the characteristics of chlorine isotope effects of two model organochlorines, i.e., tetrachloroethylene and trichloroethylene, during fragmentation/CID on EI-double focusing magnetic-sector high resolution MS (EI-HRMS), EI-quadrupole MS (EI-qMS) and EI-triple-quadrupole MS/MS (EI-MS/MS) that were all coupled with gas chromatography (GC). The patterns of chlorine isotope ratios measured by different instruments were different, showing different characteristics of chlorine isotope effects. On EI-HRMS, both normal and inverse chlorine isotope effects were observed; while on EI-qMS, only normal isotope effects were found, and on EI-MS/MS merely inverse isotope effects were discovered. On EI-HRMS and EI-qMS, inter-ion and intra-ion chlorine kinetic isotope effects (Cl-KIEs) might participate in isotope fractionation processes and contribute to inverse inter-ion and normal intra-ion isotope effects, respectively, and the dominant isotope effects determined the direction of observed (apparent) isotope effects. On EI-MS/MS, both inter-ion and intra-ion Cl-KIEs were inverse at high collision energies (including 30 and 45 eV) and contributed to normal inter-ion and inverse intra-ion isotope effects, respectively, and the inverse intra-ion isotope effects were the dominant, leading to the inverse apparent isotope effects in all dechlorination reactions. Particularly, the chlorine isotope ratios measured by EI-qMS exponentially decreased with the increasing Cl atoms of ions, implying that the intra-ion isotope effects rose as broken C–Cl bonds increased. Moreover, fundamentals of chlorine isotope effects in the stepwise multibond dechlorination reactions were revealed with the aid of density functional theory calculations. The different measurement timescales and geometries of the mass spectrometers and the difference in EI-induced fragmentation and CID might result in the different characteristics of chlorine isotope effects. This study may be conducive to elucidating isotope fractionation behaviors and fragmental pathways of organochlorines on EI-MS(/MS), and provides new insights into the mechanisms of dechlorination reactions losing multiple chlorine atoms.