Volume 39, Issue 4 (July 1994)
Soot as an Indicator in Fire Investigations: Physical and Chemical Analyses
The possibility of determining the combustion products (or accelerants) at the seat of a fire by the analysis of corresponding soot samples was investigated. Twenty liquid fuels (principally petroleum derivatives) and twelve plastic materials (from seven different polymer groups) were individually burned over one hour under controlled laboratory conditions. The soot produced was collected on glass plates and subsequently submitted to a sequence of physical and chemical analyses. Twelve casework samples (soot deposits on glass fragments collected at the fire scene) and five control samples (blind trials prepared in the laboratory) were submitted to the same analytical procedure. A total of 49 soot samples were considered.
Macroscopic (35× magnification) and microscopic (TEM) studies were conducted on each soot sample. Digitized micrographs were processed in order to obtain certain physical parameters serving to characterize the size and form of the soot aggregates: perimeter, surface area, circularity and principal surface moments ratio. These data were transformed and used as variables for a discriminant analysis carried out with an SPSS program. Furthermore, the soot aggregates were characterized by their fractal dimension.
The chemical composition of the soot samples was explored using three chromatographic methods: GC-FID, GC-MS, and pyrolysis-GC. Two studies were conducted: a comparison of the total chromatographic profiles obtained by GC-FID and pyrolysis-GC, and a comparison based upon qualitative and semi-quantitative analyses of 11 polycyclic aromatic hydrocarbons (PAH's) in order to determine the value of these compounds as potential markers for accelerants used at the start of a fire.
The combination of physical and chemical parameters permitted the differentiation of most of the laboratory-prepared soot samples. The discriminating power was higher for the chemical analyses, with soot samples resulting from the combustion of plastic materials being the easiest to identify. Microscopy nevertheless provided interesting information concerning specific soot forms or elements. The combined results obtained by the analytical methods employed permitted the construction of a dichotomic table that can be used for the classification of soot samples taken from the scene of a fire. Additional research is required before such techniques can be routinely applied in casework.