Forensic Aspects of Fire Investigation Chapter 12 Forensics.

Forensic Aspects of Fire Investigation

Chapter 12

Forensics

Fire: rapid oxidation with evolution of heat and light.

The Chemistry of Fire: Oxidation

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 productsEx: 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

Cause of Fire: Undetermined Intentional

Natural Accidental

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

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

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.

Searching the Fire Scene

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