Contribution to the development of selective chemical ionization mass spectrometric (CI-MS) techniques for the detection of biogenic volatile organic compounds (BVOCs)
Tandem mass spectrometry
Biogenic volatile organic compounds
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Non-methane biogenic volatile organic compounds (BVOCs) emitted from vegetation constitute about sixty percent of all volatile organic compounds emitted and represent a total worldwide emission of around 1.15 Pg C per year. Plants emit a large variety of BVOCs, dependent on characteristics of both the plant itself and its surroundings. Emitted BVOCs can rapidly react with the main atmospheric oxidants (OH●, O3 and NO3●) and, in the presence of NOX from anthropogenic origin, BVOC oxidation is known to result in harmful oxidation products, such as ozone (O3) and secondary organic aerosol (SOA). In this regard, BVOCs can be classified according to their reactivity in less reactive BVOCs, such as methanol, acetone and acetaldehyde and highly reactive BVOCs, such as isoprene (C5H8), monoterpenes (C10H16, MTs) and sesquiterpenes (C15H24, SQTs). Especially the latter are important SOA precursors and have therefore become a hot topic in the atmospheric science community in recent years. Fast and sensitive measuring techniques that are able to selectively measure BVOCs are required to determine the importance of each individual BVOC in ecological and atmospheric processes. Gas chromatography mass spectrometric (GC-MS) and chemical ionization mass spectrometric (CI-MS) techniques are most frequently used for BVOC measurements and have complementary strengths and weaknesses. GC-MS has an unparalleled selectivity, but has a relatively low temporal resolution and can only be used discontinuously, while CI-MS is an on-line technique with a high temporal resolution, but is less selective. CI-MS techniques use soft ionization of neutral BVOC molecules in the gas phase through fast exothermic ion/molecule reactions with a reagent ion that does not react with the major constituents of air. Except for its reaction with ammonia and water vapor, the hydronium ion (H3O+) fulfills these criteria and typically reacts by non-dissociative proton transfer with many BVOCs (M), resulting in the corresponding characteristic MH+ product ion. Therefore, it is commonly used as reagent ion in the commercially available proton transfer reaction mass spectrometer (PTR-MS) and selected ion flow tube mass spectrometer (SIFTMS) instruments. After ionization, unreacted reagent ions and BVOC product ions are separated according to their mass-to-charge (m/z) ratio and subsequently detected. The unambiguous detection of BVOCs with CI-MS techniques using a certain reagent ion is therefore only possible when the individual ion/BVOC reactions involved result in product ions at different m/z values. Even when the reactions of BVOCs result in product ions at the same m/z value, as is often the case for isomers, more selective detection of BVOCs can be realized by using tandem mass spectrometric (TMS) instrumentation provided the controlled fragmentation of BVOC product ions showing the same m/z result in significantly different fragmentation patterns. The focus of the research presented in this work concerns the detection of isomeric unsaturated alcohols (C5H9OH and C6H11OH), monoterpenes (C10H16), linalool (C10H17OH) and sesquiterpenes (C15H24) using CI-MS techniques through fundamental studies on ion/BVOC reactions in a SIFT instrument and collision-induced dissociation (CID) of specific reagent ion/BVOC product ions in a tandem mass spectrometric (TMS) instrument.