Fire and Explosives

Fire and Explosive Investigation

Arson investigations often present complex and difficult circumstances to investigate:

• The perpetrator has thoroughly planned the act.

• The perpetrator is not present during the act.

• The destruction is so extensive.

Oxidation

• Chemically, fire is a type of oxidation, which is the combination of oxygen with other substances to produce new substances.

  • Not all oxidation reactions proceed in a manner that one associates with a fire; e.g., rusting.

• An oxidation reaction is associated with the concept of energy.

• Energy is the ability of a system to do work.

  • Steam can turn a turbine to generate electrical energy.

  • energy takes many forms; e.g., heat and light.

• All oxidation reactions are examples in which more energy is liberated than what is required to initiate the reaction.

  • These are known as exothermic reactions.

Combustion

• To start a fire, the ignition point (minimum temperature needed to ignite fuel spontaneously) must be reached.

• The heat and light released when a substance burns is known as heat of combustion.

• Once combustion starts, energy in the form of heat and light (flame) is liberated. A portion of this energy is used to sustain the fire.

  • Fire is a chain reaction.

To initiate and sustain a fire, the following are required:

• A fuel must be present.

• Oxygen must be available in sufficient quantity to combine with the fuel.

• Heat must be applied to initiate the combustion, and sufficient heat must be generated to sustain the reaction.

Physical state of the fuel

• A fuel achieves a reaction rate with oxygen sufficient to produce a flame only when it is in the gaseous state. Thus, rusting is not accompanied by a flame.

• A liquid burns when the temperature is high enough to vaporize the fuel.

  • The flash point is the lowest temperature at which a liquid produces enough vapor to burn.

    • i.e. gasoline has a flash point of -40°C

• A solid such as wood burns only when exposed to heat hot enough to decompose it into gaseous products (pyrolysis).

• Glowing combustion, or smoldering, is burning located at the fuel-air interface, such as a cigarette, the embers of a wood fire, or a charcoal fire.

Heat Transfer

Conduction

• the movement of heat through a solid object.

• Poor conductors are called insulators.

• During a fire, heat may transported through metals, such as nails, bolts, and fasteners to a location far from the initial heat source, creating a new fire location.

Radiation

• is the transfer of heat energy by electromagnetic radiation.

• A surface exposed to the heat of a fire may burst into flames when the surface reaches the ignition temperature.

  • paper at 451°F

Convection

• is the transfer of heat energy by the movement of molecules within a liquid or gas.

  • Like a convection oven

• In a structural fire, hot gases move to the upper portion of the structure causing surfaces to pyrolyze and burst into a fire

The Fire Scence

• The arson investigator needs to begin examining a fire scene for signs of arson as soon as the fire has been extinguished.

• Most arsons are started with petroleum-based accelerants.

  • gasoline, lighter fluid, kerosene, turpentine, butane, and others

• The search of the fire scene must focus on finding the fire's origin, which may be most productive in any search for an accelerant or ignition device.

Indicators 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 accelerant onto the surface

• Normally, a fire has a tendency to move in an upward direction. Thus the probable origin will most likely be the lowest point showing the most intense characteristics of burning.

• Evidence of severe burning found on the floor (as opposed to the ceiling) of a structure is indicative of a flammable liquid

• Discovery of an ignition device:

  • The most common igniter is a match.

  • Arsonists can construct many other types of devices to start a fire, including burning cigarettes, firearms, ammunition, a mechanical match-striker, electrical sparking devices, and a “Molotov cocktail.”

Flashover

• occurs when all the combustible fuels simultaneously ignite to engulf the entire structure.

• A fire that starts in one area of a structure could, through flashover, create the illusion of more unrelated fires, a sign mistaken for arson.

• Irregular patterns are common in post-flashover conditions.

• If the presence of ignitable liquids is suspected to have caused a fire pattern, supporting evidence from the laboratory for the presence of accelerant residues must confirm its existence.

• Many factors can contribute to the deviation of a fire from normal behavior.

  • Using burn patterns such as depth of char, a V-shaped pattern, or low intense burn area, as indicators of a fire's origin can prove to be misleading when a flashover has occurred.

Searching for Accelerants

• Combustible liquids are rarely entirely consumed in a fire.

• The search for traces of flammable liquid residues may be aided by the use of a sensitive portable vapor detector or “sniffer.”

Collection of fire scene evidence

• At the suspect point of origin of a fire, collect ash, soot, and porous materials which may contain excess accelerant.

• Store them in airtight containers such as new paint cans or wide-mouth glass jars, leaving an airspace to remove samples.

• Never use plastic containers to store fire scene evidence.

