Fragmentation and Reaction Rate Constants of Terpenoids Determined by Proton Transfer Reaction-mass Spectrometry

Abstract

Monoterpenes and oxygenated monoterpenes emitted by plants are involved in producing photochemical oxidants and secondary organic aerosols in the atmosphere. In the present study, the fragment patterns and the reaction rate constants of some of these compounds have been determined by proton transfer reaction-mass spectrometry (PTR-MS). Five monoterpenes (myrcene, camphene, α-phellandrene, α- and γ-terpinene) and two monoterpene alcohols (linalool and cineole) were observed to produce dominant ions of m/z 81 and 137 along with some minor m/z 67 and 95 ions. Myrcene, which is a straight-chained compound, produced an m/z 69 ion along with the above mentioned ions. A monoterpene ketone, thujone, has been found to produce a fragment ion, m/z 93. Since the m/z 69 and 93 ions are the protonated molecular ions of ubiquitous isoprene and toluene, respectively, these respective fragment ions may interfere with the quantification of both isoprene and toluene. We have also revealed that the proton transfer reaction rate is faster in the oxygenated monoterpenes (rate constant: 2.6–3.5×10⁻⁹ cm³ s⁻¹) than in the common monoterpenes (2.2–2.4×10⁻⁹ cm³ s⁻¹). This suggests that the calibration equation obtained from the relationship between the concentration and the produced ion count (m/z 81+137) of selected monoterpenes may underestimate the total concentration of monoterpene alcohols and monoterpenes.