Chapter 11. Arson and Explosives Section 1: Fire and Arson The Combustion Reaction – Flaming and Glowing Combustions Investigating Suspicious Fires –

Chapter 11

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

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


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


• 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


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


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


• 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

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

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

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

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


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


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


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

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


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


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


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


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


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


• 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


• 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

Classification of Ignitable Liquids


• 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

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

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

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

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


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


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

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)

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

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


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


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


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

Documents

Chapter 11. Arson and Explosives Section 1: Fire and Arson The Combustion Reaction – Flaming and Glowing Combustions Investigating Suspicious Fires –