Unit 5: Cases Involving Arson and Explosives

Chemistry of Fire

1. Fundamental Concepts of Combustion

Combustion is an exothermic, self-sustaining chemical reaction involving the rapid oxidation of a fuel in the presence of an oxidizer, releasing heat and light energy.

Fire Tetrahedron Components: Fuel + Oxidizer (Oxygen) + Heat + Uninhibited Chemical Chain Reaction

If any single element of the fire tetrahedron is removed, the combustion process ceases.

2. Key Thermal Parameters

• Flash Point: The lowest temperature at which a liquid fuel produces sufficient vapor near its surface to form an ignitable mixture with air, producing a brief flash upon application of an ignition source.
• Fire Point: The temperature at which a fuel produces sufficient vapors to sustain continuous burning for at least 5 seconds after ignition.
• Autoignition Temperature: The minimum temperature required to initiate self-sustained combustion of a substance without an external spark or flame.

3. Thermal Properties Summary

Collection and Preservation of Arson Evidence

1. Evidence Packaging Guidelines

Ignitable liquid residues (ILRs) consist of volatile organic compounds (VOCs). Improper packaging leads to vapor loss through evaporation or diffusion.

• Approved Containers: Clean, unused paint cans with friction-fit lids; specialized vapor-impermeable bags (e.g., Nylon bags); glass jars with PTFE-lined caps.
• Prohibited Containers: Standard polyethylene plastic bags (volatile compounds permeate plastic or leach plasticizers into the sample).

2. Sampling Strategy

1. Collect samples from the point of origin, porous materials (carpets, wood, soil), seams, and unburned edges.
2. Collect negative control samples from unaffected areas of the same material to rule out background matrix interference.
3. Fill containers to no more than 60-70% capacity to leave an adequate headspace volume for analytical sampling.

Analysis of Fire Debris

1. Vapor Extraction Methods

Prior to instrumental identification, volatile residues must be isolated from solid fire debris matrixes.

Analysis of Ignitable Liquid Residue

1. Instrumental Analysis

Gas Chromatography-Mass Spectrometry (GC-MS) is the standard analytical technique for identifying ignitable liquid residues (ILRs).

2. Classification of Ignitable Liquids

Accelerants are classified according to ASTM E1387 and E1618 standards based on boiling point ranges and chemical composition:

• Gasoline: Highly aromatic composition with characteristic alkylbenzene and naphthalene patterns.
• Medium Petroleum Distillates (MPD): Mineral spirits, paint thinners (carbon range C8-C13).
• Heavy Petroleum Distillates (HPD): Diesel fuel, kerosene, heating oil (carbon range C9-C23).
• Isoparaffinic Products: Commercial solvents dominated by branched alkanes.
• Oxygenated Solvents: Alcohols, ketones, and esters.

Scientific Investigation and Evaluation of Clue Materials

1. Determining Point of Origin

Fire investigators evaluate physical indicators to locate the origin of the fire:

• V-Patterns: Upward and outward spread of fire on vertical surfaces pointing back down to the origin point.
• Depth of Charring: Deeper charring on wooden structures indicates longer exposure to thermal energy or presence of accelerants.
• Alligatoring: Large, shiny char blisters indicate rapid, intense burning, often associated with accelerants, though not definitive on their own.
• Low-Level Burning: Floor burning or puddle-shaped burn patterns suggest liquid accelerant pour patterns.

Information from Smoke Staining

1. Smoke Deposition Patterns

Soot deposition provides crucial evidence regarding fire dynamics, ventilation, and movement:

• Lines of Demarcation: Clean boundaries indicate sudden thermal changes or structural barriers during the fire.
• Directionality of Flow: Heavy soot coatings on specific sides of structural members indicate the directional path of smoke and hot gases.
• Thermal Fracturing: Crazed or web-like glass cracking patterns indicate rapid heat buildup, while soot coatings on inside glass surfaces indicate slow, oxygen-starved smoldering.

