  Revelation of the effects of isotope fractionation during fragmentation in
electron ionization mass spectrometry (EI-MS) on compound-specific isotope
analysis of chlorine and bromine (CSIA-Cl/Br) may be of crucial significance,
yet a theoretical basis for elucidating the effects is absent. This study
provides a solid theoretical deduction regarding the isotope fractionation
taking place during fragmentation in EI-MS. Both intermolecular and
intramolecular isotope fractionations present in dehalogenation processes,
influencing the isotope ratios of detected ions. In general, intermolecular
isotope fractionation positively affects the isotope ratios of precursor ions
but inversely impacts those of the corresponding product ions. Molecular ions
are normally affected by one type of intermolecular isotope fractionation only
during the dehalogenation from parent ions to their product ions. While product
ions can be impacted by two types of intermolecular isotope fractionations,
namely, the inverse intermolecular isotope fractionation from parent ions to
their product ions and the normal intermolecular isotope fractionation from the
product ions to their further dehalogenated product ions. For a compound having
position-distinct isotope ratios, if dehalogenation reacts on only one
position, the intermolecular isotope fractionation taking palace in this
reaction is normal for the detected precursor ion, but ineffective to the
product ion isotope ratio in comparison with that of the total precursor ions.
On the other hand, intramolecular isotope fractionation positively affects the
isotope ratios of product ions only. The chlorine isotopologues of the observed
precursor ions are deduced to never obey binomial distribution, regardless of
what modes of the initial isotopologue distributions of the total precursor
ions are.
