Forensics Unit 10 Terms

vapor concentration technique

A charcoal strip is placed in airtight debris container when it is heated; strip absorbs vapors; strip is washed with solvent to recover accelerant; solvent is then injected into chromatograph for analysis.

things needed to initiate and sustain a fire

A fuel (in vapor/gaseous form) must be present; oxygen in sufficient quantity to combine with the fuel; enough heat applied to initiate the combustion; sufficient heat generated to sustain the reaction.

accessibility to high explosives

Accessibility of military high explosives to terrorist organizations; very common constituents of homemade bombs.

classification of explosives

According to speed at which explosives decompose; low explosives or high explosives.

ignition devices used by arsonists

Arsonists construct or use many different types of devices to start a fire: burning cigarettes, firearms, ammunition, mechanical match-striker, electrical-sparking devices, and "Molotov cocktail."

collection of fire scene evidence

At suspect point of origin of a fire and other relevant locations; ash, soot, and porous materials that may contain excess accelerant should be collected and stored in airtight containers; new paint cans or wide-mouth glass jars; leaving an airspace to remove samples; never use plastic containers to store fire scene evidence; keep chain custody.

fire scene

Begin examining a fire scene for signs of arson as soon as the fire has been extinguished: find point of origin, search for ignition device and accelerant.

blast injuries in confined spaces

Bombings in confined spaces (buildings and bases) cause irregular, high-pressure waves and unpredictable patterns of injury.

solid fuel

Burns only when exposed to heat hot enough to decompose into gaseous products (pyrolysis).

liquid fuel

Burns when the temperature is high enough to vaporize the fuel; flash point varies for different fuels.

pyrolysis

Chemical change of solid fuel brought about by the action of heat; solid fuel decomposes into gaseous form and favors combustion.

substrate control

Collection of all materials suspected of containing volatile liquids must be accompanied by sampling of similar but uncontaminated control specimens from another area of scene.

arson investigations

Complex and difficult to investigate; the destruction is extensive' often limited to detecting and identifying relevant chemical materials and reconstructing and identifying ignitor mechanisms; ultimate determination of the cause of a fire is made by a trained investigator.

heat transfer - fire spread mechanisms

Conduction, convection, and radiation.

blast wave

Consists of two parts: a shock wave of high pressure followed closely by a blast wind or air in motion.

lab analysis of bombing scenes

Debris collected at explosion scenes will be examined microscopically for unconsumed explosive particles; recovered debris must be thoroughly rinsed with organic solvents and analyzed by different testing procedures: color-spot tests, thin-layer chromatography, chromatography/mass spectrometry, and infrared spectrophotometry (confirmatory identification tests may be performed on unexploded materials).

explosion scene

Entire bomb site must be systematically searched; great care given to recovering any trace of a detonating mechanism and any other item foreign to the explosion site; objects located at or near the origin of the explosion must be collected for laboratory examination; often a crater is located at the origin: loose soil and other debris must be preserved from its interior for laboratory analysis.

indicators of arson

Evidence of separate and unconnected fires (not explained by typical heat transfer); presence of accelerants and/or ignition devices.

mechanics of explosions of a conventional bomb

Explosion generates blast wave that spreads out from point source; as outward energy dissipates a reversal of wind back toward the blast and under-pressurization occur; damage decreases exponentially with distance from the point source of the blast.

point of origin

Fire has a tendency to move in an upward direction: probably origin will most likely be the lowest point, showing most intense burning.

fire scene processing

First: record and document scene; then: collection of fire scene evidence.

ion mobility spectrometer

For screening objects for the presence of explosive residues; a sample is introduced into an ionization chamber, where bombardment with radioactive particles converts the sample to ions; the ions move into a drift region where ion separation occurs based on the speed of the ions as they move through an electric field; explosive substances can be characterized by the speed at which they move through the electric field.

TATP: Triacetone triperoxide

Homemade explosive used by terrorist organizations; made by combining acetone and peroxide in the presence of an acid; its existence has led to the banning of most liquids on commercial aircraft.

burning pattern on the floor associated with accelerants

Irregularly-shaped pattern on the floor resulting from the pouring of accelerant onto the surface; evidence of severe burning found on the floor (as opposed to the ceiling) of a structure is indicative of a flammable liquid; combustible liquids are rarely entirely consumed during a fire.

flash point

Lowest temperature at which a liquid fuel produces enough vapor to burn.

insulators

Materials that are poor conductors.

ignition temperature

Minimum temperature needed to spontaneously set fuel on fire (ignite).

