Arson and Explosives

Using Forensic Chemistry to Identify Substances

Arson – Incendiary Fires• Accelerant – something used to promote

and spread a fire– Solid accelerants include paper, trash,

highway flares, black powder, paraffin and an oxidizer, or sugar and chlorate.

– Liquid accelerants include petroleum products, alcohols, paint thinners, industrial solvents, and ether. Either “sloshed about” or used in firebomb (Molotov cocktail).

– Gaseous accelerants include propane and natural gas (disconnected gas line)

Incendiary Device

• Combine a means of ignition with a time delay.– Candle is allowed to burn down to come into

contact with accelerant.– Fuses, flares, matches with lit cigarette, chemical

mixes, electronic devices.

Collection of Potential Evidence

• Use clean, vapor-tight containers.

– Screw-cap glass jars, metal cans, unused paint cans with tight-fitting lids

• Care must be taken to avoid loss of evidence due to evaporation, and to avoid cross-contamination of exhibits

Search for Evidence

• Some materials will survive the fire• Start at point of origin (determined by arson

investigator)– Wires, batteries, bottles, wax, soap (used to “gel”

flammable liquid), ash residues, unburned residual liquids

Testing Evidence• Gas Chromatography

– Heat airtight container to drive volatile residues from collected evidence

– Remove vapor with syringe and inject into GC– Unburned liquids can be removed from

evidence by steam distillation, vacuum distillation, solvent extraction, solvent rinsing, or air flushing.

– Compare resulting chromatogram with those of known substances

Explosives• 1920s - dynamite bombs used by political

anarchists aimed at rich and powerful, also used by unions

• 1930s – the mob used stink bombs in theaters and restaurants to “persuade” them to sell

• November 1, 1955 – midair explosion of United Airlines flight 629 – first bomb aimed at “general public”

Bomb Investigations

• Investigation of flight 629 was very similar to how such investigations are carried out today

• Large grid was established (pieces were scattered over a 5-mile radius)

• Wreckage was pieced together, and it was determined what type of bomb was used (from residues) and where it exploded

Types of Explosives• Low explosives – cause relatively small damage

– Explode at a few thousand feet per second– Emit low frequency sound (puff or boom)– Examples are gasoline and gunpowder

• High explosives – cause large damage– 25,000 feet per second– High-frequency blast– Examples are dynamite and nitroglycerin

• Military Explosives – TNT (trinitrotoluene), RDX, PETN (Pentaerythritol Tetranitrate C5H8N4O12)

• Commercial Explosives – black powder, ANFO (ammonium nitrate-fuel oil), dynamite, nitrostarch

• Improvised Explosives – usually low explosives (require confinement) – homemade black powder, fuel mixed with an oxidizer.

Crime Scene Searches

• Focus on blast seat• Diligent search for remnants must be

performed– Explosive residues, metal fragments, fuse or

blasting cap, wire and/or insulation, electrical tape, batteries, clocks or timers

• FBI has extensive databases (batteries, detonators, accessories, timers, radio-control devices

Laboratory Tests

• Microscope – locate unconsumed explosive• Acetone is used to extract soluble explosives

from debris – TLC or HPLC• Evidence is screened with an “explosives

detector” – a special GC that identifies known explosives.