Monoterpene chemical speciation with high time resolution using a fastGC/PTR-MS: Results from the COV3ER experiment on Quercus ilex

Abstract

Monoterpenes (MTs) represent an important family of biogenic volatile organic compounds (BVOCs) in terms of amount and chemical diversity. This family has been extensively studied using gas chromatography (GC) and proton transfer reaction-mass spectrometry (PTR-MS). Upon recent advances with Fast Gas Chromatography (FastGC), it was also commercialized with proton transfer reaction-time of flight-mass spectrometry (PTR-ToF-MS) instruments. The combination of both techniques showed promising results in the near real-time separation of isomers, with the need of further improvements. In this study, a FastGC prototype was coupled to a conventional PTR-MS (PTR-QuadMS). Extensive laboratory experiments were performed, in order to test the system's performance and to optimize its operational parameters for MT separation. The detection limit was determined to be around 0.8-1.7 ppbv, depending on the MT. The system was afterwards deployed during a three-week field campaign in a mixed holm oak (Quercus ilex) forest known for its important MT emissions. MTs were measured in the incoming and the outgoing air of dynamic enclosures installed on the branches of four different trees. Three chemotypes of holm oak trees could be distinguished showing consistently different proportions of the emitted MTs throughout the measurement campaign: pinene-type, myrcene-type and limonene-type. Measurements showed a systematic diel variation in emissions typical of light and temperature-dependent, de novo-synthesized VOCs. The results demonstrated the feasibility of the FastGC/PTR-MS system for continuous measurements from dynamic chambers in the field, whereas further improvements would be necessary to lower the detection limit for ambient air measurements.