1. Arson and Combustion
Forensic science begins at the crime scene.

2. Role of the forensic scientist.
Establish a modus operandi and a suspect to which the physical evidence can be linked.
In practice, at an Arson scene……..
detect and identify relevant chemical
materials collected.
Reconstruct and identify igniters or
detonating mechanisms.

3. Chemical reaction of fire.
Oxidation (or combustion) ∆T
CH4 + 2O2  CO2 + 2H20
reactants products
But, methane mixed with oxygen does NOT produce fire!………….Why?
Methane + oxygen  carbon dioxide.

4. Oxidation of iron Not all oxidations produce heat or flames.
Rust is oxidized iron!

5. Additional energy must be added to “get the process going”.
Energy can take many forms:
Heat, electrical, mechanical, nuclear, light, chemical etc.
Energy can also be converted from one form to another .

6. Chemical Energy
Chemical reactions involve the making and breaking of chemical bonds.
To break bonds…….absorb energy
To form bonds……create energy
Draw chemical structure of methane and oxygen.
Some bonds require more energy to break than others…eg. Double bond in oxygen
Think about energy……a chemical reaction is just “energy in/energy out”

7. Chemical reactions
The quantity of heat energy in a chemical reaction comes from the breaking and forming of chemical bonds.

8. EXOTHERMIC: when energy is given off or liberated in a reaction
ENDOTHERMIC: if a reaction requires more energy than it will liberate.
Think of the energy requirement as an “invisible energy barrier”……..draw the diagram with the wee fence. CH4+2O2  CO2 + 2H2O

9. Heat of Combustion
If more energy is liberated than is required to break the different bonds….excess energy.
Excess energy is given off as heat and is called the heat of combustion.
Source:
kJ/g
kcal/g
Btu/llb
Propane
49.9
11.9
21,000
Coal 15
3.6
6,000
Wood
15-27
8,000-14,000
Natural gas
54 approx
13 approx
23,000

10. Ignition Temperature
“Energy barrier” for methane + oxygen to react is high, so a HIGH temperature is required to “jump start” this reaction.
Table 11-2 in textbook shows a table of ignition temps.

11. Common igniters Most common igniter is a lighted match BUT,
Must consider other potential ignition sources eg. Electrical discharges, sparks, chemicals etc.
Must also consider the rate or speed of the reaction.

12. Physical state & temperature of the fire.
Fuel will react ONLY when in the gaseous state.
The vapor burns when it mixes with oxygen and combusts as a flame.
Temperature MUST be high enough to vaporize the fuel.

13. Flash point Lowest temperature at which a liquid gives
off sufficient vapor to form a mixture with
air that will support combustion.
(solid fuel’s are more complex….chemical breakdown process is called pyrolysis)

14. What is “spontaneous combustion”?
Result of a natural heat-producing process in a poorly ventilated area or container.
So, do we always need air (containing oxygen) for combustion/oxidation reactions to occur?
Eg. Bacteria multiply and produce heat in hay in barns causing fires
YES!!!!!!!!!!

15. Summary 3 requirements for combustion 1. Fuel 2. Available oxygen
3. Heat to initiate combustion
with sufficient heat to sustain
the reaction.

16. Fire Causes

17. Common Accelerants

18. Searching the Fire Scene
Necessity for immediate investigation takes precedence over the requirement to obtain a search warrant.
Focus on finding the origin of the fire,
then,
are they separate or connected?
Locate containers that held the accelerant
Evidence of signs of breaking and entering
Time is working against the investigator……………petroleum based accelerants will evaporate rapidly.
Often “streamers” are used to connect seperate fires
Once the point of origin is located it should be PROTECTED and RECORDED (notes, sketches, photographs)

19. Identifying the Accelerant.
Field Detection
Portable vapor detector (“sniffer”)
Trained “sniffer” dogs.
Portable vapor detector (“sniffer”) – records the presence of vapor as an increase in temperature which is measured as deflection on the meter.
Dogs are trained to recognize smells of different hydrocarbons.

20. Collection and Preservation of Arson Evidence
1.Collect 2-3 quarts of ash and soot debris
2. Package in airtight container.
3. Controls from similar but uncontaminated areas at the fire scene.
Ash should contain all areas though to be points of origin and substances thought to be accelerants.
No loss due to evaporation/usually use clean paint cans (low cost and airtight). NOT plastic bags as plastic reacts with the hydrocarbons.
Common ignitors : match, cigarette, firearms, mechanical/electrical sparking device, Molotov cocktail (can often be recovered)

21. Vapor concentration
Increases the sensitivity 100 fold over the “headspace” conventional technique.

22. Analysis of Flammable residues.
G.C is the most sensitive and reliable method. (If we have a mixture we must use GC-MS.)
Heat the airtight container and trap accelerant in the headspace//remove with a syringe //inject in GC and identify by RETENTION TIME.
DISADAVANTAGE……size of the syringe limits the volume of vapor so use vapor concentration

24. Explosives
Undergo RAPID, EXOTHERMIC oxidation reactions and produce large quantities of gas.
Explosives MUST have their own source of oxygen.
Detonation occurs so rapidly that oxygen in the air cannot participate in the reaction………..therefore explosives MUST have their own source of oxygen.

