Detection of Large Aromatics in Flames by REMPI-MS

Abstract

A new experimental technique for detecting large polycyclic aromatic hydrocarbons (PAH) is described. This technique complements presently employed methods that derive information about large PAH in the mass range between 500 and 1500 u from studying PAH ions. First experiments have been carried out in flat premixed laminar flames at low pressure. The experimental technique comprises sampling of the flames by means of a differentially pumped molecular beam sampling system. The PAH present in the molecular beam are ionized in a resonant multiphoton ionization process (REMPI) at 259 nm. The PAH ions are analyzed by a time-of-flight mass spectrometer. REMPI time-of-flight mass spectrometry offers several advantages: it is very specific, i.e. only distinct groups of molecules are ionized at a fixed wavelength. Fragmentation can be almost completely avoided. In the first experiments which are presented in the paper PAH up to C19H12 could be observed. The experimental technique offers the possibility of studying large aromatic hydrocarbons that are likely soot precursors and offers thereby some more insight into the mechanism of soot formation.