ONLINE MEASUREMENT OF BENZENE AND TOLUENE IN DILUTE VEHICLE EXHAUST BY MASS-SPECTROMETRY

Tandem mass spectrometry (MS/MS), using either an atmospheric pressure ionization (API) or a low-pressure chemical ionization (LPCI) source, has the potential for measuring concentrations of individual hydrocarbon species in real time. In this study, a triple quadrupole mass spectrometer is used to monitor two environmentally important exhaust hydrocarbons species, benzene and toluene. Real-time data for concentrations ranging from 5 ppb to 30 ppm can be measured, with a time resolution of as little as 100 ms/data point. The target compounds are quantitated in dilute vehicle exhaust monitored over the Urban Dynamometer Driving Schedule (UDDS) of the Federal Test Procedure (FTP). Two ionization methods (API and LPCI) are compared. The analytical results obtained with the two ionization methods are also compared to time-integrated off-line analyses by capillary gas chromatography. Of the real-time API- and LPCI/MS measurements, 94% (36 out of 38) fell within 2 standard deviations of the off-line GC/FID analyses. Real-time plots of tailpipe concentration levels of benzene and toluene over the UDDS show much higher concentrations for toluene during the first 90 s of the cold-start test (bag 1) and the hot-start test (bag 3), before the catalyst has reached full-performance temperature (light-off). However, higher concentrations of benzene relative to toluene are consistently observed after catalyst light-off. Considering the high ratio of toluene to benzene in the fuel (7:1), this observation is consistent with recent suggestions that benzene is produced in the catalytic converter by hydrodealkylation of alkylbenzenes. After catalyst light-off, most of the benzene and toluene occurs in spikes corresponding with transitions (accelerations and deceleration) in the vehicle operation.