chapter 11. arson and explosives section 1: fire and arson the combustion reaction – flaming and...
TRANSCRIPT
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Chapter 11
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Arson and Explosives
• Section 1: Fire and Arson
• The Combustion Reaction – Flaming and Glowing Combustions
• Investigating Suspicious Fires – Arsonists’ Motives
• Investigation of Fire Scenes
• Recovery of Ignitable Liquid Residues from Suspicious Fire Scenes
• Laboratory Analysis of Debris and Other Samples – Recovery of Ignitable Liquid Residues
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I. The Combustion ReactionFlaming and Glowing Combustion
• Combustion is a rapid oxidation reaction, the combination of fuel and oxygen to form carbon dioxide, water, and heat
• Chemical reactions that give off heat are called exothermic reactions
• Incomplete combustion reactions produce poisonous carbon monoxide
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I. The Combustion ReactionFlaming and Glowing Combustion
1. Necessary Components for Combustion:
• Combustion requires a number of components as shown by the fire triangle, tetrahedron, and pentagon– The fire triangle shows the essential components as: fuel, oxygen, &
heat
– The fire tetrahedron adds free radical reactions (chain reaction)
– The fire pentagon adds an ignition source
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I. The Combustion ReactionFlaming and Glowing Combustion
• There are two ways to interrupt a combustion reaction: adding water to absorb heat or adding fire retardants to interrupt the chain reaction process
• There are two major types of combustion: – Flaming combustion - both the fuel and oxygen are in the
gaseous phase
– Glowing combustion - the fuel is solid and only oxygen is in the gaseous phase
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I. The Combustion ReactionFlaming and Glowing Combustion
2. Nature of Fuels:
• Common fuels can be classified as solids, liquids or gases– Gases – fuels include hydrogen gas, natural gas, methane,
and propane
– Liquids – fuels include gasoline, fuel oil, kerosene, and ethanol
– Solids – fuels include wood, coal, charcoal
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I. The Combustion ReactionFlaming and Glowing Combustion
3. Characteristics of Fuels:
• Flash point – is the lowest temperature at which a liquid produces enough vapor to be ignited by a small flame
• Fire point (self - ignition temperature) – is the temperature at which there is enough heat to cause combustion even in the absence of a source of ignition
• Flammable Range – is a measure of the percentage of fuel that, when mixed with air, is needed to sustain combustion
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I. The Combustion ReactionFlaming and Glowing Combustion
• Relative Vapor Density – a property of compounds relating vapor density to molecular weight. Most materials when vaporized are much heavier than air
• Pyrolysis of Solid Fuels – the process by which solid materials are decomposed by heat, forming smaller molecules that can support flaming combustion
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II. Investigating Suspicious FiresArsonists’ Motives
There are several motives for most arson fires:
• economic, revenge, intimidation, and extortion
• Examples:
• Burning of buildings to collect on the artificially inflated insurance value
• Setting fire to a place in revenge or to teach the owner a “lesson”
• Setting fire to a business that refuses to pay for protection
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III. Investigation of Fire ScenesFires are investigated to determine the cause of the fire and its origin
1. Burn Patterns:
• Since heat travels upward, pyrolysis occurs in materials above the area of combustion, producing a burn pattern that looks like an “inverted cone” or “V” pattern
• The inverted cone is a strong indicator that the point of origin of the fire lies at the point of the cone or base of the V
• Locating the point or points of origin is critical to the initial investigation of possible causes of the fire
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III. Investigation of Fire Scenes
2. Search for Causes:
• An important objective of the scene investigation is to determine the cause of the fire
• Determining the cause of a fire is basically a reconstruction
• Accidental causes include electrical short circuit, cooking accidents, and careless smoking
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IV. Recovery of Ignitable Liquid Residues from Suspicious Fire Scenes
1. Ignitable Liquid Residues:
• An ignitable liquid is one which can be ignited in the presence of air
• Ignitable liquids are not usually completely consumed in a fire as the liquids soak downwards while the fire burns upwards
• Ignitable liquids and accelerants are not completely interchangeable terms
• An accelerant is a flammable liquid or solid that may have been used to start or sustain a suspicious fire
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IV. Recovery of Ignitable Liquid Residues from Suspicious Fire Scenes
2. Searching for Places to Collect Debris:
• Residual ignitable liquids will most likely be found at or near the point(s) of origin
• Instruments such “Sniffers”, can be used to detect hydrocarbon vapors and help to locate possible sources of ignitable liquids
• Dogs that are trained to detect the odor of common accelerants can also be used
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IV. Recovery of Ignitable Liquid Residues from Suspicious Fire Scenes
3. Collection & Packaging of Debris Samples:
• Samples should be collected from the point(s) of origin
• Fire debris samples must be packaged in airtight containers, such as clean metal cans
• Plastic bags may leak or be punctured
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IV. Recovery of Ignitable Liquid Residues from Suspicious Fire Scenes
4. Collection of Other Physical Evidence:
• Empty cans or containers at or near the scene may have trace amounts of liquid
• Ignition devices
• Fingerprints, footprints, toolmarks, blood etc.
