Forensic Science -

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• Forensic Science -
• Arson and Explosives

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I. The Chemistry of Fire

• Oxidation The combination of oxygen with other
  subtances to produce new substances.
• One example is the burning of methane (natural
  gas)
• CH4 2O2 CO2 2H2O

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• This reaction requires a spark to initiate.
• Combustion the rapid combination of oxygen with
  another substance accompanied by the production
  of noticeable heat and light.
• Exothermic reaction a chemical reaction in
  which energy is released or given off.
• Ignition temperature the minimum temperature at
  which a fuel will spontaneously ignite.

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• The oxidation of a fuel differs from the
  oxidation of a substance like iron which oxidizes
  into rust.
• 4Fe 3O2 2Fe2O3
• Rusting and burning are both oxidation
  processes. The difference between them is the
  rate or speed at which the reaction takes place.

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• A fuel will achieve a reaction rate with oxygen
  sufficient to produce a flame only when it is the
  gaseous state, for it is only in this state that
  molecules can collide frequently enough to
  support a flaming fire.
• How then do liquids and solids burn?
• In a liquid fuel the temperature must be high
  enough to vaporize the fuel.

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• Flash Point the minimum temperature at which a
  liquid fuel will produce enough vapor to burn.
• Once flash point is reached, the fuel can be
  ignited by some outside source to start a fire.
• Solids will burn only when exposed to heat that
  is hot enough to decompose the solid into gaseous
  product.

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• Pyrolysis the decomposition of organic matter
  by heat.
• Flammable range - the entire range of possible
  gas or vapor fuel concentrations in air that are
  capable of burning.
• A fuel can burn without the presence of a flame.

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• Glowing combustion - burning at the fuel air
  interface. Examples are a red-hot charcoal or a
  burning cigarette.
• For most reactions, a 10ºC rise in temperature
  doubles or triples the reaction rate.
• Spontaneous combustion - a fire caused by a
  natural heat-producing process in the presence of
  sufficient air and fuel.

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• Explosives are substances that undergo a rapid
  exothermic reactions, with the production of
  large quantities of gases. It is this sudden
  buildup of gas pressure that contitutes the
  nature of an explosion. Detonation occurs so
  rapidly that oxygen in the air cannot participate
  in the reaction thus many explosives must have
  their own source of oxygen.

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• Oxidizing Agent - a substance that supplies
  oxygen to a chemical reaction.
• The three requirements that must be satisfied if
  combustion is to be initiated and sustained are
  
  1. A fuel must be present.
  2. Oxygen must be available in sufficient
  quantity to combine with the fuel.
  3. Heat must be applied to initiate
  the combustion, and generated to sustain it.

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II. Searching the Fire Scene

• Accelerants any material used to start or
  sustain a fire. The most common accelerants are
  combustible liquids.
• Because any petroleum residues that remain after
  a fire may evaporate in a few hours or days, a
  search warrant is not required.

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• A search of a fire scene focuses on finding the
  origin.
• Some telltale signs of origin may include
  streamers, accelerant containers, ignition
  devices, or time-delay devices.
• Nothing should be moved until pictures, sketches
  and notes are taken.

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III. Collection Preservation of Arson Evidence

• Two to three quarts of ash and soot debris must
  be collected at the point of origin.
• Should include all porous materials and all other
  substances likely to contain flammable residues.
  (Wood flooring, rugs, upholstery, and rags.)
• New, clean paint cans with friction lids are good
  containers. Wide mouthed glass jars are also
  useful for packing materials.

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• Cans and jars should be filled one-half to
  two-thirds full, leaving an air space in the
  container above the debris.
• Plastic polyethylene bags are NOT suitable for
  packing speciments because they react with
  hydrocarbons and permit vapors to be depleted.
• Collect uncontaminated samples of materials from
  the scene to provide substrate control.
• If a perpetrator is arrested within a few hours,
  their clothing may contain residual traces of
  accelerant.

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IV. Analysis of Flammable Residues

• The gas chromatograph is the most sensitive and
  reliable instrument for detecting and
  characterizing flammable residues.
• Most arsons are initiated by petroleum
  distillates such as gasoline and kerosene.
• Hydrocarbons - any compound consisting of only
  carbon and hydrogen.

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V. Types of Explosives

• Explosion a chemical or mechanical action
  resulting in the rapid expansion of gases.
• The speed at which explosives decompose varies
  greatly from one to another and permits their
  classification as high and low explosives.

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• Low explosive explosive with a velocity of
  detonation less than 1,000 meters per second. For
  example, black powder and smokeless powder.
• In the low explosive the speed is called speed of
  deflagration (burning). This is characterized by
  a very rapid oxidation producing heat, light and
  a subsonic pressure wave.

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• High Explosive Explosive with a velocity of
  detonation greater than 1,000 meters per second.
  For example, dynamite and RDX.
• In the high explosive, the speed is called the
  speed of detonation. Detonation refers to the
  creation of a supersonic shock wave within the
  explosive charge.
• This shock wave causes the chemical bonds of the
  explosive charge to break apart, leading to the
  new instantaneous buildup of heat and gases.

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• The most widely used low explosives are black
  powder and smokeless powder.
• Black powder is a relatively stable mixture of
  potassium or sodium nitrate, charcoal, and
  sulfur. Unconfined, it merely burns. It becomes
  explosive and lethal when confined.
• Safety fuses usually consist of black powder
  wrapped in a fabric or plastic casing.
• The safest and most powerful low explosive is
  smokeless powder.

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• High explosives are classified into two groups by
  their sensitivity.
• Primary explosives a high explosive easily
  detonated by heat or shock.
• Usually used to detonate other explosives through
  a chain reaction and are referred to as primers.
• Often found in blasting caps.

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• Secondary explosives are relatively insensitive
  to heat, shock, or friction, and will normally
  burn rather than detonate if they are ignited in
  small quantities in the open air.
• Includes dynamite, TNT (trinitrotoluene), PETN
  (pentaerythritol tetranitrate), RDX
  (cyclotrimethylenetrinitramine)

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• Nitroglycerin-based dynamite has been replaced by
  ammonium nitrate based explosives. These
  explosives mix oxygen-rich ammonium nitrate with
  a fuel to form a low-cost and very stable
  explosive.
• RDX is the most popular and powerful of the
  military explosives. It is often found in the
  form of a pliable plastic of doughlike
  consistency known as C-4.
• PETN is often used in a detonating cord to
  connect a series of charges so that they will
  detonate simultaneously.

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VI. Collection Analysis of Explosives

• Most important is collection of samples from the
  scene containing undetonated residues.
• EGIS is a portable explosive residue detector.
• There are also chemical tests for common
  explosives.