Ascertaining Hydrogen-Abstraction Reaction Efficiencies of Halogenated Organic Compounds in Electron Impact Ionization Processes by Gas Chromatography–High-Resolution Mass Spectrometry

Abstract

H-abstraction reactions occurring on electron ionization mass spectrometry (EI-MS) are a longstanding 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 13 C6-hexachlorobenzene ( 13 C6-HCB) and 13 C6-hexabromobenzene ( 13 C6-HBB) as exemplary compounds by gas chromatography high resolution mass spectrometry (GC-HRMS). The Habstraction efficiencies were evaluated with the MS signal intensity ratios of ions with Habstraction relative to the corresponding original ions (without H-abstraction). Ion source temperatures, EI energies and numbers of heavy isotope atoms ( 37 Cl or 81 Br) 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 13 C6-HCB and 13 C6-HBB, respectively. The overall H-abstraction efficiencies involving all measured ions of 13 C6-HCB and 13 C6-HBB were 0.004 and 0.128, respectively. With increasing ion source temperatures, the H-abstraction efficiencies firstly increased to a summit and then began to linearly decrease. EI energies and emission currents could impact the H-abstraction efficiencies, but showed no certain tendency.The H-abstraction reactions were inferred to belong to ion-molecule reactions, and the siloxanes bleeding from the GC column might be the hydrogen source. Some strategies were proposed Page 3 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. Keywords:Hydrogen-abstraction efficiencies; Halogenated organic compounds; Electron ionization mass spectrometry; Ion-molecule reactions; Chlorine and bromine isotope effects; Gas chromatography-high resolution mass spectrometry Up to now, reported studies concerning H-abstraction reactions on EI source mainly focused on quinones. 20,21 While halogenated organic compounds (HOCs), e.g., organochlorines and organobromines, may also abstract hydrogen atoms on EI source, in view of their lone electron pairs of halogen atoms (e.g., Cl and Br). HOCs are a large category of chemicals produced by both natural and anthropogenic activities, [35][36][37][38] and have been contributing enormous influences to human beings and the environment, in either positive or negative ways. [39][40][41][42][43] Many HOCs are persistent, bioaccumulative and toxic pollutants, such as polychlorinated...