25. Explosives need extra oxygen!
The hot cigar!
If excess oxygen is made available (by soaking the cigar in liquid oxygen) the flame will burn hotter and longer.
This is the principle underlying explosive mixtures.

26. Explosives using potassium chlorate.
Explosive mixtures contain oxidizing agents to provide extra oxygen eg. potassium nitrate or potassium chlorate.
Explosives using potassium chlorate. OR
Have oxygen and fuel components combined in one molecule

27. Nitroglycerin

28. Fireworks are mini-explosions!
Potassium chlorate, icing sugar (the “fuel”) and a metal nitrate (NaNO3, Sr(NO3)2, Ba(NO3)2) are the ingredients for basic fireworks.
The color of the flames is dependent on the metal nitrate added (Bengal lights experiment).

29. Types of Explosives
An explosion occurs at a rapid rate but the speed of decomposition varies greatly allowing classification.
high (high speed)
….speed of detonation
low (low speed) explosives
…..speed of deflagration (burning)
Detonation…creation of supersonic shockwave within the explosive charge.
Deflagration….characterized by very rapid oxidation that produces heat, light and subsonic pressure wave.
Most bombing incidents involve homemade explosives and incendiary devices…….parts can be traced and identified.
Explosion is a product of combustion accompanied by creation of gases and heat.

30. Low explosives Usually black powder and smokeless powder.
Very accessible to the public
Ingredients:
Fuel + oxidizing agent (eg. Potassium chlorate)
Safety fuse – black powder wrapped in fabric or plastic casing.
Low explosives decompose at relatively slow rates. …causes a “propelling or throwing action” that makes them suitable for ammunition or skyrockets.
BUT,
If confined they can explode with a force as lethal as any other explosive.
Fuel can be SUGAR + potasium chlorate+ fuse -> small container (pipe)->. Bomb
Don’t always need a match……heat from a chemical reaction can cause ignition ie. Add sulfuric acid!
SMOKELESS POWDER is the safest and most powerful low explosive………….mix with NITROCELLULOSE (single based powder) or nitroglycerin and nitrocellulose ( double-based powder).

31. Two Categories of High Explosives
1. Primary Explosives
very sensitive to heat shock or friction -> violent detonation
Used as primers eg. lead azide, lead styphnate

32. majority of explosives for military or commercial use
2. Secondary Explosives
majority of explosives for military or commercial use
Burn rather than detonate in open air
Booster + main charge
Primary explosives are rarely used as the main charge due to sensitivity to heat shock or friction //primers will detonate other explosives through a chain reaction
Rarley used in homemade bombs
examples lead azide, lead styphnate, diazodinitrophenol.
2. Majority of explosives are secondary ie. dynamite., TNT, PETN, RDX. etc.
Most commercial explosives are:
AMMONIUM-NITRATE based (water gels, emulsions and ANFO explosives).
Formulation of ammonium nitrate + sodium nitrate gelled with natural polysaccaharide gum and contain a combustible material (ALUMINIUM) mixed with a gel as the explosives fuel.

33. Commercial high (secondary) explosives
Most commercial explosives are ammonium nitrate based
Booster:
Usually PETN
Main charge:
Water gel, emulsion and
ANFO explosives.
VERY STABLE.
Ammonium nitrate is fertilizer …widely available……1993 bombing of the World Trade Center used a homemade bomb (ANFO explosive).5 men arrested for the crime in their hideout were making the ANFO mixture!
ANFO explosives:
- ammonium nitrate soaked in fuel oil.
Water gels are used for blasting under wet conditions/ consistency like toothpaste.

34. Military high (secondary) explosives
Booster:
RDX: most common and very powerful.
PETN: detonating cord (primacord) used in TNT mixtures for small caliber projectiles (eg. grenades)
Main charge:
TNT: used alone or in combination for shells, bombs, grenades, demolition explosives etc.
Dynamite or binary explosives
RDX…pliable plastic dough-like consistency is known as C-4
Military dynamite contains NO NITROGLYCERIN…..mixture of TNT and RDX.
Initiating explosion for high explosives are blasting caps of copper or aluminum cases filled with lead azide and PETN or RDX as the detonating charge.

36. Collection and Analysis of Explosives
1. Look for the presence of a crater
2. Wear PPE to avoid contamination
3. Systematically search the area
- wire-mesh screens for sifting through debris
- EGIS system
All loose soil and debris from the interior as well as around the crater
Documentation//EGIS – vacuum which collects vapors from “explosive residues”
Analysis using the GC

37. Final analysis back at the Lab!
Microscopic examination
Rinse explosives in solvent
TLC
HPLC
GC-MS
Microscopy//might distinguish between black powder and smokeless powder due to their shape
Rinse with acetone//explosives will dissolve in the solvent
TLC//search database for Rf values to identify nitroglycerin, RDX, PETN etc
HPLC//sensitive to detect any trace explosives / occurs at room temp so explosives will not decompose.