Unit-5: Cases Involving Arson and Explosives

[Chemistry of fire. Collection and preservation of arson evidence. Analysis of

fire debris. Analysis of ignitable liquid residue. Scientific investigation and
evaluation of clue materials. Information from smoke staining.
Classification of explosives – low explosives and high explosives. Synthesis and
characteristics of TNT, PETN and RDX. Mechanism of Explosion process. Blast
waves. Searching the scene of explosion. Post blast residue collection and
analysis.]

Chemistry of fire
Fire is the rapid oxidation process with release of heat, light, and various
reaction products.

1. Fuel can be any combustible

material in any state of matter-solid,
liquid, or gas such as furniture,
paper, kerosene, diesel, petrol,
alcohol, turpentine oil, etc.
2. Oxygen is available from the
atmosphere.
3. Heat is the energy necessary to
increase the temperature of the fuel to
a point where sufficient vapors are
given off for ignition to occur.
Flammable liquids are those that have
a flash point below 1000F. Figure1 is a Fire Triangle that represents the
Combustible liquids are those that 3 elements needed for fire to occur: Heat, fuel
and oxygen are all essential elements for a
. have a flash point above 1000F fire to occur.

Types of Fire:
(a) CLASS “A” FIRES
A class “A” fire can involve any material that has a burning ember or leaves an ash.
Some examples of class “A” fires are wood, paper, or pulp. The adopted method for
quenching fire of class “A” is to remove the heat. Water is considered to be most
common agent, but other agents such as foam and dry chemical can be effectively
used.
 Table 1.

Arson
Arson is the crime of intentionally and maliciously setting fire to buildings,
wild land areas, vehicles or other property with the intent to cause
damage. Arson often involves fire deliberately set to the property of
another or to one’s own property so as to collect insurance compensation.
Arson is also called incendiary fire.

Motives for Arson:

(i) Revenge: Personal or professional vendetta
(ii) Vandalism: Crime of opportunity
(iii) Profit: Monetary gain by claiming insurance
 (iv) Crime Concealment: Destroying evidence to cover another crime
(v) Excitement: Sense of power or seeking recognition
(vi) Extremism: Social protests against governments or corporations

Collection and preservation of arson evidence

Proper evidence collection and chain of custody are the responsibility of
the scene investigator or a designated evidence technician who selects and
assembles all evidence collection equipment and materials.
As evidence is identified, the investigator may work with a photographer
and/or a schematic artist. The investigator is responsible for proper
documentation of evidence prior to collection, maintaining an evidence log
and numbering system, packaging and preserving all evidence collected,
and maintaining custody at the fire scene and during transportation to a
properly secure storage area. The investigator is also responsible for
arranging for laboratory analysis requests and transmitting evidence to
the laboratory.
Evidence Containers
Fire investigators should carry a supply of various evidence containers, including
both one-quart and one-gallon clean, unused metal evidence cans, or the
equivalent, in which to store residue samples. A good practice is to seal a one-
quart evidence can and place it inside a one-gallon evidence can. Then, seal the
one-gallon can before placing the cans in your vehicle. This saves space and
prevents contamination. Seal the can with only hand pressure to eliminate
contamination from outside vapors. Open the cans just prior to physically
collecting the sample at the collection site.

 When evidence is expected to be subject to analysis, as in an arson

investigation, it is necessary to be able to establish that the item seized
is the same item that was analyzed. Consider the following advice from
Melville, Manual of Criminal Evidence, Second Edition, Denver D.A.
office:
"Whenever any piece of evidence must be passed from hand to hand to set up the
chain of evidence in a case, it is essential that every person who has anything to do
with the matter must be prepared to testify as to
1) when and how such piece of evidence came to him,
2) what he did with it while it was in his possession, and
3) when, why, how and to whom he delivered it."
 Extraction Techniques
There is no single best extraction method for the analysis of fire debris.
Selection of an extraction technique is dependent on a variety of factors,
many of which are beyond the control of the examiner. The ideal extraction
technique would be sensitive yet nondestructive – that is, it would not
fundamentally alter the evidence in such a way that it could not be
retested. It should allow for the extraction process to recover any ignitable
liquid of interest from the debris, but should minimize the recovery of
chemicals related to the substrate or its combustion and pyrolysis
products.

