PTR-MS fragmentation patterns of gasoline hydrocarbons

Product ion yields for a suite of hydrocarbons associated with motor vehicle exhaust including alkenes, alkanes, cycloalkanes, and aromatic compounds are reported for a PTR-MS instrument operated at an E/N ratio of 80 and 120 Td. At 120 Td many of the compounds tested underwent dissociative proton transfer reactions complicating the interpretation of the vehicle exhaust mass spectrum. However at 80 Td most aromatic and alkene compounds tested yielded M + H productions at near 100% yield and alkyl substituted cyclohexanes and C-9-C-12 branched alkanes yielded M-H ions as the major product ion. The PTR-MS response factors to dimethyl and trimethyl cyclohexane compounds were about a third of that expected if they reacted at the collisional rate limit with H3O+. Analysis of gasoline at 80 Td showed that the major peaks in the mass spectrum could be reasonably accounted for from known hydrocarbon abundance and their product ion yields, including the alkyl substituted cyclohexane compounds which yielded M-H ions at m/z 97, 111, and 125. Operation at lower E/N ratios would enable the PTR-MS to measure alkyl substituted cyclohexanes and larger alkane compounds in urban air at their M-H product ion. (C) 2015 Elsevier B.V. All rights reserved.