Spectroscopic and compositional signatures of pah-loaded mixtures in the soot inception region of a premixed ethylene flame

The determination of unsubstituted polycyclic aromatic hydrocarbons (PAH) in the 200–300 u mass molecular range has usually failed to account for the total mass of condensed species in rich flames, with the inference that higher molecular weight species are present. In this paper, the feasibility and reliability of various techniques, including chromatographic and spectroscopic methods, were assessed. In particular, UV-VIS and fluorescence spectra of the PAH-loaded mixtures, collected from a rich premixed ethylene flame (C/O=0.8), are reported and used to fingerprint their overall aromatic character at the beginning of the soot formation region. High-pressure liquid chromatography was used to isolate the aromatic fraction from the other high-molecular-weight species. The aromatic fraction was found to be about 70% by the mass of the total condensed species. Both chromatographically separated fractions were subjected to spectroscopic analysis. In addition, the hydrogen content of these fractions was evaluated by Fourier transform-infrared (FT-IR) analysis, calibrated in such a way that aliphatic and aromatic hydrogen content could be evaluated separately, and their trends followed in the soot inception region. UV-VIS and fluorescence spectroscopy, along with the hydrogen content of the fractions, showed significant differences on passing through the region in which the first soot particles appear. Speculations have been made on their nature and relationship with soot inception.