Molecular species adsorbed on soot particles issued from low sooting methane and acetylene laminar flames: A laser-based experiment

Recent advances in the field of laser desorption/laser ionization mass spectrometry (LD/LI/MS) have renewed interest in these separation methods for fast analysis of chemical species adsorbed on soot particles. These techniques provide mass-separation of the desorbed phase with high selectivity and sensitivity and require very small soot samples. Combining LD/LI/MS with in situ measurements of soot and gaseous species is very promising for a better understanding of the early stage of soot growth in flames. In this work, three lightly sooting laminar jet flames (a methane diffusion flame and two premixed acetylene flames of equivalence ratio (ϕ) = 2.9 and 3.5) were investigated by combining prompt and 50 ns-delayed laser-induced incandescence (LII) for spatially resolved measurements of soot volume fraction (fv) and laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAH). Soot and PAH calibration is performed by two-colour cavity ring-down spectroscopy (CRDS) at 1064 and 532 nm. Soot particles were sampled in the flames and analysed by LD/LI/Time-of-flight- MS. Soot samples are cooled to −170 °C to avoid adsorbed phase sublimation (under high vacuum in the TOF-MS). Our set-up is novel because of its ability to measure very low concentration of soot and PAH together with the ability to identify a large mass range of PAHs adsorbed on soot, especially volatile two-rings and three-rings PAHs. Studied flames exhibited a peak fv ranging from 15 ppb (acetylene, ϕ = 2.9) to 470 ppb (acetylene, ϕ = 3.5). Different mass spectra were found in the three flames, each exhibiting one predominant PAH mass; 202 amu (4-rings) in methane, 178 amu (3-rings) in acetylene,ϕ = 2.9 and 128 amu (2-rings) in acetylene, ϕ = 3.5. These variations with flame condition contrasts with other recent studies and is discussed. The other PAH masses ranged from 102 (C8H6) to 424 amu (C34H16) and are well predicted by the stabilomer grid of Stein and Farr.