Abstract
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: α-pinene, δ-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. δ-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.
Highlights
Proton-transfer-reaction mass spectrometers (PTR-MSs) are commonly used for real-time monitoring of volatile organic compounds (VOCs) in air, including highly reactive monoterpenes (C10H16) and sesquiterpenes (C15H24) emitted from forests [1,2,3,4,5,6,7]
The PTR-MS response to VOCs depends on different factors, including how effectively the instrument can collect and detect measured VOC, and on the extent to which the reactions between H3O+ ions and VOCs are occurring in the drift tube
The PTR-MS is commonly operated at high E/N values to minimize clustering of the reagent and product ions with water, but it promotes increased fragmentation of many trace VOCs due to the increased number of energetic collisions occurring inside the drift tube
Summary
Proton-transfer-reaction mass spectrometers (PTR-MSs) are commonly used for real-time monitoring of volatile organic compounds (VOCs) in air, including highly reactive monoterpenes (C10H16) and sesquiterpenes (C15H24) emitted from forests [1,2,3,4,5,6,7]. Other potential sources of error in Eq (1) include the uncertainty in the transit time of ions through the drift tube and variation in ion transmissions as a function of m/z This variation must be determined before every measurement campaign, which is typically done using a gas cylinder containing several compounds with varying masses that do not fragment inside the PTR-MS, for example different aromatic compounds. Due to the main disadvantages (cost, time, standard preparation) of existing PTR-MS calibration systems, we used a dynamic dilution system—a cost-effective and simple option to calibrate the PTR-MS, and to determine product ion distributions for the studied terpenoid compounds. We generated gas-phase standards for twelve different terpenoid compounds with the dynamic dilution system to quantify product ion distributions for two different reduced electric field (E/N) settings. These results provide useful information to PTR-ToF-MS users that will improve analytical quantification of BVOC measurements
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