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 O(2)(+*), NO(+), H(3)O(+), 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 C(n)H(2n+1)(+) 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 C(7) and C(8) alkanes react with NO(+) to form a C(4)H(10)NO(+) 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 C(5)-C(8) give a product ion corresponding to C(4)H(7)(+)(M), which we suggest is attributed to addition of [M - H](+) to C(4)H(8) 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.