FSI 226 - Chapter 17 Arson + Explosives

T/F: Arson investigations are often complex and difficult

True

Why are arson investigations complex and difficult?

• They are well planned by the perpetrator

• The perpetrator is often no longer present

• The destruction is extensive

What must difficult investigations establish?

• Motive

• Suspect

• Modus operandi: Offender’s pattern of operation

What is fire?

A transformation process that uses oxygen in combination with other substances to produce heat and light

What is oxidation?

The known process that the combination with other substances produces new substances

Examples of oxidation?

• Burning of methane (combustion reaction)

  • CH₄ + 2O₂ → CO₂ + 2H₂O

• Rusting of iron

  • 4Fe + 3O₂ → 2Fe₂O₃

T/F: All oxidation reactions require fire

False

What separates oxidation reactions that require fire to proceed?

The amount of energy needed

What is energy?

The ability or potential of a system or material to do work

What is combustion?

A type of oxidation reaction in which more energy is liberated than is required to break the chemical bonds between atoms

What do combustion reactions produce?

Water and carbon dioxide

What are the two types of combustion reactions?

Exothermic and endothermic

What is an exothermic reaction?

Excess energy is liberated as heat and light (heat of combustion)

What is an endothermic reaction?

More energy is required to start the reaction than the amount of energy liberated

What are other forms of energy?

• Heat

• Electrical

• Mechanical

• Nuclear

T/F: A criminalist’s function is rather limited to detecting and identifying relevant chemical materials collected at the scene and reconstructing and identifying igniter mechanisms

True

What is ignition temperature?

The minimum temperature at which a fuel spontaneously ignites

T/F: All oxidation reactions produce a flame

False

Why do some oxidation reactions create a flame and others don’t?

Due to the speed of the reaction (the faster molecules move, the greater the number of collisions, the faster the rate of reaction)

What are the two factors that describe the rate of collisions?

1. The physical state of the fuel (solid, liquid, gas)

2. Fuel temperature

T/F: The fuel has to be in a gaseous state

True

What is flash point?

The minimum temperature at which a liquid fuel produces enough vapor to burn

What is pyrolysis?

The decomposition of solid organic matter by heat

What is glowing combustion?

Combustion on the surface of a solid fuel in the absence of heat high enough to pyrolyze the fuel (ex: charcoal, cigarettes)

What are the three requirements for combustion?

1. Fuel

2. Oxygen

3. Heat

What are the three ways of heat transfer?

1. Conduction

2. Radiation

3. Convection

What is conduction?

Heat moving through a solid

What is radiation?

The transfer of heat energy by electromagnetic radiation

What is convection?

The transfer of heat energy by the movement of molecules within a liquid or gas

T/F: In a structural fire, the flames move to the upper portion of the structure to continue burning

True

What must investigators wait for before examining the fire scene for potential signs of arson?

For the fire to be completely extinguished

What is the focus of examining a fire scene?

Finding the fire’s origin

What is an accelerant?

Any material used to start or sustain a fire

What is a common base for accelerants?

Petroleum

What are some telltale signs of arson?

• Evidence of separate and unconnected fires

• The use of “streamers” to spread the fire from one area to another

• An irregularly shaped pattern on the floor resulting from the pouring of an accelerant onto the surface

T/F: Fire does NOT have a tendency to move in an upward direction

False

T/F: The most probable location of origin is at the lowest point with intense characteristics of burning

True

What are some common ignition devices?

• Matches

• Burning cigarettes

• Firearms

• Ammunition

• Mechanical match-striker

• Molotov cocktail

What may you find when a Molotov cocktail has been used?

Shards of glass or the rag used

What is flashover?

The transitional phase in some compartment fires in which temperatures rise to a level sufficient to cause the ignition of all combustible items in the compartment

T/F: Flashover can be mistaken for arson due to the creation of irregular patterns

True

What is included in burn patterns?

• Depth of char

• A v-shaped pattern

• Low intense burn area

T/F: Fires always consume all combustible liquids

False

What is a “sniffer”?

A portable vapor detector that can be used to find traces of flammable liquid residues

T/F: Ash, soot, and porous materials should be collected at the suspect point of origin

True

What type of packaging should be used for fire scene evidence?

• Metal paint cans

• Glass jars

What is substrate control?

The collection of all materials suspected of containing volatile liquids accompanied by a thorough sampling of similar but uncontaminated control specimens from another area of the fire scene

T/F: The easiest way to recover accelerant residues from fire scene debris is to heat the airtight container

True

Why is it beneficial to heat the container with the fire scene evidence?

The vapor/headspace can be removed with a syringe or SPME fiber and directly injected into the GCMS

What is gas chromatography?

Separates the hydrocarbon components and produces a chromatographic pattern

What is the ILRC?

Ignitable Liquids Reference Hydrocarbon Collection

T/F: Complex chromatographic patterns can be simplified by GCMS

True

What is an explosive?

Substances that undergo a rapid oxidation reaction, producing large quantities of gases

What can explosives be classified as?

Low and high

Characteristic of low explosives

Decompose at a slower rate (deflagration)

What is black powder?

A low explosive that is comprised of potassium/sodium nitrate, carbon, and sulfur

What is smokeless powder?

A low explosive comprised of a mixture of nitroglycerin and nitrocellulose

Characteristic of high explosives

Have a high velocity (detonation)

What is a primary explosive?

A high explosive that is easily detonated by heat, shock, or friction

What is a secondary explosive?

A high explosive that is relatively insensitive to heat, shock, or friction

T/F: The primary explosive does NOT have to detonate the secondary explosive

False

Where is the point of origin in an explosion often located?

By a crater and loose soil

What can be used to monitor explosive residue?

An ion mobility spectrometer

How should explosive evidence be packaged?

In air-tight containers