• Ignitable liquid residues help to determine that the fire was intentionally set, but seldom helps to identify the arsonist
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V. Laboratory Analysis of Debris andOther Samples
1. Preparation of Liquid Samples:
• Liquid samples are simply drawn into a special syringe and injected into a gas chromatograph (GC)
2. Preparation of Fire Debris samples:
• There are four commonly used methods to separate or concentrate any ignitable liquids from fire debris samples: heated headspace, steam distillation, carbon strip or tube absorption, and solvent wash
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V. Laboratory Analysis of Debris andOther Samples
i. Heated Headspace:
• The paint can containing the fire debris is punctured with a nail, the hole covered with tape, and the can incubated for 1+ hours at 70-80 °C
• After removing the can from the oven, the headspace vapor is quickly sampled with a gas-tight syringe and immediately injected into a GC
• Advantages: simple with minimal handling
• Disadvantages: not for trace amounts of ignitable liquid residues
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V. Laboratory Analysis of Debris andOther Samples
ii. Steam Distillation:
• Debris is placed into a flask and either water or ethylene glycol added
• The flask is attached to a distillation apparatus and brought to a boil
• The condensate is collected, and if any ignitable fluids are present, it will separate into a second discernible liquid phase
• Disadvantages: time-consuming and risk of contamination
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V. Laboratory Analysis of Debris andOther Samples
iii. Carbon Strip or Tube Absorption:
• Uses activated carbon to absorb hydrocarbon vapors
• A strip is suspended above the debris & the can is heated to vaporize the residues
• Hydrocarbons are removed from the strip by a solvent wash, which can be analyzed directly by GC
• A very sensitive technique
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V. Laboratory Analysis of Debris andOther Samples
iv. Solvent Wash:
• Fire debris is placed into a flask, solvent added, mixed and allowed to sit for a period of time
• The solvent is separated from the debris, then analyzed as with the other approaches
• Works best for high boiling point ignitable liquids that cannot be easily vaporized
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V. Laboratory Analysis of Debris andOther Samples
3. Laboratory Examination of Prepared Samples:
• Gas chromatography separates mixtures of compounds by a partitioning process between a mobile and a stationary phase
• Ignitable fluids such as gasoline produce a very complex pattern of peaks
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V. Laboratory Analysis of Debris andOther Samples
• Different ignitable liquids will produce very different GC patterns
• A library of patterns is obtained from known standards of all possible accelerants
• With fire debris samples, many of the most volatile components tend to be lost
• The GC chromatograms can be compared to give an indication of the type of accelerant used, but the GC should not be considered an identification technique
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V. Laboratory Analysis of Debris andOther Samples
• Individual peaks from the GC can be identified using a mass spectrometer (MS)
• As compounds reach the end of the GC column, they pass into the MS
• The MS bombards the compounds with high energy electrons, fragmenting the compounds and yielding a mass spectrum that can be used to identify the compound
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Classification of Ignitable Liquids
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V. Laboratory Analysis of Debris andOther Samples
• Comparison specimens are samples of the surface or substratum on which ignitable liquid residues might be present
• Natural and synthetic materials may contain volatile compounds that produce GC peaks when heated or extracted
• Specimens should be collected from unburnt areas of the fire scene
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Arson and Explosives
Section 2: Explosives & Explosion Incidents
• Characteristics of Explosives and Explosions
• The Three Major Classes of Explosives
• The Explosive Train or Device
• The Role of the Scene Investigator
• Laboratory Analysis of Explosives and Explosive Residues
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I. Characteristics of Explosives and Explosions
