The Chemistry of Fire
The air we breathe is about 21% oxygen. Fire requires an atmosphere
with at least 16% oxygen.
FUEL
HEAT
OXYGEN
The Chemistry of Fire
Combustion -
The Chemistry of Fire
The Chemistry of Fire
A fuel will produce a flame only when it is in the gaseous state.
Flashpoint:
Accelerant
Flashpoint (oC)
Acetone
-20
Gasoline
-46
Kerosene
52-96
Mineral Spirits
40-43
Turpentine
32-46
Pyrolysis:
BASIC METHOD OF A FIRE INVESTIGATION
Fire investigation
Point of Origin (POO)
Interior Examination
◦ Defined as where the fire
o Work backward in relation
originated.
to fire travel and from least
to most damage.
◦ Cause of fire may be near the
POO.
◦ Fire usually burns longer at
o Ceiling damage may lead to
POO.
POO.
◦ If accelerants or ignition devices
used, they may be present at the
o In accidental fires, floor
POO.
damage is limited in respect
◦ Multiple POO’s MAY indicate
to the ceiling damage.
arson.
◦ “V” patterns usually point to the
POO.
◦ Extensive ceiling damages may
be present above the POO.
patterns

Char Patterns – Created by very hot fires that
burn very quickly and move fast along its
path, so that there can be sharp lines
between what is burned and what isn't.
V-Patterns -

Inverted Cone Pattern:

Alligatoring Pattern:

Spalling Pattern:

Streamer Pattern:

Arc Damage Pattern:

Pool or plant:

Glass –

Chimney Effect - Since fire burns upwards, there can be a "chimney
effect" If the roof is not entirely burnt, and the fire investigator finds
such a hole, the origin of the fire could be directly underneath.

Colour of smoke – Determine what type material was burning.

Colour of flames – Indicates at what temperature the fire was burning.

Melting points of metals: Each metal has a unique melting point
Fire investigation
What clues might a fire investigator gain from
these photographs?
What are Common Motives for Arson?
•
Arson indicators: what do you think?
General Arson Indicators
Owner Occupant Indicators
Arson indicaters: What do you think?
Vehicle Indicators
Arson Indicators Commercial
Arson investigation
Evidence of Accelerants
Large amounts of damage
 Unusual burn patterns
 High heat stress
 Multiple sites of origin
 “Sniffers” Portable detectors
 Human smell at 1 part /10 million
 Portable gas chromatographs
 Chemical tests
 Canines
 Detect change in oxygen level on a
semiconductor

 Dogs can detect 0.01µL of 50%
evaporated gasoline 100% of the time.
 0.01µL is about the size of a
thousandth of a drop.
A portable
hydrocarbon
detector
Vehicle Fires
Arson investigation
Discovering Bodies
 often bodies will be part buried in debris and often difficult to
recognize.

If fire threatens to destroy evidence and the body needs to be
removed, the following details should be noted;
o Location of the body, Position of the body, the nature and position
of article around the body

Carbon Monoxide asphyxiation most common cause of death in fatal
fires
Methods for Isolating and Concentrating
Accelerants
Evidence Containers
The evidence container should
have the following:
Investigating suspects
Clothing Items
 Impact or transfer patterns - actions
Symptoms of ignitable liquid use
Burn injuries to the hands,
face, legs or hair of a
suspect/witness.
Methods for Isolating and Concentrating
Accelerants
A. Steam Distillation
B. Solvent Extraction
C. Head Space Analysis
D. Vapor Concentration on Charcoal
E. Solid Phase Extraction
HW. Read each of the types of analysis
Steam Distillation
Solvent Extraction
It relies on variations in the solubilities of different
compounds in different substances. A solvent will be
chosen that does not mix with the compound in which the
substance of interest is currently dissolved, so that, when
left undisturbed, they will form two separate layers, as with
oil and water. It is also important that the compound to
be extracted should have greater solubility in the solvent
that has been added, and that this should not dissolve any
unwanted substances in the original mixture.
Once added, the two liquids may be shaken together for a
time then allowed to stand for a while, so that they
separate out. The choice of solvent to be used will
depend on the chemical and physical properties of all the
substances in the mixture. The process may need to be
carried out in several stages, using different solvents.
Head Space Method
A liquid or solid sample is placed into a vial, sealed, and heated to a
specific temperature. All of the components that are volatile at or below
the pre-set temperature escape from the sample to form a gaseous
"headspace" above the sample.
After a certain period of time, the headspace gas is extracted from the vial
and injected into a gas chromatograph which separates the various
components of the sample based on size and/or polarity.

