A Mass Spectrometry Study of Alkanes in Air Plasma at Atmospheric Pressure

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 O2 •, NO, H3O, 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 H], which is accompanied by M• only in the case of n-octane. Ionic fragments of CnH2n1 composition are also observed, particularly with branched alkanes: the relative abundance of such fragments with respect to that of [M H] decreases with increasing concentration of M, thus suggesting that they react with M via hydride abstraction. The branched C7 and C8 alkanes react with NO to form a C4H10NO ion product, which upon collisional activation dissociates via HNO elimination. The structure of t-Bu(HNO) is proposed for such species, which is reasonably formed from the original NO(M) ion/molecule complex via hydride transfer and olefin elimination. Finally, linear alkanes C5–C8 give a product ion corresponding to C4H7 (M), which we suggest is attributed to addition of [M H] to C4H8 olefin formed in the charge-transfer-induced fragmentation of M. The results are relevant to applications of nonthermal plasma processes in the fields of air depuration and combustion enhancement.