Forensic Aspects of Fire Investigation

Forensic Aspects of Fire Investigation Chapter 12 Forensics

The Chemistry of Fire: Oxidation Fire: rapid oxidation with evolution of heat and light.

The Chemistry of Fire: Oxidation Arson - The crime of maliciously, voluntarily, and willfully setting fire to the building, buildings, or other property of another or of burning one's own property for an improper purpose, as to collect insurance.

Chemistry of Fire • Oxidation • The fundamental chemical reaction of fire • Oxygen combines with other substances to make new products • Ex: CH4 + 2O2→ CO2 + H2O

Energy & Combustion • Energy – ability to do work • Many forms: heat, electrical, potential, kinetic, nuclear, light, chemical • In fires the energy of the fire is provided from chemical energy when bonds are broken • All oxidation reactions are exothermic which means: ___________________. • This released energy is sometimes called the heat of combustion • A combustion reaction is one where a substance BURNS in the presence of oxygen.

Heat • Not only do chemical reactions sometimes release energy, but they require energy in order to start breaking the bonds • This energy is typically HEAT • Once combustion is started, it releases enough heat to keep the reaction going

Speed of Reaction • In order to produce fire, oxidation reactions must happen quickly • Many thing will speed up a reaction. In forensics we look at: • The physical state of fuel. The fuel (wood, methane, gasoline) must be in the gaseous state for fire to occur. • Fuel temperature. Fuel must be at a high enough temperature to be vaporized (turned into a gaseous state)

Fuel-Air Mix and Types of Combustion • Fuel-Air Mix – the mixture of the gaseous fuel and oxygen in the air must be within certain limits • Different for different fuels • If the mix is right it is said to be in the flammable range • Glowing Combustion – (smoldering) combustion on the surface of a solid fuel that is not hot enough to vaporize • Ex: glowing coals • Spontaneous Combustion – fire starts without any obvious source of energy. Very limited circumstances. • Ex: Hay stored in a poorly ventilated barn. Bacteria grows on the hay and produce heat. Heat builds up and ignites the hay.

Three Requirements to Initiate and Sustain Combustion • A fuel must be present. • Oxygen must be available in sufficient amounts to combine with the fuel. • Heat must be applied to initiate the combustion, and sufficient heat must be generated to keep the reaction going.

Transfer of Heat • Heat tends to move from a hot location to a cold location in one of three ways: • Conduction – through direct contact. As the solid object heats up, the atoms start to move faster. As they bump into each other they transfer energy(heat). • Ex: putting a metal spoon into a pot on the stove. • Convection – transfer through the movement of molecules in a liquid or a gas. • Ex: water heating on a stove. As the water heats up it rises. Once it is at the top of the pot it is further from the stove and cools. As it cools it falls. Once it is at the bottom it starts to heat again and the cycle repeats. • Radiation – transfer through electromagnetic waves • Ex: heat from the sun. all hot surfaces emit SOME radiant heat

Searching the Fire Scene Cause of Fire: • Natural • Accidental • Undetermined • Intentional Who investigates the fire? Often, the Fire Department will have an investigative team to determine the cause/origin of a fire. (major events = ATF)

Searching the Fire Scene • Clues that it may have been arson: • Containers for holding accelerants such as gasoline or kerosene • Ignition device • Irregular pattern on floor or ground from accelerant (meaning that the fire didn’t spread the way it would have naturally) • Sign of breaking and entering and theft • Scene needs to be search as soon as possible because accelerant residues may evaporate quickly

Searching the Fire Scene What evidence can be collected? • Paper/Trash – not much evidence • Accelerant – rapid start • Chemical device (road flares, propellant) • Electronic device for delayed ignition Unburned portions of chemical and electronic devices may be left behind.

Locating the Fire’s Origin • This is the first step in investigating a fire • May discover in the process more than one origin, which proves the fire was arson • Most fires tend to move • Upward • Outwards in a V-shape (see p 435 Figure 12-8) • If a flammable liquid was used • There may be more charring and more severe burning in the lower parts of the room and furniture • Once located the origin needs to be protected and sketches and photographs need to be done

Searching for Accelerants • Usually there is SOME trace of the accelerant left after the fire • May have been absorbed by carpet, plaster, etc • Search may be aided by the use of a portable vapor detector • Accelerants generally are volatile, which means they vaporize at room temperature. The detector senses the gaseous substance in the air. • Dogs can also be trained to detect the presence of accelerants

Collection & Preservation of Arson Evidence • Evidence must be packaged in airtight containers to prevent the accelerant traces from evaporating • Ex: unused paint cans, glass jars, tightly sealed bags • Only fill cans and jars ½ to 2/3 full so there is some air space

Collection & Preservation of Arson Evidence • Reference samples must be taken of items like carpet and drapes from an area where there was NO accelerant. • Igniters – matches and other devices used to start the fire are important evidence • The suspect’s clothes will usually contain traces of the accelerant used

Analysis of Flammable Residue • The gas chromatograph is used frequently because it separates the components of liquid accelerants easily • Can compare to patterns of known petroleum products • Headspace Technique – heat the airtight container the debris are in and the accelerant will vaporize into the left over air space. The gas can then be removed with a syringe through the top of the can and injected into the gas chromatograph.

Analysis of Flammable Residues • Vapor Concentration – the headspace technique only allows a small sample to be tested (the volume of the syringe). • A charcoal strip is placed into the container and the container is heated. The strip absorbs the vapor released by the accelerant. The vapor is then ‘washed’ off with a solvent and injected into the gas chromatograph • Advantages: can find much smaller amount of accelerant. Much more sensitive

Analysis of Flammable Residue • Gas Chromatography/Mass Spectrometry (GCMS) – sometimes the GC is not enough to identify the accelerant used. • Possibly due to using a combination of chemicals or after-effects of the fire itself. • In this case, GCMS is used

Forensic Aspects of Fire Investigation