Speciation of monoterpenes at atmospheric relevant concentrations without sample preconcentration

Volatile organic compounds (VOC) are exchanged between the Earth’s surface and the atmosphere. Their role in biosphere-atmosphere-climate interactions have been extensively studied in various and often inherently interdisciplinary research topics. Crucial to the understanding of these processes is the fast quantification of VOCs both primarily emitted as well as products and intermediates. Monoterpenes are an important class of VOCs contributing 15% to the global VOC emissions from terrestrial ecosystems. The most dominant isomeres of monoterpenes in the atmosphere are α-pinene, β-pinene, limonene, myrcene, camphene, and sabinene that are emitted in variable fractions depending on sources and conditions. In the atmosphere, monoterpenes may undergo rapid conversion via reactions with hydroxyl- and nitrate-radicals and ozone, forming oxidation products that eventually condense to secondary organic aerosol at high yields. Reaction rates of the different isomers with OH, O₃ and NO₃ spread up to orders of magnitude (0.8, 4.4 and 2.3 orders, respectively). Therefore it is of uttermost importance to quantify individual isomers to understand the reactivity of the mix of monoterpenes in an air mass.    

Proton-transfer-reaction mass-spectrometry (PTR-MS) is a well characterized analytical method for the real-time quantification of VOCs including monoterpenes. The drawback of PTR-MS is the detection of VOCs on a chemical composition level, hence, only the sum of monoterpenes can be measured. To solve this problem, we herein introduce an optimized fast gas-chromatographic pre-separation solution (fast-GC) that is seamlessly integrated to an ultrasensitive FUSION PTR-TOF 10 (IONICON Analytik, Austria). 

The improved fast-GC design allows for fast and precise heating rates leading to highest stability and repeatability of GC-runs. A temperature ramp can be completed as quick as 90 s providing  resolutions sufficient for a good separation of several monoterpene isomeres. A fast-GC run can be triggered approximately every 5-10 min; the time in between runs is used to measure the ambient sample without GC separation in real-time PTR-MS mode. With the high sensitivity (> 40 000 cps/ppbV) and lowermost limit of detection (< 1 pptV in 1 s) of FUSION PTR-TOF 10, no preconcentration prior to injection of the sample into the fast-GC is required.

In this presentation we show a thorough characterization of this fast-GC FUSION PTR-TOF 10. To demonstrate the capabilities of the system in a real-world application we sampled ambient air in Innsbruck, Austria, with fast-GC runs triggered every 10 minutes continuously over the course of several weeks. Despite the low ambient concentrations of the sum of monoterpenes in sub-ppbV levels, the presented method was able to separate and quantify 7 monoterpenes.