Laser desorption–ionization time-of-flight mass spectrometry for analyses of heavy hydrocarbons adsorbed on soot formed behind reflected shock waves

Abstract

During the last decade, the laser desorption/ionization time-of-flight mass spectrometry (LDIToFMS) development has provided the opportunity to improve the understanding of the soot formation process. Combining LDIToFMS and shock-tube allows the study of adsorbed compounds on aged soot formed at constant and well defined temperature. The present work investigates effects of fuel structure (with 2-methyl-2-butene (2M2B)), fuel molecular composition (with thiophene, C4H4S) and the presence of oxygen (with toluene in pyrolysis and at an equivalence ratio of 18) on the composition of the adsorbed phase. The evolution, with the temperature behind reflected shock waves, of the soot induction delay time and the soot yield was also investigated from highly diluted mixtures in argon. LDIToFMS results show that PAHs were detected on soot. The mass range of PAHs is dependant on the fuel and on the temperature. The amount of species adsorbed on soot surface was found to be inversely proportional to the soot yield. A dominant sequence of benzenoid PAHs, with a 24-amu increment between peaks, is observed whatever the initial fuel or condition (temperature, presence of oxygen). A second sequence of five membered ring PAHs was also observed with a 12-amu increment between peaks of the benzenoid sequence. The signal intensity of this five membered ring PAHs sequence depends strongly on the temperature, the presence of oxygen and the structure the fuel. The presence of a sulphur atom in the structure of the initial fuel leads to the detection of small amounts of compounds adsorbed on soot which contain sulphur atom(s) and with a PAH-like structure.