arXiv subjects

Yujuan Fan

Publications and source records attributed to Yujuan Fan.

5 recordsLinked to original sources

arXiv2019

Observation of inconsistent carbon isotope compositions of chlorine-isotopologue pairs of individual organochlorines by gas chromatography-high resolution mass spectrometry

This study investigated the consistency/inconsistency of carbon isotope compositions of chlorine-isotopologue pairs, e.g., 12C235Cl4 vs. 12C13C35Cl4, of individual organochlorines including two chloroethylenes, three polychlorinated biphenyls, methyl-triclosan and hexachlorobenzene. The raw carbon isotope ratios were measured by gas chromatography-high resolution mass spectrometry. Data simulations in terms of background subtraction, background addition, dual 13C-atoms substitution, deuterium substitution and hydrogen-transfer were conducted to confirm the validity of measured carbon isotope ratios and their differences. Inconsistent carbon isotope ratios derived from chlorine-isotopologue pairs of individual organochlorines were observed, and the isotopologues of each organochlorine were thus inferred to be non-randomly distributed. Mechanistic interpretation for these findings was tentatively proposed according to a basic principle in clumped-isotope geochemistry, reaction thermodynamics and kinetics, along with isotope effects occurring on electron ionization mass spectrometry. This study sheds light on the actual carbon isotope compositions of chlorine-isotopologue pairs of organochlorines, and yields new insights into the real distributions of carbon and chlorine isotopologues. The inconsistent carbon isotope compositions of chlorine-isotopologue pairs are anticipated to benefit the exploration of formation conditions and source identification of organochlorine pollutants.

physics.chem-ph↗

arXiv2019

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

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.

physics.chem-ph↗

arXiv2019

Ascertaining hydrogen-abstraction reaction efficiencies of halogenated organic compounds on electron ionization mass spectrometry

H-abstraction reactions occurring on electron ionization mass spectrometry (EI-MS) are a long-standing and crucial topic in MS research. Yet some critical relevant mechanisms are controversial and ambiguous, and information about the EI-induced H-abstraction reactions of halogenated organic compounds (HOCs) is completely in the dark. This study provides a systematic investigation of H-abstraction reactions of HOCs taking place on EI source using 13C6-hexachlorobenzene (13C6-HCB) and 13C6-hexabromobenzene (13C6-HBB) as exemplary compounds by gas chromatography high resolution mass spectrometry (GC-HRMS). The H-abstraction efficiencies were evaluated with the MS signal intensity ratios of ions with H-abstraction relative to the corresponding original ions (without H-abstraction). Ion source temperatures, EI energies and numbers of heavy isotope atoms (37Cl or 81Br) of isotopologues were investigated in terms of their effects on the H-abstraction efficiencies. The H-abstraction efficiencies of individual isotopologues generally decreased from the first to the last isotopologues of respective ions, and those of individual ions were different from each other, with the highest values of 0.017 and 0.444 for 13C6-HCB and 13C6-HBB, respectively. The overall H-abstraction efficiencies involving all measured ions of 13C6-HCB and 13C6-HBB were 0.004 and 0.128, respectively. EI energies and emission currents could impact the H-abstraction efficiencies. The H-abstraction reactions were inferred to belong to ion-molecule reactions. Some strategies were proposed for eliminating or alleviating the interference triggered by the H-abstraction reactions on EI-MS in identification of halogenated organic pollutants (HOPs). Our findings provide a better understanding for the EI-induced H-abstraction reactions of HOCs, and may benefit identification of HOPs in environmental analysis, especially for novel HOPs.

physics.chem-ph↗

arXiv2017

Chlorine and Bromine Isotope Fractionation of Halogenated Organic Compounds in Electron Ionization Mass Spectrometry

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.

physics.chem-ph↗

arXiv2017

Chlorine and Bromine Isotope Fractionation of Halogenated Organic Pollutants on Gas Chromatography Columns

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.

physics.chem-ph↗