Laser ionization time-of-flight mass spectrometry combined with residual gas analysis for the investigation of moderate temperature benzene oxidation

The simultaneous use of 10.5-eV vacuum ultraviolet photo ionization time-of-flight mass spectrometry and residual gas analysis using standard electron ionization for the study of the oxidation of a typical aromatic hydrocarbon fuel is shown to provide reliable estimates of species concentration, including radical reaction intermediates. The advantages and limitations of the combined techniques are discussed, with special emphasis on the factors bringing about uncertainties in the concentration measurements and spurious signals arising in quantitative photo ionization time-of-fight mass spectrometry. These factors are shown to be minimized and controllable under appropriate experimental conditions. The results for oxidation of benzene in an argon diluent at 800–1200 K and 30 ms mean residence time provide simultaneous semi-quantitative measurements of the broad spectrum of both stable and labile product masses. Especially interesting features include observation of biphenyl and C5H5 concentrations at low benzene conversions. The data are also consistent with the previously proposed mechanism for phenoxy radical decomposition to CO and cyclopentadienyl radical. Although alternative pathways cannot be ruled out without further structural identification, especially of the C5H5 species, these data provide an opportunity for checks of detailed models for these reaction conditions.