Ion chemistry of chloroethanes in air at atmospheric pressure

Ion chemistry at atmospheric pressure is of major relevance to novel method s for the abatement of volatile organic compounds (VOCs) that employ non-th ermal plasmas. For this reason, positive and negative APCI (atmospheric pre ssure chemical ionization) mass spectra of all six di-, tri- and tetrachlor oethanes diluted in air (500-1500 ppm) at atmospheric pressure were investi gated at 30 degreesC and at 300 degreesC. Spectral changes due to collision al activation of the ions achieved by increasing AV, the potential differen ce between sampling and skimmer cones, are informative of structures and io n-molecule reactions. Positive ion chemistry of the chloroethanes (M) can, in general, be ascribed to CC and C-Cl cleavages of the molecular ion, M+.- , never detected but likely formed via exothermic charge exchange from prim ary ions of the APCI plasma. Exceptions to this characteristic pattern were observed for 1,1-dichloroethane and 1,1,2,2-tetrachloroethane, which give [M - H](+) and [M - HCl](+) species, respectively. It is suggested that bot h such species are due to ionization via hydride transfer. Upon increasing DeltaV, the [M - HCl](+) ion formed from 1,1,2,2-tetrachloroethane undergoe s the same fragmentation and ion-molecule reactions previously reported for trichloroethene. A nucleophilic reaction of water within the [C2H4Cl+](H2O )(n) ionic complexes to displace HCl is postulated to account for the [C2H5 O+],(H2O)(m) species observed in the positive APCI spectra of the dichloroe thanes. Negative ion spectra are, for all investigated chloroethanes, domin ated by Cl- and its ion-neutral complexes with one, two and, in some cases, three molecules of the neutral precursor and/or water. Another common feat ure is the formation of species (X-)(M)(n) where X- is a background ion of the APCI plasma, namely O-2(-),O-3(-) and, in some cases, (NO)(2)(-). Pecul iar to 1,1,1-trichloroethane are species attributed to Cl- complexes with p hosgene, (Cl-)(Cl2C=O)(n)(n = 1,2). Such complexes, which were not observed for either the isomeric 1,1,2-trichloroethane or for the tetrachloroethane s, are of interest as oxidation intermediates in the corona-induced decompo sition process. No conclusions can be drawn in the case of the dichloroetha nes, since, for these compounds, the ions (Cl-)(Cl2C=O)(n), and (Cl-)(M)(n) happen to be isobaric. Copyright (C) 2001 John Wiley & Sons, Ltd.