Ion chemistry of chloroethanes in air at atmospheric pressure

Ion chemistry at atmospheric pressure is of major relevance to novel methods for the abatement of volatile organic compounds (VOCs) that employ non-thermal plasmas. For this reason, positive and negative APCI (atmospheric pressure chemical ionization) mass spectra of all six di-, tri- and tetrachloroethanes diluted in air (500-1500 ppm) at atmospheric pressure were investigated at 30 °C and at 300 °C. Spectral changes due to collisional activation of the ions achieved by increasing δV, the potential difference between sampling and skimmer cones, are informative of structures and ion-molecule reactions. Positive ion chemistry of the chloroethanes (M) can, in general, be ascribed to C-C and C-Cl cleavages of the molecular ion, M+•, never detected but likely formed via exothermic charge exchange from primary 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 both such species are due to ionization via hydride transfer. Upon increasing δV, the [M - HCl]+• ion formed from 1,1,2,2-tetrachloroethane undergoes 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 [C2H5O+](H2O)m species observed in the positive APCI spectra of the dichloroethanes. Negative ion spectra are, for all investigated chloroethanes, dominated 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 feature is the formation of species (X-) (M)n where X- is a background ion of the APCI plasma, namely O2 -,O3 - and, in some cases, (NO)2 -. Peculiar to 1,1,1-trichloroethane are species attributed to Cl- complexes with phosgene, (Cl-)(Cl2C=O)n(n = 1,2). Such complexes, which were not observed for either the isomeric 1,1,2-trichloroethane or for the tetrachloroethanes, are of interest as oxidation intermediates in the corona-induced decomposition process. No conclusions can be drawn in the case of the dichloroethanes, since, for these compounds, the ions (Cl-)(Cl2C=O)n and (Cl-)(M)n happen to be isobaric.