Forensic Science in Arson Investigations: Unraveling the Chemistry of Combustion

1. Arson and Combustion

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?

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.

6. Chemical Energy • Chemical reactions involve the making and breaking of chemical bonds. • To break bonds…….absorb energy • To form bonds……create energy

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.

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: www.repp.org/repp_pubs/articles/envImp/Table4

10. Ignition Temperature • “Energy barrier” for methane + oxygen to react is high, so a HIGH temperature is required to “jump start” this reaction.

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.

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?

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

19. Identifying the Accelerant. Field Detection • Portable vapor detector (“sniffer”) • Trained “sniffer” dogs.

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.

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.)

24. Explosives • Undergo RAPID, EXOTHERMIC oxidation reactions and produce large quantities of gas. • 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. 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)

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.

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. 2. Secondary Explosives • majority of explosives for military or commercial use • Burn rather than detonate in open air • Booster + main charge

33. Commercial high (secondary) explosives Most commercial explosives are ammonium nitrate based Booster: Usually PETN Main charge: Water gel, emulsion and ANFO explosives.

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

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

37. Final analysis back at the Lab! • Microscopic examination • Rinse explosives in solvent • TLC • HPLC GC-MS