A mass spectrometry study of alkanes in air plasma at atmospheric pressure

Abstract

The positive APCI-mass spectra in air of linear (n-pentane, n-hexane, n-heptane, n-octane), branched [2,4-dimethylpentane, 2,2-dimethylpentane and 2,2,4-trimethylpentane (i-octane)], and cyclic (cyclohexane) alkanes were analyzed at different mixing ratios and temperatures. The effect of air humidity was also investigated. Complex ion chemistry is observed as a result of the interplay of several different reagent ions, including atmospheric ions O 2 ϩ• , NO ϩ , H 3 O ϩ , and their hydrates, but also alkyl fragment ions derived from the alkanes. Some of these reactions are known from previous selected ion/molecule reaction studies; others are so far unreported. The major ion formed from most alkanes (M) is the species [M Ϫ H] ϩ , which is accompanied by M ϩ• only in the case of n-octane. Ionic fragments of C n H 2nϩ1 ϩ composition are also observed, particularly with branched alkanes: the relative abundance of such fragments with respect to that of [M Ϫ H] ϩ decreases with increasing concentration of M, thus suggesting that they react with M via hydride abstraction. The branched C 7 and C 8 alkanes react with NO ϩ to form a C 4 H 10 NO ϩ ion product, which upon collisional activation dissociates via HNO elimination. The structure of t-Bu ϩ (HNO) is proposed for such species, which is reasonably formed from the original NO ϩ (M) ion/molecule complex via hydride transfer and olefin elimination. . For air pollution control, different types of reactors and power supplies are being developed and tested for production of nonthermal plasmas by means of electrical discharges [3]. Following an initial phase devoted to prove the feasibility and competitiveness of the process in terms of energy and cost efficiency, research is presently focusing on the characterization of the chemistry-i.e., the products being released and the underlying chemical mechanisms-to develop a more efficient and cleaner process [5].In nonthermal plasmas, which are conveniently produced by electric corona discharges in air at atmospheric pressure, high-energy electrons induce ionization, excitation, and dissociation of the bulk gas molecules, N 2 and O 2 . The resulting reactive species, ionic and neutral as well as thermalized electrons, can attack molecules of organic pollutants present in the air, such as hydrocarbons and other volatile organic compounds (VOCs), and initiate a chain of reactions leading eventually to their oxidation. It is generally accepted that nonthermal plasma-induced VOC oxidation proceeds via VOC-derived organic radicals, R • , which are trapped by molecular oxygen and undergo further reactions as described for their tropospheric oxidation [6], leading eventually to CO 2 . As for the origin of such VOC-derived radicals, i.e., the nature of the initiation steps of nonthermal plasma-induced VOC decay, reactions of VOC molecules with neutrals and radicals, notably O( 3 P) and • OH, are usually envisioned. However, increasing numbers of literature reports suggest that in some cases ionic initiation steps might prevail [7][8][9...

HPLC/APCI Mass Spectrometry of Saturated and Unsaturated Hydrocarbons by Using Hydrocarbon Solvents as the APCI Reagent and HPLC Mobile Phase

“…[M -H] + was the most abundant ion formed upon APCI of pentane, hexane, and cyclohexane (with nitrogen sheath gas), as expected [31], accompanied by only minor fragmentation. For example, pentane also formed ions of m/z 57 and 43, although their relative abundances are very low.…”

Ambient ionization and direct identification of volatile organic compounds with microwave‐induced plasma mass spectrometry

“…The possibility of the formation of ammonium in microwave plasma has been described in literature . Another ion observed in lower abundance is O 2 H + , formed in endothermic ion/molecule reactions . MIPI belongs to chemical ionization because of a series of reactions of these reagent ions with samples.…”

The unexpected formation of [M − H]+species during MALDI and dopant-free APPI MS analysis of novel antineoplastic curcumin analogues

“…+ ions was significantly reported with alkanes during chemical ionization (CI) [14,15] and APCI-MS. [3] Subsequently, [M À H] + ion formation has been detected with a wide range of compounds with various ionization techniques, such as ESI, [16,17] MALDI, [18][19][20][21][22] APPI, [23] desorption APPI, [24] desorption ESI [24] and direct analysis in real time. [25] Several mechanisms have been proposed to explain the formation of [M À H] + during MS-analysis that could be summarized into three main mechanisms: (1) loss of hydrogen molecule from the protonated [M + H] + ion, (2) hydrogen transfer from the analyte radical cation or (3) hydride abstraction from the neutral analyte molecule [14][15][16][17][18][19][20][21][22][23][26][27][28] (Scheme 4).…”