Classification of Explosives

1. Broad Categorization

Explosives are substances that undergo rapid chemical decomposition, producing large volumes of gas and high thermal energy.

Deflagration: Subsonic combustion propagation through thermal conductivity (speed < 343 m/s).

Detonation: Supersonic shock wave-driven propagation through chemical reaction front (speed > 1000 m/s).

2. Explosive Types Comparison

Synthesis and Characteristics of TNT, PETN, and RDX

1. Trinitrotoluene (TNT)

• Chemical Formula: C7H5N3O6
• Characteristics: Yellow crystalline aromatic nitro compound; highly stable, insensitive to mechanical shock, melting point approximately 80.1°C, detonation velocity approximately 6,900 m/s.
• Conceptual Chemical Synthesis: Produced conceptually via multi-stage electrophilic aromatic nitration of toluene using nitrating acid mixture (nitric acid and sulfuric acid): Toluene + HNO3/H2SO4 → Mononitrotoluene → Dinitrotoluene → Trinitrotoluene.

2. Pentaerythritol Tetranitrate (PETN)

• Chemical Formula: C5H8N4O12
• Characteristics: Nitrate ester derivative; white crystalline powder, highly sensitive to shock and friction compared to TNT, detonation velocity approximately 8,400 m/s. Used in detonating cord and booster charges.
• Conceptual Chemical Synthesis: Produced conceptually by the esterification nitration of pentaerythritol alcohol using concentrated nitric acid: C(CH2OH)4 + 4 HNO3 → C(CH2ONO2)4 + 4 H2O.

3. Cyclotrimethylenetrinitramine (RDX)

• Chemical Formula: C3H6N6O6
• Characteristics: Nitramine heterocyclic high explosive; white crystalline solid, high stability in storage, high energy output, detonation velocity approximately 8,750 m/s. Primary constituent in plastic explosives (e.g., C-4).
• Conceptual Chemical Synthesis: Produced conceptually by the nitrolysis of hexamethylenetetramine (hexamine) using concentrated nitric acid or nitric acid-ammonium nitrate mixtures.

Mechanism of Explosion Process

1. Explosion Dynamics

An explosion involves a sudden conversion of chemical potential energy into mechanical work through four steps:

1. Rapid Chemical Reaction: Highly exothermic decomposition occurring within microseconds.
2. Gas Generation: Extremely rapid conversion of solid or liquid reactants into hot gaseous products (CO, CO2, H2O, N2).
3. Thermal Expansion: Reaction heat expands generated gases to thousands of times their original volume.
4. Pressure Wave Propagation: Expanding gases compress surrounding air, establishing a propagating pressure front.

Blast Waves

1. Wave Structure

A blast wave consists of a steep-fronted shock wave moving outward from the detonation epicenter.

• Positive Pressure Phase: Immediate pressure spike reaching peak overpressure (Pmax), causing primary blast injury and structural displacement.
• Negative Pressure Phase (Suction): Partial vacuum created behind the advancing blast front causing reverse air flow, pulling debris back toward the detonation seat.

Formula for blast shock pressure over time t:

P(t) = P0 + Pmax * (1 - t / T) * e^(-k * t)

Searching the Scene of Explosion

1. Field Operations Protocol

1. Safety Assessment: Render safe procedures for secondary unexploded devices, structural collapse, and toxic gas hazards.
2. Perimeter Establishment: Outer perimeter established at 1.5 times the distance to the farthest detected fragment.
3. Locating the Seat of Explosion: Identify the blast crater or point of primary impact.
4. Systematic Search Patterns: Utilize grid, spiral, or line search patterns from the perimeter inward to the epicenter.

Post Blast Residue Collection and Analysis

1. Evidence Collection Techniques

• Swabbing non-porous surfaces (metal, plastic) using solvent-moistened cotton or glass fiber swabs (e.g., isopropanol or acetone).
• Collecting soil samples from the epicenter/crater and reference samples from outside the blast zone.
• Preserving fragment evidence, detonator wires, and timer components for trace analysis.

2. Analytical Techniques