1. Solvent extraction
The technique of solvent extraction is based on one of the simplest and
most well known chemical principles: like dissolves like. The technique of
solvent extraction requires that the analyst use an appropriate solvent to
dissolve any ignitable liquids that may be present in the sample being
tested. It is a simple, quick, inexpensive, and uncomplicated technique that
requires little in the way of equipment and has long been applied to the
analysis of fire debris for the recovery of accelerants. Solvents are selected
based on their ability to recover common ignitable liquids. Because the
most frequently encountered accelerants are composed of hydrocarbons
derived from crude oil, nonpolar solvents are most effective. Commonly
used solvents include pentane, carbon disulfide, diethyl ether, and
chlorinated solvents.
Solvent extraction offers several advantages to the fire debris analyst.
 It can take only minutes to perform, allowing the analyst to begin the
instrumental portion of the analysis nearly immediately.
 It also offers the distinct advantage of minimal discrimination in
extracting residue from the sample matrix.
 The solvent extraction method recovers all miscible components of
the ignitable liquid residue more or less equally. This allows for the
extract to better represent the actual composition of the ignitable
liquid residue present in the sample matrix.

Solvent extraction offers some disadvantages as well.

 It can often require fairly substantial amounts of hazardous solvents,
which can pose risks to health and safety.
 In addition, this technique, although sensitive, is less sensitive than
some of the available techniques.
  It often requires concentration of the extract, which may result in the
loss of the more volatile ignitable liquid components and the
concentration of low-level impurities present in the solvent.
 It often co-extracts substantial amounts of matrix-related
components.

Fig. 2 Solvent Extraction Method

2. Headspace sampling
Like solvent extraction, the technique of headspace sampling is quick and
simple, easy to perform, and requires minimal equipment. The technique
of headspace sampling relies on the physical property of the volatility of
ignitable liquids. Volatility refers to a liquid’s tendency to have a rich
vapor phase, and to evaporate readily. In headspace sampling, the sample
of fire debris may be heated in order to enrich the vapor above the sample,
or it may be done at ambient temperature. To perform the technique of
headspace sampling, a portion of the vapors above the sample of fire
debris is removed, and directly introduced to the instrument being used
for analysis – generally, the gas chromatograph (GC).

The headspace sampling method offers several advantages as a

method for the recovery of ignitable liquids.
 The application of the technique is rather simple; there is no special
equipment needed, other than a suitable syringe for collecting and
introducing the sample.
 No solvents are used, thereby offering two advantages: there are
reduced health, safety, and environmental hazards, and there is no
added solvent, which may interfere with the products being recovered
from the sample matrix.
  Because this sampling method only removes a relatively small portion
of the headspace vapors, the evidence is essentially unchanged and
the technique is considered to be nondestructive.
 Headspace sampling can provide important information about some
types of samples; however, because of its inability to recover the full
range of commonly encountered ignitable liquids, it should not be
used as the sole sampling technique.

The disadvantages associated with the method are

 Only a small portion of the headspace vapors being sampled is that
this technique offers no means of concentrating low levels of ignitable
liquids. Therefore, it is one of the least sensitive techniques still in
modern use.
 The headspace technique is also not suitable for the recovery of all
types of ignitable liquids.
 Because it is based on the collection of vapors, this technique may be
useful for lighter (more volatile) ignitable liquids, but will not efficiently
recover all classes of ignitable liquids, such as those classified as
heavy or medium.

