Oligomeric carbon and siloxane series observed by MALDI mass spectrometry during the analysis of soot formed in fuel rich flames

Oligomeric carbon and siloxane series have been observed by matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS), during the analysis of the dichloromethane (DCM)-soluble fractions of condensable material recovered from fuel-rich flames. Laser desorption (LD) spectra showed a pattern of oligomeric dimethyl-siloxane structures with a spacing of 74 u. The siloxane series appears to have originated as contamination of samples by silicone oil used to lubricate connections of polymer tubing. This was confirmed by extracting silicone tubing and silicone grease with DCM followed by MALDI-MS analysis. A series of peaks with a mass spacing of 24 u was also observed, superimposed on the continuum of unresolved organic ions. This oligomeric series appears to correspond to polycyclic aromatics separated by (mainly) ethylene bridges. Thus LDMS appears to have revealed a series of soot precursors, intermediate between polycyclic aromatics and particulate soot, which was not detected by MALDI-MS. More detailed work is necessary to define these species with precision.