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 , 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 C n H +1/+2n 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 C4H +7 (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.
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Marotta, E., Paradisi, C. A mass spectrometry study of alkanes in air plasma at atmospheric pressure. J Am Soc Mass Spectrom 20, 697–707 (2009). https://doi.org/10.1016/j.jasms.2008.12.005
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DOI: https://doi.org/10.1016/j.jasms.2008.12.005