Thermophoretic sampling of large PAH (C >= 22-24) formed in flames

The study of the transition from molecular to particle-like species involved in soot inception needs experimental strategies aiming to measure high molecular mass structures that could be important also for their effect on environment and human health. In this work, we have investigated the carbon particulate selectivity involved in the thermophoretic deposition used for the first time as a direct and fast sampling of high molecular weight aromatic species formed at the inception stage in an ethylene heavily sooting flame. Gravimetric analysis along with mass spectrometry, UV-Visible and fluorescence spectroscopy showed that the dichloromethane-extract of carbon particulate matter thermophoretically deposited on the substrate was much less than that sampled by a conventional water-cooled probe as it is composed only of high molecular weight condensed phases (C >= 22-24). Thermophoretic deposition is demonstrated to pre-separate, already at the sampling step, soot precursors of high molecular weight allowing their further analysis without the interference of the more abundant light polycyclic aromatic compounds (C < 22-24). These high molecular weight aromatic species presented interesting optical properties in terms of size and fluorescence emission peaked in the blue-region that could be of interest for carbon dots application. Size exclusion chromatography of particulate coupled with UV-Visible spectroscopy showed the presence of some aggregates already at the beginning of soot formation to be taken into account as ruling the optical properties of the whole carbon particulate matter.