Chlorine and bromine isotope fractionations of halogenated organic compounds in fragmentation by gas chromatography-electron ionization high resolution mass spectrometry

Abstract

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.

Theoretical evaluation of inter-ion and intra-ion isotope effects in fragmentation: insights into chlorine and bromine isotope effects of halogenated organic compounds occurring in electron ionization mass spectrometry

“…We hypothesize that dehalogenation reactions of HOCs in fragmentation in EI-MS are stepwise and follow the following sequence: molecular ion / losing Cl/Br atoms one by one / product ions containing successive numbers of Cl/Br atom(s). 17,18 We dene an ion with n Cl/Br atoms as the precursor ion of an ion with n À 1 Cl/Br atom(s), which stem from the same compound and keep unchanged carbon skeletons; and the latter is dened as the product ion of the former. Specically, we dene a precursor ion that nally transforms to its product ion as the parent ion of the product ion.…”

“…Both chlorine and bromine have two stable isotopes with a mass difference of 2 u in the nature ( 35 Cl vs. 37 Cl, and 79 Br vs. 81 Br), and isotopologue distributions and dehalogenation processes of organochlorines and organobromines in EI-MS are similar. 17 Therefore, organochlorines and organobromines may exhibit similar characteristics of chlorine/bromine isotope effects in fragmentation in EI-MS. 17 In this study, to simplify expression, we hence principally concern chlorine isotope effects and use organochlorines to conduct theoretical derivation for elucidating the in-fragmentation isotope effects in EI-MS. In principle, the theoretical derivation results for chlorine isotope effects of organochlorines are also applicable for bromine isotope effects of organobromines.…”

“…15,16 Both inter-ion and intra-ion KIEs can be present in fragmentation, and the latter can be approximately evaluated with relative abundances of fragmental ions. 14 In our recent studies, we observed chlorine and bromine isotope effects of HOCs occurring in both separation on GC and fragmentation in EI-MS. 6,7,17,18 Isotope effects in EI-MS could affect CSIA results, and thus external isotopic standards structurally identical to analytes are necessary. 17 Up to now, it remains unclear how chlorine and bromine isotope effects in EI-MS inuence relative abundances of chlorine and bromine isotopologues, and thereby raw isotope ratios measured by GC-EI-MS.…”

“…14 In our recent studies, we observed chlorine and bromine isotope effects of HOCs occurring in both separation on GC and fragmentation in EI-MS. 6,7,17,18 Isotope effects in EI-MS could affect CSIA results, and thus external isotopic standards structurally identical to analytes are necessary. 17 Up to now, it remains unclear how chlorine and bromine isotope effects in EI-MS inuence relative abundances of chlorine and bromine isotopologues, and thereby raw isotope ratios measured by GC-EI-MS. Furthermore, at present, impacts of the in-fragmentation isotope effects on isotopologue distributions of ions on EI-MS has not been revealed, neither the exact properties of the isotopologue distributions (binomial or non-binomial).…”

Energy‐dependent normal and unusually large inverse chlorine kinetic isotope effects of simple chlorohydrocarbons in collision‐induced dissociation by gas chromatography‐electron ionization‐tandem mass spectrometry

“…where RCl is the measured chlorine isotope ratio of DCM calculated as follows: I I I I R I I I I (3) which is derived from previously reported schemes of isotope-ratio calculation [17,30,31]. The calculation schemes for the intra-ion Cl-KIEs of TCE and PCE, similar to that of DCM, are detailed in the Supplementary data.…”