Using Forensic Chemistry to Identify Substances

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Presentation transcript:

Using Forensic Chemistry to Identify Substances 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.