Analyzing the solid soot particulates formed in a fuel-rich flame by solvent-free MALDI FT ICR mass spectrometry.

Abstract

RATIONALE:The compositional and structural information of the soot particles is essential for a better understanding of the chemistry and mechanism during the combustion. The aim of the present study was to develop a method to analyze such soot particulate samples with high complexity and poor solubility. METHODS:The solvent-free sample preparation MALDI technique was combined with the ultrahigh resolution FT ICR mass spectrometry for the characterization of solid soot particulates. Moreover, a modified iso-abundance plot (Carbon Number vs. Hydrogen Number vs. Abundance) was introduced to visualize the distributions of various chemical species, and to examine the agreement between the hydrogen- abstraction- carbon- addition (HACA) mechanism and the polycyclic aromatic hydrocarbon growth in the investigated flame system. RESULTS:This solvent-free MALDI method enabled the effective ionization of the solid soot particulates without any dissolving procedure. With the accurate m/z ratios from FT ICR MS, a unique chemical formula was assigned to each of the recorded mass signals. The combustion products were proven to be mainly large polycyclic aromatic hydrocarbons, together with a small amount (<5%) of oxidized hydrocarbons. CONCLUSIONS:The developed method provides a new approach for the molecular characterization of soot particulates like carbonaceous materials. The investigated soot particulates are mainly polycyclic aromatic hydrocarbons (PAHs) with no or very short aliphatic chains. The PAHs growth mechanism during combustion can be examined against the classic HACA mechanism.