• An explosion is a very rapid chemical reaction that produces heat and gaseous products
• Explosions produce a large amount of heat (an exothermic reaction) in a very short time period
• Molecular fragmentation converts the solid explosive material into an enormous number of gas molecules which will occupy a much greater volume, further enhanced by the very high temperature of the explosion
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I. Characteristics of Explosives and Explosions
• The rapidly expanding gases compress the air creating a physical force known as a shock wave
• It is the shock wave that is responsible for much of the damage associated with an explosion
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II. The Three Major Classes of Explosives
1. Low Explosives:
• Low explosives are materials that burn rapidly and will only explode if contained
• Examples include smokeless powder and fireworks (pyrothechnics)
• Smokeless powders consist of nitrocellulose and black powder
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II. The Three Major Classes of Explosives
2. Primary High Explosives:
• Are used as primers or detonators
• Are sensitive to shock, heat, and electrical spark
• Examples include nitroglycerine, and mercury fulminate & lead styphnate which are used as primers in cartridges and blasting caps
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II. The Three Major Classes of Explosives
3. Secondary High Explosives:
• High explosives do not have to be contained to explode
• Are relatively stable and safe to handle
• Initiation requires an electrical spark, fuse, intense heat, or sharp blow
• Examples include dynamite, TNT, PETN, RDX, & ammonium nitrate
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III. The Explosive Train or Device
• The explosive train is the combination of components needed for a successful explosion (i.e. a bomb)
• An explosive device or train has three primary components:– The igniter starts the event (e.g. a spark)
– The primer or detonator contains a primary high explosive (e.g. blasting cap)
– The main charge is a low explosive or secondary high explosive (e.g. dynamite)
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IV. The Role of the Scene Investigator
• The investigator must sort through all of the debris to find any portion of the explosive device and residue
• The debris is examined and any large pieces that do not appear to have been near the center of the explosion are removed
• The remaining debris is sifted through screens and examined for portions of the explosive device or items that show evidence of being close to the seat of the explosion
• Promising pieces are forwarded to the lab
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V. Laboratory Analysis of Explosives and Explosive Residues
1. Examination of an Unexploded Device:
• The device must first be rendered safe
• A detailed description is then made of each component and how they are connected to form the explosive train
• The chemical nature of the explosive ingredients is then determined, as an investigative lead and possible identification of the manufacturer
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V. Laboratory Analysis of Explosives and Explosive Residues
2. Examination of Exploded Devices & Debris
• The first step is the microscopic examination to locate small specks of explosive material as well as pieces of the original device
• If there is a lack of visible residue, selected pieces are washed with an organic solvent such as acetone, then subjected to chemical screening tests
• A second wash with water may be necessary to collect any inorganic residues
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V. Laboratory Analysis of Explosives and Explosive Residues
2. continued ...
• Chromatography is used to separate and clean components of interest for subsequent instrumental methods of analysis
• TLC or other chromatography methods are often used for this purpose
• Organic residues such as TNT are identified using IR spectroscopy or GC/MS
• Inorganic residues are examined using IR spectroscopy, X-ray diffraction or fluorescence
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V. Laboratory Analysis of Explosives and Explosive Residues
3. Examination of the Device or other Evidence
• Explosive devices are a combination of objects assembled in a particular way
• Examination of the type of wire, the timing devices, and the containment device can provide investigative leads
• Tool marks and fingerprints can be used to associate a person or workshop to a device