Identification of large polycyclic aromatic hydrocarbons in carbon particulates formed in a fuel-rich premixed ethylene flame

Abstract

Large polycyclic aromatic hydrocarbons were identified in carbon particulate sampled in a fuel-rich premixed ethylene flame. The particulate was extracted with dichloromethane (DCM) in order to separate the soluble organic species (DCM-extract) from the solid carbon (soot). After DCM extraction soot was re-extracted with N-methyl pyrrolidone (NMP) obtaining the NMP-extract. Both the DCM-extract and NMP-extract were further fractionated by size exclusion chromatography in selected molecular weight (MW) ranges. Large polycyclic aromatic hydrocarbons obtained by regular incorporation of C 2 and/or C 2H 2 unit (24/26 rule) occurring in both odd and even series of carbon atom number of polycyclic aromatic hydrocarbons, were identified by laser desorption ionization–mass spectrometry (LDI–MS) analysis of the lighter MW fractions of both the DCM-extract and NMP-extract (100–400 u MW of the DCM-extract and 200–600 u fractions of the NMP-extract). The LDI–MS spectra of the heaviest MW fractions of DCM-extract and NMP-extract (600–2000 u and 600–5000 u fraction) showed a continuous spectrum of masses typical of polymeric structures. The UV–visible absorption and emission spectral analysis corroborated the assignment of lighter and of heavier fractions of DCM-extract and NMP-extract to PAH and to polymeric aromatic structures, respectively.