Ch 16 Arson

Fuel temp

*ignition temperature of fuel is always considerably higher than flash point* -the rate of chemical reaction increases when temp raised -10C(18F) rise in temp =doubles/triples

Telltale signs of Arson

-"streamers" to spread fire (trail of gasoline or paper) -fire moves up-> look at lowest point with most intense burn -v shaped pattern -more severe burns on floor than ceiling indicates accelerants

Searching Fire Scene

-#1 priority =origin of fire -objects of suspicion=containers, ignition devices, irregular shaped patterns on floor -nothing should be touched or moved before notes, sketches and photographs taken Considerations: -drafts&winds -collapsing floors and roofs -stairways, elevator shafts

Collection/Preservation

-2-3 quarts of ash and soot debris from point of origin -include all porous materials and other substances thought likely to contain flammable residues (wood flooring, rugs, upholstery, rags)

Analysis of Flammable residue

-GC-most sensitive instrument for detecting flammable residue -headspace technique -vapor concentrations -sometimes GC-MS is better bc it can simplify chromatographic pattern by filtering out ions

Endothermic Reaction

-a chemical transformation in which heat energy is absorbed from the surroundings

Spontaneous Combustion

-a fire caused by natural producing process in the presence of sufficient air and fuel *in a poorly ventilated area or container

Physical state of fuel

-achieves reaction rate with oxygen sufficient to produce a flame only when its in the gaseous state -bc molecules collide frequently enough to support a flaming fire (ex: conversion of iron to rust proceeds slowly bc iron atoms cant achieve gaseous state)

Substrate control

-all materials suspected of containing volatile liquids must be accompanied by a thorough sampling of similar but uncontaminated control specimens from another area of the fire scene -helps decipher how substance would react to burning ex:plastic floor tiles, carpet, linoleum and adhesives produce volatile hydrocarbons when burned which can be mistaken for an accelerant

Timeliness of Investigation

-any accelerant residue after a fire is extinguishable may evaporate w/i a few days/hours -safety and health concerns -degradation of accelerants in soil and vegetation --> freeze them *the need to begin an immediate investigation takes precedents over warrants*

Hydrocarbon

-any compound consisting of just carbon and hydrogen -most petroleum distillates(gasoline and kerosene) compounded on them *when vapor injected into GC separates into components*

Accelerant

-any material used to start or sustain a fire *most arsons started w/ petroleum-based accelerant (gasoline/kerosene)*

Classifying ignitable liquids

-by boiling point range -related to the # of carbon molecules in mixture of hydrocarbon that makes up carbon molecule -or use retention times of known hydrocarbon substances -cant identify gasoline brands bc changing market and agreements

Vapor Concentrations

-charcoal coated strip placed inside container with debris, then heated to 60C for hour -charcoal absorbs accelerant vapor (significant quantity gets trapped over time) -wash with carbon disulfide and inject in GC (100x more sensitive than headspace)

Oxidation

-combination of oxygen with other substances to produce new substances -some forms appear as fire others do not -ex: o2 and metals-->rust -some require a match to initiate, while others are produced at room temp (bc of energy)

Searching for accelerants

-combustible liquids not usually entirely consumed during a fire -liquid will seep into porous surface....enough remain unchanged to permit its detection in crime lab **water doesn't interfere with the lab methods used to detect and characterize flammable liquid residues**

Flammable Range

-concentration range between upper and lower limits -entire range of possible gas or vapor fuel concentrations in air that are capable of burning EX:gasoline 1.3-6% (so gas must be at least 1.3% and no more than 6% in mixture)

Headspace technique

-heat airtight container w/ sample--> any volatile residue in debris will be driven off and trapped in enclosed air space then removed with syringe

Igniter of Fuels

-match is most typical -electrical discharges -sharks and chemicals (all of these have temps higher than most fuels ignition temps) -burning cigarettes -firearms -ammunition -mechanical match striker -molotov cocktail(glass w/ flammable liquid and cloth rag)

Insulators (poor conductors)

-materials with electrons firmly attached to their molecules ex: wood, plastic, paper *heat emanating from it doesnt spread well, and dont cause ignitions far from the initial source*

