warm-up december 1, 2014 what is combustion? what are the three types of heat transfer?
TRANSCRIPT
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Warm-UpDecember 1, 2014
What is combustion? What are the three types of heat transfer?
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Objective
SWBAT identify the point of origin and be able to interpret identifying marks of accelerants in the debris of a fire.
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Agenda
1. OKC Bombing Documentary
2. Debris of Fire
3. Arson
4. Burn Patterns
5. Exit Slip
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The Problem with Arson
One of the fastest growing crimes in US Arson for its own sake – pyromania Arson to cover up other crimes such as
murder or embezzlement Not hard to determine if a fire is arson Difficult to determine who caused the fire
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What Is Fire?
Essentially a combustion reaction with a fuel, oxygen and sufficient temperature. This is the fire triangle.
2C8H18 + 25O2 = 18H2 O + 16CO2
An explosion and a fire involve the same process. – The only difference is the speed of the reaction.– Combustion may cause an explosion if confined
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The Fire Triangle
oxygen
heat fuel
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Types of Fires
Burning or flaming fire– Sufficient oxygen and fuel– Flames and smoke visible
Smoldering fire– Oxygen deprived– Can burst into explosive flame if oxygen is
suddenly supplied
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OKC Documentary6. Why did the FBI dismiss John Doe 2 and
what do the body shop witnesses say?7. Why was McVey already in jail? 8. Why does Jones think that the FBI stopped
searching for John Doe 2? 9. Why does Jones think that McVey and
Nicholls could not have been the mastermind behind the bombing?
10. What were comparisons with the 1993 World Trade Center bombing?
11. Why is there a theory that Islamic terrorists were involved?
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Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved.www.CrosscuttingConcepts.com
Fire Debris
• Fire debris is submitted to laboratories for analysis by the fire marshal, crime scene investigators, forensic scientists, and insurance investigators.
• Investigators determine the best locations at the scene to collect samples, based on suspicious details.
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Suspicious Circumstances
• The presence of:– ignitable liquid (combustible)– accelerants (increase the rate of combustion)
• These are frequently detected by canines trained to detect ignitable liquids by smelling the fire debris.
• Investigators can also detect the presence of these liquids from pour patterns that remain on the burnt substrate after the fire.
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Pour Patterns
• Pour patterns often are characterized by intermixed light, medium, and heavy burning in a puddle shape that corresponds to the shape of the original pool of the ignitable liquid.
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Suspicious Circumstances
• Other indicators of ignitable liquid:– unnatural flame movement (downward or too
fast)– gapping of wood or floor seams (caused by
pooling of liquid)– damage with no identifiable point of origin– burned out flooring beneath appliances and
furniture– Inverted cone shaped burn patterns on
vertical surfaces
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Fire Debris Sampling
• Sample areas likely to contain traces of ignitable liquid:– lowest regions of burned area– insulated areas within the pattern– porous substrates in contact with the pattern
• cloth• paper products• wood
– seams or cracks– lightly burned edges of the pattern
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Collecting Fire Debris
• Once an appropriate area has been identified for sampling, samples are collected for later analysis.
• Samples are collected in a tightly sealed container (glass jar or metal can).
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Collecting Fire Debris
• Sample should fill 2/3 of the container.
• Top 1/3 of container is used for sampling headspace.
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Passive Headspace Sampling
• In passive headspace sampling the container is heated to volatilize any ignitable liquids remaining in the sample.
• Activated charcoal is suspended in the headspace to absorb the volatilized liquid.
• The charcoal is then removed from the sample container and the liquid is removed from it by solvent extraction.
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Solvent Extraction
• Carbon disulfide is frequently used in solvent extraction because it produces excellent desorption of most accelerants.
• It also produces a low detector response when analyzed by a gas chromatograph using a flame ionization detector.
• Unfortunately, carbon disulfide is a health hazard because it can cause nervous system damage.
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GC-MS
• After the solvent is extracted it is analyzed with a gas chromatograph – mass spectrometer (GC-MS).
• The liquid is injected into the GC and carried through the instrument by an inert carrier gas (called the mobile phase).
• The liquid then permeates a column (long thin tubing) which binds the liquid to a polymer coating on the inside (called the stationary phase). This separates the liquid components by volatility.
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GC-MS
• More volatile components move faster through the column.
• The components come off of the column separately during the elution step and enter the mass spectrometer.
• The mass spectrometer separates each component based upon the mass-to-charge ratio of their particles.
• The mass spectrum of the sample is then compared to a library of known compounds to identify the compound in the sample.
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Presumptive Testing
• GC-MS analysis can be time consuming and expensive.
• A presumptive test can be performed quickly in the field to indicate the presence of an ignitable liquid.
• Colorimetric gas detection tubes are used for this purpose.
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Presumptive Testing
• Colorimetric gas detection tubes are filled with a compound designed to react with a specific compound of interest (gasoline, diesel fuel, etc.)
• To perform the test, both ends of the glass tube are broken off.
• Air from the scene is drawn through the tube with a pump.
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Presumptive Testing
• In the presence of specific vapors, the compound within the tube will change color.
