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Forensic Aspects of Fire Investigation

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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 arson 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 Endothermic 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 Iron Rusting
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 Flammable Range
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

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