analysis of fire debris and arson investigations

arson in the United States

• when arson is incendiary/intentional can be many possible motives

• insurance, fraud, cover up other crimes, hate crime, pryomania, revenge

Forensic Chemistry

• arson investigations almost always part of forensic chemistry unit of forensic laboratory

• arson investigation

• explosive and incendiary residues

fire investigator

• primary role of fire investigator is to determine cause of fire (not necessarily who did it)

• accidental (explainable, possibly due to negligence)

• natural (lightning, other weather)

• incendiary (intentional act)

• unknown (unable to be identified)

fire tetrahedron

needs

• heat

• oxygen

• and fuel

this is all a chain reaction

chemistry of fire

• oxidation: the combination of oxygen with other substances to produce new substances

• pyrolysis : the decomposition of solid organic matter by heat

types of heat transfer

conduction: movement of heat through a solid object

radiation: transfer of heat energy by electromagnetic radiation

convection: transfer of heat energy by movement of molecules within a liquid or gas

causes of fire

• fuel source is a critical aspect starting a fire (and risk of a fire breaking out)

flash point

lowest temperature of a liquid at which still emits a vapor to form an ignitable mixture with air

• gasoline

• aerosol

• acetone

• candles

• alcohol

auto-ignition temperature

minimum temperature required to ignite a gas or vapor in air without a spark or flame being present

accelerant

anything that increases the spread and intensity of a fire

fire investigation

in suspected arson case, there are specific steps and/or procedures for first responders, investigators, and lab personnel

• for specific investigative questions around arson, subset of procedures

point of origin

usually critical for arson investigation as it can easily indicate whether crime was committed

• typically examining the spatial patterns of burned and unburned materials

• fire follows the path of least resistance, sometimes creates a v-shaped pattern

• function of materials, geometry of scene

what physical evidence can you collect from an arson scene?

• accelerants

• any object that has come into contanct with a fuel/accelerant could still have that compound on it or compounds related to it

evidence collection

• packing evidence that might contain flammable liquids and/or gases can be tricky

• dissipates with time and eventually evaporate if not stored properly

• evidence is then sent to the forensic laboratory, normally the chemistry unit

evidence analysis

• once compounds are extracted they are characterized by gas chromatography

• separates compound mixtures and identifies individual components by molecular weight

• separation based on interactions with a column, stationary vs. mobile phase

• ignitable liquids are usually classified rather than identified

• laboratory does NOT determine if arson was committed, only if ignitable liquids are present with evidence and/or crime scene

ignitable liquids

8 categories:

• gasoline

• petroleum distillates

• isoparaffinic products

• aromatic products

• naphthenic-parafiinic products

• normal alkanes products

• oxygenated solvents

• miscellaneous products

light, medium, and heavy usually refer to the moleduclar weight of compounds in the sample

can be several reasons for no chemical evidence after arson

• no ignitable liquid used

• all liquid consumed in the fire

• fire fighting washed away products

• compounds evaporated prior to analysis (due to improper packaging or too much time before evidence collection)

three types of explosions

• chemical: oxidation reactions

• nuclear: fusion or fission reactions

• mechanical: physical reaction, usually high pressure gases

chemistry of explosions

• fire vs explosion: explosion is accompanied by rapid expansion of gases

• deflagration: very rapid oxidation reaction accompanied by low-intensity pressure wave

• detonation: extremely rapid oxidation reaction accompanied by a violent disruptive effect and an intense high-speed shock wave

low explosives

• explosives with a velocity of detonation less than 1000 m/s

• produce a propelling action, which is what makes them good propellants for ammunition: black powder mixture of potassium nitratre or sodium nitrate, charcoal, and sulfur

• smokeless powder: nitratred cotton or nitrocellulose (single-based) or nitroglycerin mixed with nitrocellulose (double based)

• chlorate mixtures: potassium chlorate

• gas-air mixtures: natural gas, methane

high explosives

• explosive with veloctiy of detonation greater than 1000 m/s

• dynamite, TNT, ammonium nitrate, RDX

• primary explosives: ultra sensitive to heat, shock, friction: used to detonate other explosives

• secondary explosives: burn rather than detonate, most hight explosives

connection and analysis of explosive evidence

• need to collect any undetonated residues of the explosive left at the scene

• sign of explosive is the presence of a crater

• collect any loose soil or debris, objects near site of detonation

• detonating mechanism?

• collection and preservation is much the same as in fire investigations