Selected ion flow tube (SIFT) studies of the reactions of H3O+, NO+ and O2+ with six volatile phytogenic esters

The selected ion flow tube (SIFT) was used to study the reactions of the three SIFT-MS precursor ions H3O+, NO+ and O2+ with six phytogenic esters: hexyl acetate, phenethyl acetate, benzyl acetate, methyl salicylate, methyl benzoate and benzyl benzoate. These compounds are emitted into the atmosphere by various species of plants and play a role in communication between individual plants and also amongst different species. Thus, it is necessary to know the rate constants and branching ratios of the different ion products of these reactions for identification and quantification by SIFT-MS. The results of this study show that the reactions of H3O+ with the esters proceed via proton transfer, which is non-dissociative for methyl salicylate, entirely dissociative for benzyl benzoate and partly dissociative for the remaining four esters. All protonated esters readily associate with water molecules with the notable exception of methyl salicylate. All six NO+ reactions result in formation of adduct ions in parallel with charge transfer or with reactions leading to fragment ions. All six O2+ reactions proceed by charge transfer with the production of one or two major fragment ions; the parent molecular radical cation is formed in three reactions only. Kinetic library entries allowing unambiguous quantification of the six esters by SIFT-MS are presented together with their experimental validation.

Phytogenic esters: hexyl acetate, phenethyl acetate, benzyl acetate, methyl salicylate, methyl benzoate and benzyl benzoate emitted by plants can now be quantified by SIFT-MS.Figure optionsDownload high-quality image (150 K)Download as PowerPoint slideResearch highlights▶ Protonated hexyl acetate, phenethyl acetate and benzyl acetate efficiently associate with H2O. ▶ Protonated methyl salicylate does not associate with H2O. ▶ Benzyl benzoate reacts with H3O+ by incorporating the O atom into the product ion. ▶ New kinetic library allows simultaneous SIFT-MS quantification of the six esters released by plants.