Flash Point

-minimum temp @ which a liquid fuel produces enough vapor to burn -once reached the fuel can be ignited by some outside source of temperature to start a fire

Conduction

-movement of heat through a solid object *heat always travles from hot areas of a solid to cold ones

Heat

-once combustions starts enough heat is liberated to keep the reaction going fire becomes chain reaction absorbing portions of its own liberated heat to generate even more heat *Fire burns until either oxygen or fuel is exhausted*

Packaging/ preservation

-packaged in airtight containers so residues dont evaporate -use new clean paint cans with friction lids-->low cost, airtight, unbreakable, various sizes available -wide mouthed glass jars w/ airtight lids also good *only fill cans+jars 1/2 -2/3 full (air space above debris) -dont use plastic polyethylene bags...they can react with hydrocarbons and allow vapors to be depleted *fluids found in open bottles/cans must be collected and sealed...might contain trace amounts of liquid or vapors

Speed of reaction

-rate of speed @ which a reaction takes place -faster molecules move the greater # of collisions thus faster rate of reaction -reflected by the production of noticeable quantities of heat and light ***all oxidations liberate energy but not all are accompanied by a flame--> thus doesn't explain all the characteristics of reaction

Exothermic Reaction

-reaction in which more energy is liberated than is required to break the chemical bond between atoms *reaction in which heat is liberated* -heat is liberated during combustion

Glowing Combustion

-smoldering -combustion on surface of a solid fuel in the absence of heat high enough to pyrolyze the fuel -woods carbonaceous residue continues to smolder long after the flame itself is extinguished.

Conductors of heat

-solids whose atoms or molecules have loosely held electrons ex: metals *heat may be transported through them--> any fuel in contact with one may be ignited (creating a new fire location)

Detection using "sniffer" sensitive portable vapor detector

-sucks in air surrounding sample -air is passed overheated filament combustible vapor persists it oxidizes -immediately increases temp of filament -registered as a deflection on the detector's meter *not conclusive--> screening device)

Ignition Temperature

-the minimum temperature @ which a fuel spontaneously ignites

Convection

-transfer of heat energy by movement of molecules with liquid or gas -boiling water~as molecules on bottom heat and move faster they break apart becoming less dense causing them to move up -denser cooler water molecules then migrate to the bottom of pot

Radiation

-transfer of heat energy from a heated surface to a cooler surface by electromagnetic waves -hot surface emits electromagnetic waves of various wavelengths--> moves in a straight line from 1 object to another *this heat is a key role in understanding how a fire spread through a structure* -surface that face fire are exposed to this heat and burst into flames when surface reaches ignition temperature

ILRC Ignitable liquid reference collection

-useful collection showing chromatographic patterns =500 ignitable liquids

3 requirements must be satisfied to initiate and sustain combustion

1. a fuel must be present 2. oxygen must be available in a sufficient quantity to combine with fuel 3. heat must be applied to initiate the combustion and sufficient heat must be generated to sustain reaction

Factors that influence the rate of these collisions

1. physical state of fuel 2. fuel temp

Modus operandi

an offender's pattern of operation

Why is burning so rapid?

as fire spreads it raises the temp of fuel-air mix thus increasing rate of reaction; generating more heat again increasing the rate of reaction

3 mechanisms of heat transfer

conduction, radiation, convection

Fuel-Air Mix

if fuel concentration is too low or too high combustion doesnt occur

Combustion

rapid combination of oxygen with another substance accompanied by production of noticeable heat and light -*energy comes from heat*

Energy

the ability or potential of a system or material to do work -quantity of heat from a chemical reaction is from breaking and forming of chemical bonds -molecules most absorb it to break apart their chemical bonds then they liberate it when their bonds are reformed -forms: heat, electrical, mechanical, nuclear, light, chemical

Gas Chromatography

separates hydrocarbon components of these liquids and produces a chromatographic pattern of the product

Pyrolysis

the decomposition of solid organic matter by heat *a solid burns only when exposed to heat intense enough to turn a solid into gaseous products*

Chemistry of fire

transformation process in which oxygen is united with some other substance to produce a noticeable quantities of heat and light (a flame)