Size exclusion chromatography of particulate produced in fuel-rich combustion of different fuels

Carbonaceous particulates formed in the combustion of gaseous and liquid fuels were collected from a premixed laminar flame and a spray flame operating under fuel-rich combustion conditions. Particulate collected in the flame was extracted with dichloromethane (DCM) to separate condensed species (CS) soluble in DCM from solid carbonaceous material (soot), insoluble in DCM. These samples were examined by size exclusion chromatography (SEC) using N-methyl-pyrrolidone (NMP) as eluent, with detection by light scattering and UV-visible absorbance. Size exclusion chromatography of soot and related materials has provided a diagnostic tool for a more thorough characterization of these materials than has been hitherto available. The data show that there are step-changes in structures from the small near-planar PAH molecules, to large components with elution times corresponding to polystyrenes of mass above 200 000 u. The SEC analysis of soot, carried out on Mixed-A column with light scattering detection, showed an intense peak corresponding to molecular masses much larger than the highest mass polystyrene standard (15 x 10(6) u). The DCM-soluble fraction of the ethylene soot gave two peaks, the former corresponding to a molecular mass of the order of 1 X 10(6) u, whereas the second peak corresponded to masses ranging from several hundreds to about 200 u on the basis of polystyrene calibration; part of the material in this range was found to be polymiclear aromatic hydrocarbons (PAH) (128-300 u) as detected by gas chromatography-mass spectrometry. Using a column with a better resolution regarding the smaller molecules (Mixed-D) and connected to an UV-visible absorption detector, the molecular weights of soot and CS were found to be in good agreement with those obtained by using the previous SEC system. The molecular weight ranges of soot and CS were found independent of both the combustion system and fuel used even though some differences could be observed in the relative proportions of the different molecular masses.