Characterization of the Autoxidation of Terpenes at Elevated Temperature Using High-Resolution Mass Spectrometry: Formation of Ketohydroperoxides and Highly Oxidized Products from Limonene.

Low-temperature experiments on the oxidation of limonene-O 2 -N 2 mixtures were conducted in a jet-stirred reactor (JSR) over a range of temperatures (520-800 K) under fuel-lean conditions (equivalence ratio φ = 0.5) with a short residence time (1.5 s) and a pressure of 1 bar. Collected samples of the reaction mixtures were analyzed by (i) online Fourier transform infrared spectroscopy (FTIR) and (ii) Orbitrap Q-Exactive high-resolution mass spectrometry after direct injection or chromatographic separation using reversed-phase ultra-high-performance liquid chromatography (RP-UHPLC) and soft ionization (with positive or negative heated electrospray ionization and atmospheric-pressure chemical ionization). H/D exchange using deuterated water (D 2 O) and a reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH) were performed to probe the presence of OH, OOH, and C═O groups in the oxidized products. A broad range of oxidation products ranging from water to highly oxygenated products containing five and more O atoms were detected (C 7 H 10 O 4,5 , C 8 H 12 O 2,4 , C 8 H 14 O 2,4 , C 9 H 12 O, C 9 H 14 O 1,3-5 , C 10 H 12 O 2 , C 10 H 14 O 1-9 , C 10 H 16 O 2-5 , and C 10 H 18 O 6 ). Mass spectrometry analyses were only qualitative, and quantification was performed with FTIR. The results are discussed in terms of reaction routes involving the initial formation of peroxy radicals, H atom transfer, and O 2 addition sequences producing a large set of chemical products, including ketohydroperoxides and more oxygenated products. Carbonyl compounds derived from the Waddington oxidation mechanism on exo - and endo -double bonds (C═C) were observed in addition to their products of further oxidation. Products of the Korcek mechanism (carboxylic acids and carbonyls) were also detected.