A selected ion flow tube study of the reactions of H3O+, NO+ and O2+• with seven isomers of hexanol in support of SIFT-MS

A selected ion flow tube, SIFT, study has been carried out of the reactions of H3O+, NO+ and O2+
• with seven structural isomers of hexanol with the common molecular formula C6H14O and molecular weight 102 that are commonly met in food science studies. The main objective is to provide the kinetic data, i.e., the rate constants and the product ion branching ratios to be included in the SIFT-MS kinetics database that would allow the separate identification and quantification of these compounds. The specific compounds involved are the primary alcohols 1-hexanol, CH3(CH2)5OH; 2-ethyl-1-butanol, (C2H5)2CHCH2OH; 4-methyl-1-pentanol, (CH3)2CH(CH2)3OH; secondary alcohols 2-hexanol, CH3(CH2)3CH(OH)CH3; 4-methyl-2-pentanol, (CH3)2CHCH2CH(OH)CH3; 3-hexanol, CH3(CH2)2CH(OH)CH2CH3 and a tertiary alcohol 3-methyl-3-pentanol, (CH3CH2)2C(CH3)OH. The reactions of H3O+ proceed via dissociative proton transfer invariably producing C6H13+ hydrocarbon ions (m/z 85). The reactions of NO+ proceed predominantly via hydride ion transfer producing C6H13O+ ions (m/z 101) for the primary and secondary hexanols, with a minor fragmentation channel for the primary hexanols, when a H2O molecule is lost from the major product ion resulting in C6H11+ ions (m/z 83), and a minor channel representing the process of hydroxide ion transfer producing C6H13+ ions (m/z 85) for secondary alcohols. The latter reaction is also the dominant process for the tertiary alcohol. O2+
• reacts by dissociative charge transfer in all reactions resulting in multiple product ions analogous to electron ionization. The differences in product ion branching ratios and fragmentation patterns can thus assist SIFT-MS identification of the different hexanol isomers in cases when they are not present in complex mixtures.

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► Dissociative proton transfer from H3O+ produces C6H13+ ions only.
► Hydride ion transfer to NO+ produces mostly C6H13O+ ions that associate with H2O with varied rates.
► O2+
• reacts by dissociative charge transfer analogous to electron ionization.
► Rate constants useful for absolute quantification of the total concentration of all hexanol are similar for different isomers.
► Quantification of various isomers of hexanol in the vapours emitted from foods and other products is therefore possible by SIFT-MS.