Reaction Products and Reactive Species

An important laboratory application of PTR-MS, in relation to elucidating atmospheric chemistry, is the measurement of reaction products that are difficult to observe or quantify by other analytical techniques. This includes products resulting from reactions involving the important tropospheric and highly reactive hydroxyl radical, OH, which significantly affects the lifetime of many VOCs in the atmosphere [10,179,180] and is the most common atmospheric radical involved in oxidation pathways. 

Reactions of OH with isoprene have been investigated by Zhao et al. [179], In this case, PTR-MS was used to detect C4 (C4H7O2 +, mlz 87) and C5 (C5H9O2, miz 101) hydroxy-carbonyls resulting from OH-initiated oxidation of isoprene. From these measurements the hydroxycarbonyl yields (F) were determined according to the equation  

In order to explore possible causes of missing OH reactivity, Kim et al. used the comparative reactivity method to investigate OH reactivity within a branch enclosure for four different tree species (red oak, white pine, beech and red maple) [183], Their results showed that isoprene and monoterpenes, and in one case (red maple) a sesquiterpene (a-famesene, detected using GC-MS), could account for the measured OH reactivity. Thus no unknown biogenic VOCs could have been contributing to the OH reactivity in any significant way, and therefore no noteworthy difference between measured and calculated OH reactivity was observed. This is in contrast to the tropical forest study by Sinha et al. mentioned above [182]. 

An interesting instmmental development has been described by Hanson et al. [184], who demonstrated the ability of PTR-MS to detect peroxy radicals. These radicals were produced in a laminar flow reactor via an initial reaction of atomic chlorine with an alkane to produce an alkyl radical, which was followed by reaction with O2 to generate the peroxy radical. The PTR-MS instrument was operated under extremely low E/Nconditions (18-34 Td), which ensured that H30+(H20) cluster ions (n 0) dominated rather than bare H3O+ reagent ions, because alkanes do not react with protonated water clusters. The detection of methyl and ethyl peroxy radical species was found to be adversely affected by water vapour, whereas that for the cylcohexyl peroxy radical was found to be much less affected. A particularly powerful feature of this work was the ability to map the formation of product distributions in organic reaction systems involving peroxy radicals. 

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