Module 3 - Arson and Explosives
Lesson 2 - Investigating Arson Fires
Collection and Analysis of Fire Scene Evidence
The physical evidence collected from a fire scene is analyzed and interpreted by forensic lab specialists. These forensic experts use specialized scientific equipment or techniques to help determine the cause of the fire. The most common service provided by laboratories to fire investigators is the analysis of fire debris for suspected accelerants. Arsonists often use an excess of accelerant to start a fire. Accelerant residues remain, and they may be detected by laboratory analysis. Unfortunately, if only a small amount of accelerant is used or if the accelerant is poured onto highly flammable substrates such as paper or plastic, laboratory analysis may be ineffective because no residues are left in the debris.
The appearance of a flame under normal gravity conditions depends on convection. As soot rises to the top of a flame, it is cone-shaped and appears yellow. In zero gravity, such as in outer space, convection no longer occurs, and the flame appears round, spherical, and blue.
Collection of Evidence from a Fire Scene
Proper collection and storage of evidence is crucial to ensure that evidence is not contaminated. The ideal containers for fire scene evidence suspected of containing volatile accelerants are clean metal paint cans, sealable glass jars, or plastic bags. Fire investigators use several unique ways to find and collect physical evidence.
Accelerant Detection Dogs: Dogs used in arson cases are trained to search for any type of accelerant that may have been used to start a fire. Accelerant detection dogs search the entire fire scene. If an accelerant was used, they alert their handlers to this by barking or sitting. The site that the accelerant detection dog identifies is usually the ignition point of the fire.
Photo Ionization Detectors (PID): Hydrocarbons such as gasoline, kerosene, and paint solvents used as accelerants can be detected using PIDs. A PID is a sensitive portable device that can detect hydrocarbon vapours at concentrations of 100 parts per billion (although similar devices used in a laboratory may be able to detect hydrocarbon vapours in the one part per trillion range). The PID helps narrow the location of the hydrocarbon, and then this evidence may be collected and analyzed further.
The PID contains an ultraviolet lamp that emits photons, which are absorbed by hydrocarbon compounds. Photoionization occurs in a PID when hydrocarbon molecules absorb light energy, which causes them to break apart and emit an electron(s) thus creating positively charged ions. Electrodes in the PID, which generates an electrical current that is converted into a digital measurement reading, collect these ions. PID results are almost immediate; however, they cannot identify the type of hydrocarbon used so a sample must be collected and analyzed for further identification. PIDs may also give a false positive reading for water vapour, especially in humid conditions.
Portable Arson Sampler (PAS): PASs are used to collect any smouldering vapours or residue from a fire scene. A PAS consists of a series of special glass sampling tubes about the size of short pencils and containing an absorbent material (usually charcoal) that readily absorbs vapours. Inside a PAS sampling tube, vapours are separated from unwanted debris and kept away from metal surfaces where they might decompose. The entire sample is then sent to a laboratory for analysis and identification.
Solid-Phase Microextraction (SPME): SPME uses fibres that work like ‘chemical dipsticks’ to capture unknown vapours or residue. The fibres inside a syringe are coated with a polymer that readily absorbs thousands of compounds when exposed to the environment for a time. After the fibres have absorbed evidence, they are sent to a lab for analysis and identification.
Photons
- the particles composing light
Ions
- an atom or a group of atoms that has acquired a net electric charge by gaining or losing one or more electrons