Ion chemistry of chloroethanes in air at atmospheric pressure

Abstract

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

An atmospheric pressure chemical ionization study of the positive and negative ion chemistry of the hydrofluorocarbons 1,1‐difluoroethane (HFC‐152a) and 1,1,1,2‐tetrafluoroethane (HFC‐134a) and of perfluoro‐n‐hexane (FC‐72) in air plasma at atmospheric pressure

“…8 Consistently, the same proposal is advanced that m/z 45 is the oxygencontaining species C 2 H 5 The occurrence of this oxygen-containing ion and the clear involvement of water are of special significance with regard to the focus of our research, as they provide evidence for ionic oxidative decay routes of VOCs in non-thermal plasmas in air. It was therefore important to characterize such a process better.…”

“…Clusters of M with other anions of the background are also seen in addition to a second series of ion-neutral complexes due to the association of F with neutral molecules of 1,1-difluoroethane and/or water. As reported earlier for Cl complexes with chloroethenes and chloroethanes, such ion-molecule complexes undergo sequential losses of neutral ligands when V cone is increased as a result of collisional activation: 7,8 at V cone ½ 70 V, F is observed as the free anion (Fig. 5(b)).…”

“…As for CF 3 O , which is reminiscent of the CCl 3 O species observed in the APCI spectra of 1,1,1-trichloroethane, 8 the results of the SIFT study cited above 21 indicate that it is not formed via reaction of 1,1,1,2-tetrafluoroethane with either O or O 2 . Therefore, alternative possibilities had to be considered.…”

Novel CFCs-substitutes recommended by EPA (hydrofluorocarbon-245fa and hydrofluoroether-7100): Ion chemistry in air plasma and reactions with atmospheric ions

“…The electron affinity of HFE-7100 is not known, but dissociative electron transfer seems to be the most probable way for the formation of C 4 …”

“…Specifically, novel nonthermal plasma-based technologies are currently being developed for the removal of VOCs (volatile organic compounds) from contaminated air [2]. A major focus of our research in this field deals with the ionic reactions occurring in such air plasmas, which can be conveniently produced by corona discharges at atmospheric pressure, and investigated by means of APCI-MS analysis [3][4][5][6][7]. Finally, it is worth mentioning that the interest in such novel plasma-based treatment processes is also attributable to the possibility of abatement of ozone-safe compounds which are, however, implicated as greenhouse gases.…”