Heat container (paint can) and
stick syringe through top and pull
gas into syringe
Failure of Head Space Method
 Limited to sample by size of syringe and concentration
present in headspace at that time
Vapor Concentration Method
 A charcoal coated strip is placed into the container while the container
is enclosed and heated to 60ºC for one hour
 As the accelerant vaporizes the charcoal absorbs it
 The strip is then washed with a small volume of carbon disulfide and
the solvent is injected into the GC/MS
 100x more sensitive than headspace analysis

At the suspect point of origin of a fire, ash and soot, along with porous
materials which may contain excess accelerant, should be collected and
stored in airtight containers, leaving an airspace to remove samples.
Solid Phase Extraction (SPE)
A separation process by which compounds
that are dissolved or suspended in a liquid
mixture are separated from other compounds
in the mixture according to their physical and
chemical properties. Solid phase extraction
can be used to isolate analytes of interest from
a wide variety of matrices, including urine,
blood, water, beverages, soil, and animal
tissue.
The result is that either the desired analytes of
interest or undesired impurities in the sample
are retained on the stationary phase. The
portion that passes through the stationary
phase is collected or discarded, depending on
whether it contains the desired analytes or
undesired impurities.
http://www.youtube.com/watch?v=BCjF50wrzn4
Methods for Isolating and Concentrating
Accelerants
Accelerant lab testing
Catalytic Combustion Detectors: The most common flammable vapor
detector operates on the catalytic combustion principle.
 Flame Ionization Detector: In the flame ionization detector the sample
gas is mixed with hydrogen and the mixture is burned.
 Gas Liquid Chromatograph: The portable gas liquid chromatograph
(GLC) is one of the most common detectors in arson investigations.
 Infrared Spectrophotomer: Infrared spectophotometers can achieve
high specificity to flammable liquids and high sensitivity.
 Ultraviolet Fluorescence: This procedure consists of illuminating the
darkened fire scene with an ultraviolet lamp.


Laser ablation: This process allows the removal material from a small
sample’s surface by irradiating its surface with a laser beam.
Gas Chromatography
In the laboratory, the gas chromatograph is the most sensitive and
reliable instrument for detecting and characterizing flammable residues.
 The vast majority of arsons are initiated by petroleum distillates such as
gasoline and kerosene.
 The gas chromatograph separates the hydrocarbon components and
produces a chromatographic pattern characteristic of a particular
petroleum product.
 By comparing select gas chromatographic peaks recovered from firescene debris to known flammable liquids, a forensic analyst may be
able to identify the accelerant used to initiate the fire.
 Can tell what type of hydrocarbon is
present such as: benzene, toluene,
xylenes, alcohols, methane, pentane,
butane, ethane, hexane, etc.

Gas Chromatography
Arson labs often use a method called gas chromatography/mass
spectrometry detector (GC/MSD) to determine the composition of the
accelerants or explosives found.
MS



GC
The vial is automatically injected on the gas
chromatograph / mass selective detector (GC/MSD).
The GC will separate all of the sample’s components.
The MSD will identify the sample’s components.
Gas Chromatography
The problem with gas
chromatography and mass
spectrometry, however, is that in
order to analyze evidence, you
have to destroy it.
Laser Ablation
Laser ablation is the process of removing material from a solid (or
occasionally liquid) surface by irradiating it with a laser beam. At low laser
flux, the material is heated by the absorbed laser energy and evaporates or
sublimates. At high laser flux, the material is typically converted to a
plasma.
Laser ablation etches off only
a tiny slice of a sample with a
needlelike light beam and
cooking it in a plasma furnace
equipped with a mass
spectrometer especially
sensitive to trace elements.
Accelerant lab testing
Ignitable Liquids Were
Detected
Example:
Sample contains a medium
petroleum distillate (MPD),
some types are paint thinner
and mineral spirits.
Sample contains a mixture of
gasoline and a heavy petroleum
distillate (HPD). Some types of
HPD are diesel fuel and
heating oils.
No Ignitable Liquids
Were Detected
We can look at this in four ways…
 No ignitable liquids were used.
 Ignitable liquids were used to start
the fire, but have totally been
consumed.
 Ignitable liquids are still present;
however, not in the collected
sample.
 Ignitable liquids are still present
in the collected sample; however
they are too dilute to be detected.
Further lab analysis

Furnishings and Clothing:
◦ Are they natural fibers – cotton, linen, wool?
◦ Are they synthetic – polyester, polypropylene, nylon, acrylic, or ???
◦ Are they thermoplastic, or thermosetting, or elastomers?
◦ Melting points? Ignition temperatures?
◦ What volatile products do they generate?
◦ Foam – polyurethane, polyether, or latex?
◦ Are there fire retardants present?
Further lab analysis


General type of fuel may help identify or exclude competent ignition
sources.
Is the Fuel First Ignited Cellulosic?
◦ Cellulosic: Derived from Plants
 Wood
 Paper
 Cotton
 Linen
 Cardboard
 Susceptible to smoldering
 Crumbly grey/black ash on burning
 White smoke
Cellulosic fuels are susceptible to hot
surface/glowing ember ignition
Further lab analysis
Is the Fuel First Ignited Synthetic?
 Most Synthetics are petroleum derivatives
o Nylon
o Polyethylene
o Polystyrene
o Polyester
 Rarely ignitable by smoldering/hot surface source
 Easily ignited by open flame
 Tend to melt and shrink away from heat
 Most do not sustain smoldering combustion
 Synthetics:
◦ May be thermoplastic – melts without degrading
◦ May be thermosetting (resin) – degrades, chars, may smolder
 Other fuel types:
◦ Elastomers (rubbers): Synthetic or natural
◦ Leather – Wool – Silk: Proteinaceous (from animals)