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
3O
+, 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
4H
10NO
+ 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
4H
7+(M), which we suggest is attributed to addition of [M − H]
+ to C
4H
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.
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