On-line detection of benzene and polycyclic aromatic hydrocarbons in flames at different C/O ratio

On-line detection of benzene and polycyclic aromatic hydrocarbons (PAH) has been performed by using a time of flight mass spectrometer (TOF-MS), which has been designed and implemented for obtaining in real time complete mass spectra from molecules of few mass units up to thousands of mass units (corresponding to nanometric dimensions). The flames have been fed with different C/O ratios under and above the soot formation threshold (C/O=0.66) but in any case well above the stoichiometric limit (C/O= 0,33). The combustion source used is a premixed laminar flame. The on-line transfer of combustion gases from atmospheric pressure to the high vacuum of a mass spectrometer has been performed by a combination of a steady sampling flow from the flame with a pulsed expansion into the ionization chamber of TOF-MS (molecular beam). The validation of the technique has been performed in this work by comparing the composition profiles of benzene and PAH measured by TOF-MS with those found in the literature evaluated by means of conventional analytical techniques based on gas chromatography (GC). The good agreement obtained with the literature suggests that the apparatus can be used for the on-line monitor of combustion systems. The high sensibility and fast response of the technique should allow the application also to practical complex sources such as exhausts from engines and from industrial plants for energy production and waste disposal beside to atmospheric aerosols. The mass spectrometric analysis moreover allows to detect and identify through the fragmentation paths more hydrocarbon species with respect to conventional techniques that can be useful for studying kinetic mechanism of pyrolitic reactions.