1. Forensic Science: Arson and Explosives

2. 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

3. 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.

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

5. 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

6. 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

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

8. 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

9. 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.

10. 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

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

12. 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.

13. Analysis of Explosives

14. 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

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

16. 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).

17. 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.

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

19. 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

20. Low Explosives & Black Powder
Explosive with a velocity of detonation less than 1,000 meters per second S.O.S 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

21. 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

22. 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

23. 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.

24. Acetone Peroxide
Popular with terrorists

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

26. RDX (C4)

27. TNT
Secondary explosive. High speed detonation

28. Nitroglycerin (used to make dynamite)

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

30. 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)

31. 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.

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

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

34. 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

35. Griess test
Pink in the presence of nitrites