• The collection of all materials suspected of containing volatile liquids must be accompanied by a thorough sampling of similar but uncontaminated control specimens from another areas of the fire scene, called a substrate control.

Laboratory recovery of flammable residues

• The easiest way to recover accelerant residues from fire-scene debris is to heat the airtight container in which the sample is sent to the laboratory.

• When the container is heated, any volatile residue in the debris is driven off and trapped in the container's enclosed airspace.

• The vapor or headspace is then removed with a syringe.

• When the vapor is injected into the gas chromatograph, it is separated into its components, and each peak is recorded on the chromatogram.

• In the vapor concentration technique, a charcoal strip is placed in the airtight debris container when it is heated.

  • The charcoal strip absorbs much of the vapors during heating.

  • The strip is washed with a solvent which will recover the accelerant vapors.

  • The solvent is then injected into the gas chromatograph for analysis.

Gas Chromatography

• In the laboratory, the gas chromatograph is the most sensitive and reliable instrument for detecting and characterizing flammable residues.

• The vast majority of arsons are initiated by petroleum distillates such as gasoline and kerosene.

• The gas chromatograph separates the hydrocarbon components and produces a chromatographic pattern characteristic of a particular petroleum product

• By comparing select gas chromatographic peaks recovered from fire-scene debris to known flammable liquids, a forensic analyst may be able to identify the accelerant used to initiate the fire.

  • The chromatographic pattern of the unknown is compared to patterns produced by known petroleum products.

Accelerant Identification

• Typically a forensic analyst compares the pattern generated by the sample to chromatograms from accelerant standards obtained under the same conditions.

• The pattern of gasoline, as with many other accelerants, can easily be placed in a searchable library.

  • An invaluable reference known as the Ignitable Liquids Reference Hydrocarbon Collection (ILRC)

• Complex chromatographic patterns can be simplified by gas chromatography/mass spectrometry.

Explosions

• Explosives are substances that undergo a rapid oxidation reaction, producing large quantities of gases.

• It is this sudden buildup of gas pressure that constitutes the nature of an explosion.

• Explosives can be classified as high or low explosives

Low Explosives

• The most widely used low explosives are black powder and smokeless powder.

  • Black powder is a mixture of potassium or sodium nitrate, charcoal, and sulfur.

  • Smokeless powder consists of nitrated cotton (nitrocellulose) or nitroglycerin and nitrocellulose

• Low explosives are often confined to a container like a pipe.

• The speed of decomposition in a low explosive is called deflagration, causing the walls of the container to fragment and fly outward in all directions.

High Explosives

• Primary explosives

  • ultra-sensitive to heat, shock, or friction

  • provide the major ingredients found in blasting caps or primers used to detonate other explosives.

• Secondary explosives

  • relatively insensitive to heat, shock, or friction and will normally burn rather than detonate if ignited in small quantities in the open air.

  • This group comprises the majority of commercial and military blasting, such as dynamite, TNT, PETN, and RDX.

  • MUST be detonated by a primary explosive.

  • The speed of decomposition of high explosives is known as detonation. It’s extremely rapid, producing a supersonic shock wave creating a blast effect with an outward rush of gases at speeds as high as 7,000 miles per hour.

  • In recent years, nitroglycerin-based dynamite has all but disappeared from the industrial explosive market and has been replaced by ammonium nitrate-based explosives.

Military and Peroxide Explosives

• Triacetone triperoxide (TATP) is a homemade explosive that has been used by terrorist organizations.

  • can be made by combining acetone and peroxide in the presence of an acid.

  • Its existence has led to the banning of most liquids on commercial aircrafts.

• In many countries outside the United States, the accessibility of military high explosives to terrorist organizations makes them very common constituents of homemade bombs.

  • RDX is the most popular and powerful of the military explosives, often encountered in the form of pliable plastic known as C-4.

Collection and Analysis

• The entire bomb site must be systematically searched, with great care given to recovering any trace of a detonating mechanism or 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 and loose soil and other debris must be preserved from its interior for laboratory analysis.

• One approach for screening objects for the presence of explosive residues is the ion mobility spectrometer.

• All materials collected for the examination by the laboratory must be placed in sealed air-tight containers and labeled with all pertinent information.

• Debris and articles collected from different areas are to be packaged in separate air-tight containers.

• Some explosives can diffuse through plastic and contaminate nearby containers.

At the Lab

• Typically, in the laboratory, debris collected at explosion scenes will be examined microscopically for unconsumed explosive particles.

• Recovered debris may also be thoroughly rinsed with organic solvents and analyzed by testing procedures that include color spot tests, thin-layer chromatography, and gas chromatography/mass spectrometry.

• Confirmatory identification tests may be performed on unexploded materials by infrared spectrophotometry.