Detection of electrically charged soot particles in laminar premixed flames

A particle mass spectrometer has been used to investigate the formation of electrically charged species and soot particles in laminar premixed flames. The mass range was from 600–6 × 105 amu and extends from high molecular hydrocarbons to soot particles of 10 nm diameter. The flames were stabilized on cooled porous plate burners. Acetylene/oxygen flames were investigated at low pressure (30 mbar), and ethylene/air flames were investigated at atmospheric pressure. Soot particles could only be detected in flames showing yellow luminosity, i.e. above the critical C/O-ratio for soot formation. Both positively and negatively charged particles were found, the positive charge dominating in the low pressure acetylene/oxygen flames, the negative charge dominating in the atmospheric ethylene/air flames. With the assumption of spherical shape and constant density, the mass spectra were converted to size spectra. Usually, they show a multiple peak structure which is somewhat difficult to interpret. There are indications that particles may carry multiple (1–2) charges, and also that particles of different types may coexist beside polyaromatic and polyhedral species in the early stage of particle inception.