  Kinetic isotope effects (KIEs) taking place in mass spectrometry (MS) can
provide in-depth insights into the fragmental behaviors of compounds in MS. Yet
the mechanisms of KIEs in collision-induced dissociation (CID) in tandem MS are
unclear, and information about chlorine KIEs (Cl-KIEs) of organochlorines in MS
is particularly scarce. This study investigated the Cl-KIEs of dichloromethane,
trichloroethylene and tetrachloroethylene during CID using gas
chromatography-electron ionization triple-quadrupole tandem MS. Cl-KIEs were
measured with MS signal intensities, and their validity was confirmed in terms
of chromatograms, crosstalk effects and background subtraction influences. All
the organochlorines presented large inverse Cl-KIEs, showing the largest values
of 0.492, 0.910 and 0.892 at the highest collision energy for dichloromethane,
trichloroethylene and tetrachloroethylene, respectively. For dichloromethane,
both intra-ion and inter-ion Cl-KIEs were studied, within the ranges of
0.492-1.020 and 0.614-1.026, respectively, showing both normal and inverse
Cl-KIEs depending on collision energies. The observed Cl-KIEs generally
declined with the increasing collision energies from 0-60 eV, but were inferred
to be independent of MS signal intensities. The Cl-KIEs are dominated by
critical energies at low internal energies, while controlled by rotational
barriers (or looseness/tightness of transition states) at high internal
energies. It is concluded that the Cl-KIEs may depend on critical energies,
bond strengths, available internal energies, and transition state
looseness/tightness. The findings of this study yield new insights into the
fundamentals of Cl-KIEs of organochlorines during CID, and may be conducive to
elucidating the mechanisms of KIEs in collision-induced and photo-induced
reactions in the actual world.
