  Compound-specific chlorine/bromine isotope analysis (CSIA-Cl/Br) has become a
useful approach for degradation pathway investigation and source appointment of
halogenated organic pollutants (HOPs). CSIA-Cl/Br is usually conducted by gas
chromatography-mass spectrometry (GC-MS), which could be negatively impacted by
chlorine and bromine isotope fractionation of HOPs on GC columns. In this
study, 31 organochlorines and 4 organobromines were systematically investigated
in terms of Cl/Br isotope fractionation on GC columns using GC-double focus
magnetic-sector high resolution MS (GC-DFS-HRMS). On-column chlorine/bromine
isotope fractionation behaviors of the HOPs were explored, presenting various
isotope fractionation modes and extents. Twenty-nine HOPs exhibited inverse
isotope fractionation, and only polychlorinated biphenyl-138 (PCB-138) and
PCB-153 presented normal isotope fractionation. And no observable isotope
fractionation was found for the rest four HOPs, i.e., PCB-101,
1,2,3,7,8-pentachlorodibenzofuran, PCB-180 and 2,3,7,8-tetrachlorodibenzofuran.
The isotope fractionation extents of different HOPs varied from below the
observable threshold (0.50%) to 7.31% (PCB-18). The mechanisms of the on-column
chlorine/bromine isotope fractionation were tentatively interpreted with the
Craig-Gordon model and a modified two-film model. Inverse isotope effects and
normal isotope effects might contribute to the total isotope effects together
and thus determine the isotope fractionation directions and extents. Proposals
derived from the main results of this study for CSIA-Cl/Br research were
provided for improving the precision and accuracy of CSIA-Cl/Br results. The
findings of this study will shed light on the development of CSIA-Cl/Br methods
using GC-MS techniques, and help to implement the research using CSIA-Cl/Br to
investigate the environmental behaviors and pollution sources of HOPs.
