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

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). CHF(2)CH(3) (HFC-152a) undergoes efficient ionic oxidation to C(2)H(5)O(+), in which the oxygen comes from water present in the plasma. In contrast, CF(3)CH(2)F (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 (H(3)O(+) and NO(+)). Analogously, the perfluorocarbon FC-72 (n-C(6)F(14)) does not produce significant positive ions at 30 degrees C: at high temperature, however, it undergoes dissociative ionization to form many product ions including C(3)F(6)(+), C(2)F(4)(+), C(n)F(2n+1)(+) and a few families of oxygen containing cations (C(n)F(2n+1)OH(2)(+), C(n)F(2n)OH(+), C(n)F(2n-1)O(+), C(n)F(2n-1)O(2)H(2)(+), C(n)F(2n-2)O(2)H(+)) which are suggested to derive from C(n)F(2n+1)(+) in a cascade of steps initiated by condensation with water followed by steps of HF elimination and H(2)O addition. Negative ions formed from the fluoroethanes CHF(2)CH(3) and CF(3)CH(2)F (M) include complexes with ions of the background, O(2)(-)(M), O(3)(-)(M) and some higher complexes involving also water, and complexes of the fluoride ion, F(-)(H(2)O), F(-)(M) and higher complexes with both M and H(2)O also together. The interesting product O(2)(-)(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(-)(H(2)O). In addition, in humidified air F(-)(H(2)O)(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 V(cone) 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.

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

“…The gas phase ion chemistry in air of HFE-7100 and HFC-245fa has been investigated by APCI-MS and by studying their reactions with selected atmospheric ions (H 3 between HFC-245fa and H 3 O ϩ does not lead to oxygenation of the fluorocarbon via COO bond formation, in contrast°with°what°was°found°with°other°HFCs° [6]°and with HFE-7100. A common process is, instead, the condensation of fluorinated unsaturated cations with water followed by HF loss.…”

“…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.…”

Gas‐phase synthesis and detection of the benzenediazonium ion, C6H5N. A joint atmospheric pressure chemical ionization and guided ion beam experiment

“…Finally, we studied the reaction of phenylium ions with N 2 (process B in Scheme 1) and we were able to observe the adduct ion at m/z 105. In order to rule out any contribution from trace gas impurities we repeated the experiment by using the isotopically labeled 15 pressure in the scattering cell. There is clear evidence that the ratio I P /I 0 has a quadratic dependence on the pressure (given by the dashed line in Fig.…”

“…13 Secondly, corona plasmas are exploited for the destruction of diluted pollutants in air. 14,15 Finally, the ion chemistry of benzene and polycyclic aromatic hydrocarbons (PAHs) has attracted great interest from fundamental research aimed at understanding reaction mechanisms and their implications in the chemistry of various ionized environments (e.g. combustion systems and planetary ionospheres).…”

Gas‐phase positive ion chemistry of 1‐bromo‐1‐chloro‐2,2,2‐trifluoroethane (halothane) upon electron ionization within an ion trap mass spectrometer

“…via HF elimination. An analogous process was not observed with other fluoroethanes 19. The halogen‐substituted ethyl cations C 2 ClF 3 H + and C 2 BrClF 2 H + react with halothane via chloride abstraction, while the analogous bromine‐containing ion C 2 BrF 3 H + is unreactive under the same experimental conditions.…”