RDX

Most popular and powerful on the military explosives; often encountered in the form of pliable plastic known as C-4.

low explosives

Most widely used: black powder and smokeless powder; must be confined to a container like a pipe.

conduction

Movement of heat through a solid object; during a fire, heat may be transported through metals (nails, bolts, and fasteners) to a location far from the initial heat source, creating a new fire location.

all materials collected from bombing scene

Must be placed in sealed, air-tight containers; labeled with all pertinent information; debris and articles collected from different areas must be packaged in separate containers; avoid plastic containers: some explosives can diffuse through plastic and contaminate nearby containers.

ignition device

Object used to set something on fire or to begin burning (provides the initial spark); most common ignitor is a match.

flashover

Occurs when all the combustible fuels simultaneously ignite and engulf the entire structure.

arsonist

Perpetrator, or person that causes a fire intentionally; frequently, in arson the perpetrator has planned the act and is not present during act

effects of blasts on humans

Primary: direct effects of pressure. Secondary: effects of projectiles. Tertiary: effects due to wind. Quaternary: burns, asphyxia, and exposure to toxic inhalants.

secondary explosives

Relatively insensitive to heat, shock, or friction; will burn rather than detonate if ignited in small quantities in the open air; majority of commercial and military blasting: dynamite, TNT, PETN, and RDX; must be detonated by a primary explosive.

blast injuries

Resulting pressure effect damages organs, particularly at air-fluid interfaces; wind propels fragments and people, causing penetrating or blunt injuries; blast injuries should be suspected regardless of the distance the patient was from the blast center and/or the absence of injuries in other people who were near the patient.

glowing combustion

Smoldering; no visible flame; burning at the fuel-air interface; examples: cigarette, embers of a wood fire, and charcoal fire.

deflagration of low explosives

Speed of decomposition that causes the walls of the container to fragment and fly outward in all directions; to cause the low explosive to explode.

accelerants

Streamers: substance used to facilitate combustion and/or to spread the fire from one area to another; most arson are started with petroleum-based accelerants or petroleum distillates (gasoline and kerosene).

explosives

Substances that undergo a rapid oxidation reaction with production of large quantities of gas creating an explosion; the nature of an explosion is given by sudden buildup of gas pressure.

arson

The criminal act of deliberately setting fire to property.

detonation of high explosives

To cause the high explosive to explode: speed of decomposition of high explosives is extremely rapid; producing a supersonic shock wave and blast effect with an outward rush of gases at high speeds.

laboratory recovery of flammable residues

To recover accelerant residues from debris the airtight container in which sample is sent to lab must be heated; any volatile residue in debris is driven off and is trapped in container; vapor or headspace can be removed with a syringe and then injected into chromatograph for analysis.

combustion

To start fire, ignition temperature must be reached; heat of combustion: heat produced when a substance burns; once combustion starts, enough energy in the form of heat and light; (flame) is liberated and a portion of this energy is used to sustain the fire (fire is a chain reaction).

locate accelerant residues

Traces flammable liquid residues may be located with a vapor detector (sniffer) or a trained canine.

convection

Transfer of heat energy by movement of molecules within gas; in a structural fire hot gases move to upper portion of the structure, causing surfaces to pyrolyze and burst into a fire.

radiation

Transfer of heat energy so that a surface exposed to the heat of a fire may burst into flames when the surface reaches ignition temperature.

fire

Type of oxidation: combination of oxygen with other substances to produce new substances; produces energy: heat and light (exothermic reaction).

high explosives

Types: primary explosives and secondary explosives.

primary explosives

Ultra-sensitive to heat, shock, or friction; major ingredients in blasting caps or primers used to detonate other explosives.

chromatography

Vapor of recovered accelerant is injected into has chromatograph and separated into its components; each peak is recorded; sensitive and reliable instrument for detecting and characterizing flammable residues; compare select peaks from fire-scene debris to known flammable liquids and identify accelerant used to initiate the fire; complex chromatographic patterns can be simplified by gas chromatography / mass spectrometry.