PTR-MS analysis of reference and plant-emitted volatile organic compounds

Abstract

Proton transfer reaction-mass spectrometry (PTR-MS) was applied to the analysis of a series of volatile organic compounds (VOCs) that emit from various plants. These include a group of alcohols (methanol, ethanol and butanol), carbonyl-containing compounds (acetic acid, acetone and benzaldehyde), isoprene, acetonitrile, tetrahydrofuran (THF), pyrazine, toluene and xylene and a series of terpenes ( p-cymene, camphene, 2-carene, limonene, β-myrcene, α-pinene, β-pinene, γ-tepinene and terpinolene) and oxygen-containing terpenes (1,8-cineole and linalool). These mass spectral data were compared to an electron ionization (EI) database identifying that not all PTR-MS fragments were common to EI. PTR-MS studies of these reference compounds were utilized to identify VOCs emitted from Eucalyptus grandis leaf at a temperature range of 30–100 °C. In addition to protonated molecules (M + H) +, abundant proton-bound dimers or trimers were detected for alcohols, acetone, acetonitrile and THF. Abundant fragment ions attributed to the loss of water from these proton-bound clusters were also observed. The stability of butyl (C 4H 9 + m/ z 57) and acetyl (CH 3CO + m/ z 43) fragment ions directed the proton-transfer reactions of butanol and acetic acid. Abundant (M + H) + ions were detected for pyrazine, THF, toluene and xylene, as well as for all terpenes except those containing oxygen. For linalool and 1,8-cineole, the loss of water generated an abundant fragment ion at m/ z 137. PTR-MS fragmentation patterns for terpenes were proposed for m/ z 81 (C 6H 9 +), 93 (C 7H 9 +), 95 (C 7H 11 +), 107 (C 8H 11 +), 109 (C 8H 13 +), 119 (C 9H 11 +), 121 (C 9H 13 +) and 137 (loss of water for oxygen-containing terpenes; C 10H 17 +). The relative abundances of (M + H) + and fragments for all terpenes (except linalool) were dependent on the drift tube voltage and the optimum voltage for detection of molecular ions was different for various terpenes.