Forensic Science: Arson and Explosives

Oxidation Reactions Is the combination of oxygen with other substances to produce new substances Are reactions that exothermic (give off heat) Associated with fire Not all oxidation reactions result in flames! – rusting – metabolism Rate of reaction determines the nature of the reaction Chemicals that supply oxygen are known as oxidizing agents

Requirements for Combustion 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 a reaction. Ignition temp is the minimum temperature at which fuel vapor will ignite Accelerant: any material used to start or sustain a fire.

Gibbs free energy The exothermic nature of a reaction can be estimated by the ∆G Example - hydrogen combusted in oxygen -237.1 kJ/mole thermite reaction -847.6 Al + Fe2O3 Hess’s law can be used to detemine the overall enthalpy of the reaction and determine if it will be negative = spontaneous

Spontaneous Combustion A natural heat-producing process that may give rise to a fire in the presence of sufficient air & fuel Ex.) hay stored in barns growing medium for bacteria which generate heat Ex.) rags soaked w/ highly unsaturated oils, such as linseed oil

Known oxidizers Provide a source of oxygen or electron receptance. Will support a fire and accelerate it. Nitrate based NO3 potassium nitrate, ammonium nitrate Chlorate based ClO3 or perchlorate based ClO4 OCl− hypochlorite MnO4 permanganate I2 iodine H2O2 peroxide CrO42− chromate

Known accelerants Typically volatiles or other highly inflammable substances Gasoline, kerosine, ethanol, ether, etc. Paper, nitrated paper , wood, magnesium shavings

Gasoline Residues Are best characterized by gas chromatography Hydrocarbon: any compound consisting only of carbon and hydrogen Thin-layer chromatography is also useful for screening debris Unevaporated gasoline 90% evaporated gasoline Unevaporated kerosene Gasoline Residues

Examination of evidence recovered from an arson Presence of gasoline, kerosene or turpentine in debris Liquid gasoline may be compared for origin Criminalist may be able to reconstruct the ignition mechanism used You can use the following evidence to determine from a lab examination.

Collecting evidence Most evidence will be burnt or partially burnt. Capturing volatiles: Can be trapped with activated charcoal that is sealed in equilibrium with sample. Vocab: dynamic headspace - purge sample with argon or nitrogen gas and have volatiles trapped in a solid charcoal matrix. Can later be eluted with a solvent and injected into GC/MS. passive headspace - heats the sample and the residue collects on a carbon strip. Residue collected injected into GC/MS static headspace – heats sample and gas collected at top of container and directly analyzed by GC/MS

Activated carbon Enormous surface area 500 m2, with 1500 m2 / gr possible Holds organics with reversible van de Waals forces

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.

Analysis of Explosives

Introduction Most bombing incidents involve homemade explosive devices There are a great many types of explosives and explosive devices Lab must determine type of explosives and, if possible, reconstruct the explosive device

Goals of Investigation Identify victims Identify explosive Recover bomb and timing device Determine method of delivery

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 in the field or laboratory is the ion mobility spectrometer (IMS).

Back 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, high-performance liquid chromatography, and gas chromatography-mass spectrometry. Confirmatory identification tests may be performed on unexploded materials by either infrared spectrophotometry or X-ray diffraction.

Chemistry of Explosion Essentially a combustion reaction - like a fire Major difference is speed of reaction Damage caused by rapidly escaping gases and confinement

Types of Explosives Low explosives Escaping gases up to about 3000fps Crucial element is physical mixture of oxygen and fuel Examples are black and smokeless powders Black powder is mixture of potassium nitrate, charcoal and sulfur Smokeless powder is nitrocellulose and perhaps nitroglycerine

Low Explosives & Black Powder Explosive with a velocity of detonation less than 1,000 meters per second. S.O.S . 340 m/sec Ex.) Smokeless powder (nitrocellulose)- nonmilitary issued Black powder Flash powder Black Powder ingredients: potassium nitrate, carbon & sulfur Potassium chromate + Sulfuric acid + sugar can create a low explosive

Types of Explosives High explosives Velocity of escaping gases up to 10,000fps Oxygen usually contained in fuel molecule Two types Initiating - Sensitive, will detonate readily when subjected to heat or shock. Used to detonate other explosives in explosive train - primers and detonators Noninitiating - relatively insensitive, requires heat or shock. Includes TNT or PETN, RDX

Military Explosives - High explosives RDX (cyclotrimethlylenetrinitramine) the most powerful & popular In the form of pliable plastic of dough-like consistency composition C-4 PETN is used in TNT mixtures for small-caliber projectiles and granades

The Explosive Market In recent years, nitroglycerin-based dynamite has all but disappeared from the industrial explosive market and has been replaced by ammonium nitrate-based explosives (i.e., water gels, emulsions, and ANFO explosives). 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.

Acetone Peroxide Popular with terrorists

PETN 100 grams can blow up a car - needs a detonator Detected with reagents that target nitrates

RDX (C4)

TNT Secondary explosive. High speed detonation

Nitroglycerin (used to make dynamite)

Types of Explosives Ammonium nitrate based Water gels Emulsions ANFO’s

Primary (Initiating) High Explosives used in detonators Lead azide, lead styphnate and diazodinitrophenol Major ingredient of blasting cap Not used as main charge b/c so sensitive; however, are used to detonate noninitiating explosives PETN (pentaerythritol tetranitrate) TNT (trinitrotoluene) Dynamite (nitroglycerine in kieselgur) Smokeless powder (nitrocellulose)

Explosions Among the high explosives, primary explosives are ultrasensitive to heat, shock, or friction and provide the major ingredients found in blasting caps or primers used to detonate other explosives. Secondary explosives are 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.

Lead Azide - Used as a primer for bullets. - Can be initiated by heat or shock - Detect with reagents that detect lead

Analysis of Explosives Microscopy Thin layer chromatography Visualise with Greiss reagents Infrared spectrophotometry, terahertz spectroscopy (new) Detonator fragments

Scenario- Murrow building ANFO explosive and timer packed into a rented truck, which was then parked outside Murrah building Explosive confined to closed space such as truck is much more powerful Resulting explosion resulted in severe damage to building and loss of more than 100 lives

Griess test Pink in the presence of nitrites