Presentation on theme: "Arson. The Common Accelerants The most commonly used accelerants are petrol, kerosene, mineral turpentine and diesel. These accelerants are generally."— Presentation transcript:
The Common Accelerants The most commonly used accelerants are petrol, kerosene, mineral turpentine and diesel. These accelerants are generally complex mixtures of hydrocarbon molecules. These hydrocarbons have similar chemical properties, however their boiling points vary and cover a wide range of values. This variation causes the accelerants to alter their composition during the evaporation process. The more volatile hydrocarbons evaporate at a faster rate leaving the heavier hydrocarbons in the debris and after a period of time the accelerant becomes less volatile and less abundant.
Physical Indicators Physical indicators used to detect the presence of accelerants or localized burn patterns to floors and surfaces and overhead damage inconsistent with the naturally available fuel. Reports from fire fighters or eyewitnesses of a rapid fire or of suspicious odors can also indicate the presence of an accelerant.
These physical indicators if initially present, can often be destroyed during the course of the fire. If the roof or ceiling has collapsed, then evidence such as localized burn patterns on the floor can be concealed.
The investigator should excavate the debris around doorways or in the center of open spaces as these are areas where accelerants are normally used. If a wooden floor is involved, the investigator can hit the floor with a shovel and excavate the areas where the floor appears weakened.
Physical evidence which indicates a hot and intense fire such as a color change or spalling in concrete, melted aluminum and deformation of steel are unreliable indicators of the presence of an accelerant, as the temperature reached during the course of a fire is governed by the amount of both fuel and oxygen available.
Many combustible materials tend to burn with the same intensity as accelerants, given an appropriate supply of oxygen.
Use of one's sense of smell When one smells fire debris, they are actually sampling the headspace above the debris and noting the chemical fingerprint of the headspace. Then using one's discriminatory powers by comparing the fingerprint with those stored in one's memory, a decision can be made as to the possible presence of an accelerant.
The vapors found at fire scenes may be harmful so debris should only be smelt when necessary. Continually smelling these toxic vapors will cause the smelling senses to become less effective.
On cold still mornings when the sense of smell is quite sharp, accelerant odors can sometimes be smelt while the investigator is making his initial inspection of the fire scene. As the debris becomes disturbed during the course of the investigation the sense of smell becomes less effective due to the contamination of the atmosphere.
Sniffers Sniffers (or portable gas detectors) are best employed when toxic dust or vapors are present or if the investigator's sense of smell is impaired. They do not have the same discriminatory powers as the sense of smell as they respond to a wide variety of compounds in the headspace including non accelerant vapors.
The cheapest type of sniffer uses a detector which measures changes in the oxygen concentration. These instruments lack specificity as they respond to all types of hydrocarbons and also gases such as ammonia, alcohols, carbon monoxide, carbon dioxide and even water vapor.
A more expensive sniffer employs a detector such as a flame ionization (FID) or photo ionization (PID) which will respond to hydrocarbons but not inorganic vapors. The instrument is extremely sensitive but cannot discriminate between hydrocarbons originating from accelerants or those from burnt plastics.
Because of their high sensitivity the investigator could easily misinterpret the results and could for example believe he is following an accelerant trail when in fact the investigator is simply following a trail where a synthetic carpet has become more severely burnt.