• The concentration of the compound may be estimated but this technique is not very accurate.
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Presumptive Testing
• If the presumptive test is positive, then samples will be collected for GC-MS analysis.
• In some labs, this is used for preliminary screening of samples.
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20/20 Arson Video
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Arson
Deliberately set. Also sometimes called an incendiary fire
May involve the use of an accelerant Accelerant is a substance that causes a fire
to start that would not ordinarily start or to burn faster than in the absence of the accelerant. These are usually fuels such as gasoline
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Conditions for a Fire
A fire must have:– Oxygen– Fuel– Sufficient heat to get the fire started
Deprive the fire of one of these and it will cease or won’t start
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Conditions for a fire
Generally only gases (vapors) will burn. Solids and liquids will not burn
Sufficient heat is required to vaporize some of the fuel and break oxygen molecules apart into oxygen atoms
Fire will produce enough heat to continue vaporization and keep fire going
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Investigating the Fire Scene
Done by fire scene investigator Search for point(s) of origin Possible causes:
– Accidental (leave the stove on, accidental malfunction in appliance)
– Natural (lightening)– Deliberate (Use an accelerant, deliberately
sabotage an appliance)
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The Point of Origin
Crucial piece of evidence in determining type of fire
Often most intense location of burning Usually at a low point in the building-may
be buried under tons of rubble Smoke or damage often shows “V” pattern
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Determination of Arson
Eliminate all natural and accidental causes: leaves only arson
Multiple points of origin Fire trail Presence of accelerant in debris Presence of accelerant in observation of
fire: thick black smoke
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The Role of an Accelerant
The accelerant supplies the heat needed to get substances into the vapor phase so they can burn.
Presence of an accelerant does not necessarily mean that fire is arson.
Absence of an accelerant does not rule out arson.
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Finding Accelerants
Find point of origin Look for absorbent materials Use fire dog – capable of sniffing out
miniscule quantities of accelerants Look for fire trails
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Accelerants Accelerants are any liquid, solid or
gaseous material that will sustain or enhance flammability.
Liquid materials are commonly used because of ease of ignition and familiarity of use.
Accelerants are nearly exclusively derived from hydrocarbons.
Straight chain hydrocarbons are the backbone of the oil industry.
Hydrocarbons are molecules made up of the elements hydrogen and carbon.
Octane is a term familiar to all. It consists of a hydrocarbon having 8 carbons.
Examples:– Gasoline– Kerosene– Diesel– Lighter fluids– Charcoal starters– Automobile additives– Camping fuels
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Evidence of Accelerants Large amounts of damage Unusual burn patterns High heat stress Multiple sites of origin “Sniffers” Portable gas
chromatographs Chemical tests Canines Portable detectors Detect change in oxygen
level on a semiconductor Guides to the best place
to collect samples
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Role of the Crime Laboratory
Lab does not determine if fire is arson or not
Examines debris to determine if an accelerant may be present
Isolates and identifies accelerants This may support the fire scene
investigator in determination of arson
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Methods of Isolating and Concentrating Accelerant Residues
Heated headspacePurge and trap
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Heated Headspace
Closed container Samples headspace over residue Henry’s law - heating drives equilibrium to
vapor Favors volatile components
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Headspace Analysis
syringe
headspace
~~~~~~~~~~~~fire debris
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Purge and Trap
Closed container Uses charcoal strip or charcoal tube to trap
accelerants Uses vacuum or positive pressure with
inert gas to force accelerant vapor through charcoal trap
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Purge and Trap
vacuum
headspace
~~~~~~~~~~~~~~ Fire debris
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Analysis of Residues
Uses GC or GC/MS for comparisons– MS has advantage of identifying particular
compounds which are present in certain accelerants
Analyst should have own library of knowns for comparison
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GC of Gasoline
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50% Evaporated Gasoline
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Gasoline Residue
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Furniture Polish
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Cases
Dentist who liquidated his assets Smoldering fire Body in the trunk of the car
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Dentist Who Liquidated His Assets
Dentist had failing practice in deteriorating neighborhood
Had offer to relocate practice with partner but needed $$$$$$$$$$ to get started
Used gasoline to make fire trail to each room in office
Building destroyed but witness saw him leave building before fire started
Convicted of arson
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Smoldering Fire
Man catches wife fooling around and decides to set trap for her.
He seals up all windows and doors to house and sets a fire in living room
Fire flames first and then, having used up oxygen, settles into glowing, smoldering fire.
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Smoldering Fire
He expects wife to come home from work and enter house. This will cause influx of oxygen into house and rekindle the fire explosively, killing her instantly
She has an auto accident on the way home and is delayed. Meanwhile a neighbor smells smoke and calls fire department
They arrive at scene and recognize that whole house is sealed up and enter very carefully.
Husband is charged with arson and attempted murder
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Burn Patterns
http://www.interfire.org/res_file/patterns.asp
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Homework
Fire Debris Pre-Lab due Tuesday
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Exit SlipDecember 1, 2014
Go to m.socrative.com and enter 417101 Question: What are common accelerants
and how can you identify that one has been used in a fire?