Forensic Aspects of Fire Investigation

1. Forensic Aspects of Fire Investigation Objectives: Define oxidation. Define energy and give examples of its different forms. Describe the role of heat energy in chemical reactions. Define heat of combustion and ignition temperature. Describe the difference between an exothermic and endothermic chemical reaction.

2. Arson Investigation • criminalist must establish • motive • reason/purpose of fire • modus operandi • pattern of operation • find suspect

3. Fire Causes • accidental • faulty wiring • overheated electrical motors • improperly maintained heating systems • cigarette smoking • arson • chemical accelerants • gasoline, kerosene, etc • leave chemical traces • nonchemical caused • pile of combustibles • harder to prove as non-accidental

4. Chemistry of Fire • Oxidation • chemical reaction with oxygen combining with a substance to form new substances • with fire (heat & light) a.k.a. combustion • CH4 + 2O2 CO2 + 2H2O • without fire a.k.a. “rusting” • 4Fe + 3O2  2FeO3

5. Energy • the ability or potential to do work • heat • light • electrical • mechanical • nuclear • chemical • energy can change form but cannot be created or destroyed

6. Chemical Reactions • Exothermic Reactions • more energy is liberated than is required to rearrange atoms bonded together • energy given to surroundings • surroundings get hotter (increase in temperature) • Heat of Combustion • excess heat released during combustion • Endothermic Reactions • less energy is liberated than is required to rearrange atoms bonded together • energy taken from surroundings • surroundings get colder (decrease in temperature)

7. Forensic Aspects of Fire Investigation Objectives: Explain why the oxidation of iron to rust is not accompanied by a flaming fire. List the requirements necessary to initiate and sustain combustion. Understand the three mechanisms of heat transfer Describe how physical evidence must be collected at the scene of a suspected arson. Describe laboratory procedures used for the detection and identification of hydrocarbon residues.

8. Ignition Temperature • Energy Barrier • amount of heat (energy) needed to initiate combustion • Ignition Temperature • initial temperature that must be reached for spontaneous combustion to occur • usually provided by “igniter” • match • spark • chemical • Iron Rusting • low ignition temperature • slow small energy release

9. Speed of Reaction • faster movement more collisions faster rate of reaction • Physical State • gaseous favors greatest reaction • Fuel Temperature • solids and liquids maintain a gaseous reaction when vaporized • Flash Point • lowest temperature at which a liquid fuel will vaporize sufficiently to support combustion • 451 °F flash point of paper!!!! • pyrolysis takes place • each 10°C doubles or triples rate of reaction • stops only when fuel or oxygen is depleted

10. Fuel-Air Mixture • combustion within definite limits • low fuel (lean mixture) • high fuel (rich mixture) • Flammable Range • range of gaseous concentrations in air capable of supporting combustion • gasoline 1.3%-6.0%

11. Glowing Combustion • burning of fuels without flame • solids exposed to heat less than pyrolysis • red charcoals • cigarette • a.k.a. smoldering • “hot spots”

12. Spontaneous Combustion • natural heat producing process ignites a fire in a poorly ventilated area • limited situation rarely cause fires • hay with bacteria • oily (highly unsaturated fats) rags • Combustion requires: • fuel • oxygen • heat

13. Heat Transfer • heat moves from high to low temperature regions • trace origin • explain how fire spreads • Transfer Mechanism • conduction • radiation • convection

14. Conduction • movement through a solid • Metals are conductors • loosely held electrons better conductors • Woods/Plastics • tightly held electrons poor conductors • “insulators” • fuel contact with a conductor can spread fire

15. Radiation • heat transfer by electromagnetic radiation • hot surfaces release various wavelengths • surfaces facing “fire” often ignite from radiant heat

16. Convection • heat transfer by molecular movement in liquids/gases • hot gases expand and rise • in a fire these gases rise heating surfaces • surfaces can pyrolyze and ignite • flashover • simultaneous ignition of combustible fuels in a structure

17. Arson Investigation • accelerants (petroleum based substances) • freeze soil to stop bacteria degradation • ignition device (candle, match, cell phone) • visual indications (stains)

18. Analysis of Flammable Residues • Headspace Technique • heat container and gather vapors with syringe • Vapor Concentration • longer slower heating • absorbent charcoal strip catches accelerant • increases sensitivity 100x • GC/MS Analysis • known readouts used to ID accelerant

19. Locating Fire Origin • Fire moves up (V-shaped pattern) • Streamers – connect burnt areas • lower burning liquids flow down • Accelerant search • vapor detectors • trained canines

20. Evidence Collection and Preservation • specimens in airtight containers • “hold” vapors • Substrate Controls • sample materials NOT exposed to accelerants • burning not due to “normally” found materials • cleaning solutions or composition