3. Passive adsorption (passive headspace concentration)

The most commonly used extraction method for the recovery of ignitable
liquids from fire debris is that of passive adsorption according to reports
from recent proficiency tests and a survey of practitioners. This method
involves concentrating vapors from the headspace of a sample within a
closed system onto a suitable adsorbent, then desorbing the recovered
species. Activated charcoal, also referred to as active carbon, is the most
commonly used adsorbent, and is generally desorbed via solvent. Other
adsorption-based methods exist including solid phase microextraction
(SPME) and those that utilize an adsorbent. To use this technique,
adsorbent in an appropriate form is placed in the sample container along
with the fire debris, and the container is usually heated for a period of time.
This is the adsorption period in which vapors present in the sample
container become adsorbed onto the activated charcoal. Following the
adsorption period, the adsorbed compounds are eluted – or removed – from
the charcoal with solvent. While many solvents have been researched for
this application, the most important factors to consider are miscibility and
the solvents affinity for adsorption sites. Because of these factors, carbon
disulfide is commonly used, although other solvents such as pentane,
diethyl ether, and methylene chloride have also been used in this
application. The activated charcoal needs to be in such a form that it can be
relatively easily manipulated so that it can be placed in and removed from
the container. In addition, the active sites on the charcoal must be
accessible to the headspace vapors in the can.
The advantages offered by this technique are
 It is suitable for recovering a fairly broad range of ignitable liquids.
 This method can also recover very volatile ignitable liquids, which can
be detected and identified via GC-MS when instrument parameters are
set to collect data prior to the elution of the solvent.
 Passive adsorption offers is that it requires relatively little examiner
time. Fire debris samples can be batched and extracted
simultaneously, with the majority of the extraction time being
unattended.
 This technique allows for concentration of vapors onto the charcoal,
and elution generally occurs with a minimal volume of solvent, so it is
also one of the most sensitive techniques available.
 Studies have shown that in all but the most extreme situations, this
technique can be repeated numerous times, with no significant change
in the extracts obtained; therefore, it may be considered
nondestructive in that it does not permanently alter the parent
evidence.
All of these factors lead to the passive adsorption method being one of the
most efficient of the available techniques.

As with every technique, however, there exist some disadvantages.

 The cost per sample is relatively high compared with other sample
preparation methods.
 Samples that are extremely concentrated may saturate the available
active sites, which lead to the phenomenon of displacement.

4. Dynamic adsorption (dynamic headspace concentration)

The technique of dynamic adsorption relies on the same basic principles as

the passive adsorption extraction method. Versions of a dynamic adsorption
method have been applied in the petroleum industry and in environmental
and industrial hygiene applications. Dynamic adsorption was in common
use for the extraction of ignitable liquid residues from fire debris for a long
time prior to being almost completely replaced by the passive technique.
The dynamic technique is still considered an acceptable method for the
extraction of fire debris for the recovery of ignitable liquid residues for
Separation and Concentration of Ignitable Liquid Residues from Fire Debris
by Dynamic Headspace Concentration. Like the passive technique, the
dynamic method of extraction relies on the adsorption of vapors onto a
suitable material, most commonly charcoal, and their subsequent elution
via solvent. The critical difference between these two techniques is that
while the passive method uses a closed system, the dynamic system uses a
system in which air or an inert gas is forced through the sample container.

Advantages:

 This technique offers the capability of recovering a broad range of

ignitable liquid residues with very good sensitivity.
 Unlike its passive counterpart, however, a complete extraction – from
adsorption to elution – can often be done in less than an hour.

Disadvantages:

 The time conducting this extraction is much more labor-intensive

than the longer time spent completing the passive extraction.
 Whereas the passive extraction requires minimal examiner time
during its approximately 16 hr extraction process, and allows for
sample batching, the dynamic extraction process requires the
examiner to be present and attending the dynamic extraction.
 In addition, this method cannot be feasibly automated and is not
amenable to batching numerous samples.

