  Revelation of chlorine and bromine isotope fractionation of halogenated
organic compounds (HOCs) in electron ionization mass spectrometry (EI-MS) is
crucial for compound-specific chlorine/bromine isotope analysis (CSIA-Cl/Br)
using gas chromatography EI-MS (GC-EI-MS). This study systematically
investigated chlorine/bromine isotope fractionation in EI-MS of HOCs including
12 organochlorines and 5 organobromines using GC-double focus magnetic-sector
high resolution MS (GC-DFS-HRMS). Chlorine/bromine isotope fractionation
behaviors of the HOCs in EI-MS showed varied isotope fractionation patterns and
extents depending on compounds. Besides, isotope fractionation patterns and
extents varied at different EI energies, demonstrating potential impacts of EI
energy on the chlorine/bromine isotope fractionation. Hypotheses of inter-ion
and intra-ion isotope fractionations were applied to interpreting the isotope
fractionation behaviors. The inter-ion and intra-ion isotope fractionations
counteractively contributed to the apparent isotope ratio for a certain
dehalogenated product ion. The isotope fractionation mechanisms were
tentatively elucidated on basis of the quasi-equilibrium theory. In the light
of the findings of this study, isotope ratio evaluation scheme using complete
molecular ions and the EI source with sufficient stable EI energies may be
helpful to achieve optimal precision and accuracy of CSIA-Cl/Br data. The
method and results of this study can help to predict isotope fractionation of
HOCs during dehalogenation processes and further to reveal the dehalogenation
pathways.
