Mass spectrometric monitoring of oxidation of aliphatic C6–C8 hydrocarbons and ethanol in low pressure oxygen and air plasmas

Experimental and theoretical studies on the oxidation of saturated hydrocarbons ( n ‐hexane, cyclohexane, n ‐heptane, n ‐octane and isooctane) and ethanol in 28 Torr O 2 or air plasma generated by a hollow cathode discharge ion source were made. Ions corresponding to [M + 15] + and [M + 13] + in addition to [M − H] + and [M − 3H] + were detected as major ions where M is the sample molecule. The ions [M + 15] + and [M + 13] + were assigned as oxidation products, [M − H + O] + and [M − 3H + O] + , respectively. By the tandem mass spectrometry analysis of [M − H + O] + and [M − 3H + O] + , H 2 O, olefins (and/or cycloalkanes) and oxygen‐containing compounds were eliminated from these ions. Ozone as one of the terminal products in the O 2 plasma was postulated as the oxidizing reagent. As an example, the reactions of C 6 H 14 +• with O 2 and of C 6 H 13 + (CH 3 CH 2 CH + CH 2 CH 2 CH 3 ) with ozone were examined by density functional theory calculations. Nucleophilic interaction of ozone with C 6 H 13 + leads to the formation of protonated ketone, CH 3 CH 2 C(=OH + )CH 2 CH 2 CH 3 . In air plasma, [M − H + O] + became predominant over carbocations, [M − H] + and [M − 3H] + . For ethanol, the protonated acetic acid CH 3 C(OH) 2 + ( m / z 61.03) was formed as the oxidation product. The peaks at m / z 75.04 and 75.08 are assigned as protonated ethyl formate and protonated diethyl ether, respectively, and that at m / z 89.06 as protonated ethyl acetate. Copyright © 2016 John Wiley & Sons, Ltd.

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