A report is given on the ionization/dissociation behavior of the title compounds within air plasmas produced by electrical corona discharges at atmospheric pressure: both positive and negative ions were investigated at different temperatures using atmospheric pressure chemical ionization mass spectrometry (APCI-MS). CHF2CH3 (HFC-152a) undergoes efficient ionic oxidation to C2H5O+, in which the oxygen comes from water present in the plasma. In contrast, CF3CH2F (HFC-134a) does not produce any characteristic positive ion under APCI conditions, its presence within the plasma being revealed only as a neutral ligand in ion–molecule complexes with ions of the background (H3O+ and NO+). Analogously, the perfluorocarbon FC-72 (n-C6F14) does not produce significant positive ions at 30 °C: at high temperature, however, it undergoes dissociative ionization to form many product ions including C3F6+, C2F4+, CnF2n+1+ and a few families of oxygen containing cations (CnF2n+1OH2+, CnF2nOH+, CnF2n−1O+, CnF2n−1O2H2+, CnF2n−2O2H+) which are suggested to derive from CnF2n+1+ in a cascade of steps initiated by condensation with water followed by steps of HF elimination and H2O addition. Negative ions formed from the fluoroethanes CHF2CH3 and CF3CH2F (M) include complexes with ions of the background, O2−(M), O3−(M) and some higher complexes involving also water, and complexes of the fluoride ion, F−(H2O), F−(M) and higher complexes with both M and H2O also together. The interesting product O2−(HF) is also formed from 1,1-difluoroethane. In contrast to the HFCs, perfluoro-n-hexane gives stable molecular anions, M−, which at low source temperature or in humidified air are also detected as hydrates, M−(H2O). In addition, in humidified air F−(H2O)n complexes are also formed. The reactions leading to all major positive and negative product ions are discussed also with reference to available thermochemical data and relevant literature reports. The effects on both positive and negative APCI spectra due to ion activation via increasing Vcone are also reported and discussed: several interesting endothermic processes are observed under these conditions. The results provide important information on the role of ionic reactions in non-thermal plasma processes.

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

MAROTTA, ESTER;PARADISI, CRISTINA;SCORRANO, GIANFRANCO
2004

Abstract

A report is given on the ionization/dissociation behavior of the title compounds within air plasmas produced by electrical corona discharges at atmospheric pressure: both positive and negative ions were investigated at different temperatures using atmospheric pressure chemical ionization mass spectrometry (APCI-MS). CHF2CH3 (HFC-152a) undergoes efficient ionic oxidation to C2H5O+, in which the oxygen comes from water present in the plasma. In contrast, CF3CH2F (HFC-134a) does not produce any characteristic positive ion under APCI conditions, its presence within the plasma being revealed only as a neutral ligand in ion–molecule complexes with ions of the background (H3O+ and NO+). Analogously, the perfluorocarbon FC-72 (n-C6F14) does not produce significant positive ions at 30 °C: at high temperature, however, it undergoes dissociative ionization to form many product ions including C3F6+, C2F4+, CnF2n+1+ and a few families of oxygen containing cations (CnF2n+1OH2+, CnF2nOH+, CnF2n−1O+, CnF2n−1O2H2+, CnF2n−2O2H+) which are suggested to derive from CnF2n+1+ in a cascade of steps initiated by condensation with water followed by steps of HF elimination and H2O addition. Negative ions formed from the fluoroethanes CHF2CH3 and CF3CH2F (M) include complexes with ions of the background, O2−(M), O3−(M) and some higher complexes involving also water, and complexes of the fluoride ion, F−(H2O), F−(M) and higher complexes with both M and H2O also together. The interesting product O2−(HF) is also formed from 1,1-difluoroethane. In contrast to the HFCs, perfluoro-n-hexane gives stable molecular anions, M−, which at low source temperature or in humidified air are also detected as hydrates, M−(H2O). In addition, in humidified air F−(H2O)n complexes are also formed. The reactions leading to all major positive and negative product ions are discussed also with reference to available thermochemical data and relevant literature reports. The effects on both positive and negative APCI spectra due to ion activation via increasing Vcone are also reported and discussed: several interesting endothermic processes are observed under these conditions. The results provide important information on the role of ionic reactions in non-thermal plasma processes.
2004
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11577/1480113
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