PTR-ToF-MS product ion distributions and humidity-dependence of biogenic volatile organic compounds

Quantitative proton-transfer-reaction mass spectrometer (PTR-MS) measurements of ambient volatile organic compounds (VOCs) require proper calibration procedures. In particular, compound product ion distribution and humidity-dependent responses must be characterized. In this study, we generated twelve gas-phase terpenoid standards using a dynamic dilution system to calibrate the PTR-MS with time of-flight mass spectrometer (PTR-ToF-MS): six monoterpenes, two monoterpene derivatives, and four sesquiterpenes. The humidity-dependent response was characterized for three terpenoid compounds to compare different molecular structures: alpha-pinene, delta-limonene, and longifolene. We provide the first comprehensive summary of PTR-ToF-MS product ion distributions for twelve common biogenic volatile organic compounds using two different reduced electric field (E/N) values, 80 Td and 130 Td. Results demonstrated that neglecting to correct for individual product ion distributions of different terpenoid isomers can result in an error of up to 26% for reported mixing ratios. delta-Limonene and longifolene exhibited a small humidity-dependent response in the PTR-ToF-MS, but this did not contribute significantly to the overall measurement error. These results will improve quantification of commonly-measured biogenic volatile organic compound emissions and chemistry in the atmosphere. (C) 2018 The Authors. Published by Elsevier B.V.