5. Solid phase microextraction (SPME)

The most recent extraction technique to be added to the fire debris
analyst’s toolbox is that of SPME. It is a relatively new technique that has
been used in a variety of other separation applications, as well as in the
extraction of ignitable liquids from fire debris Extraction by SPME most
commonly involves the exposure of a fiber bearing an adsorptive coating to
the headspace above a sample, which may be heated or held at ambient
temperatures. Less frequently, the SPME fiber may be immersed directly
into an aqueous liquid. Following this adsorption step, the fiber is then
thermally desorbed directly into the instrument being used for analysis. In
this application, polydimethylsiloxane (PDMS) is most commonly used.
The application of SPME for the recovery of ignitable liquids from fire
debris offers many advantages.
 It is both quick and easy to perform.
 Extraction times for SPME are typically in the range of 5–20 min.
 The process is relatively simple. Because of the quick and simple
nature of this technique, instrumental analysis can begin with 20
min of beginning the examination, thereby allowing for very rapid
results. The SPME fiber is small, and has relatively few adsorption
sites.
  This means that the recovered sample is most often only a small
portion of the ignitable liquid residue vapors present in the can, so
the extraction may be conducted repeatedly, without significantly
altering the sample.
 Because this technique will not fundamentally change the primary
sample so that the sample can be subjected to retesting, it may be
considered a nondestructive technique, which is an important
advantage for any technique with forensic implications.
 Another advantage of this technique is that it does not require use of
a solvent in the desorption phase.
 Lack of solvent offers several advantages. There is a significantly
lower health and safety risk when no solvent is used, and there are
no expenditures for the purchase or disposal of solvents.
 In terms of analytical advantages, the lack of solvent means that
there is no concern with a solvent masking a component of an
ignitable liquid that may co-elute with the solvent, or that may be
lost within the solvent front.
 This allows SPME to be effectively applied to cases in which there
may be very volatile ignitable liquid residues.
As a result, this technique is also one of the most sensitive techniques.

Disadvantages
 There is no way to preserve a portion of the sample for later
reanalysis, which may be problematic for forensic cases.
 The purchase of several SPME assemblies is necessary for the
efficient processing of forensic casework, which requires an initial
one-time expenditure.
 The cost and fragility of the fibers may be considered to be a
disadvantage.

Information from smoke staining

Smoke staining is an important indicator in fire investigations, as it
provides information about fire behavior, progression, and possibly even
origin. Here’s how smoke staining is analyzed and used in fire
investigations:

1. Identification of Fire Patterns

Smoke staining can reveal where the fire started and how it spread. Fire
often leaves V-shaped patterns on walls or surfaces, pointing to the origin.
Smoke staining can help determine airflow patterns during the fire,
showing how heat and smoke moved throughout the space.

2. Determining Fire Duration and Intensity

The amount and thickness of smoke deposits can indicate how long the
fire burned. Heavier staining often points to prolonged exposure to smoke
or more intense burning in a particular area. The chemical composition of
soot can provide clues to the materials burned. For instance, plastics
produce oily, black soot, while wood produces lighter, flaky soot. Analysis
of these differences can give insights into the materials involved.

3. Clues about Ignition Sources

Concentrated smoke staining near an ignition source, like electrical

outlets, switches, or appliances, may indicate the source of ignition.
Objects that shielded surfaces from smoke and heat leave behind
"shadows." These shadows reveal the position of items before the fire,
helping to reconstruct the scene and potential sources.

4. Intentionality and Potential Accelerants

In cases of arson, accelerants are often poured in irregular or splash

patterns. The smoke staining can sometimes reveal these patterns,
especially if accelerants leave distinctive residue. Accelerants often cause
faster smoke spread, and the staining may reflect this through more
extensive coverage in a shorter time, differing from a natural fire
progression.

5. Environmental Influences on Smoke Staining

The presence of open doors, windows, or ventilation systems affects smoke

movement, often causing specific staining patterns or uneven distribution.
In outdoor fires or partially open structures, wind can affect smoke
direction, creating unique staining patterns. Forensic teams can study
these for clues about the fire’s origin and path.

6. Documentation and Legal Relevance

Investigators document smoke staining with photographs and sketches to

analyze patterns in court.
Smoke staining is often used in conjunction with other fire debris analysis
to support findings about fire origin, intent, and the materials involved.
Smoke staining analysis offers crucial information that can reveal a fire’s
path, duration, and whether accelerants were used, making it a vital
component of fire investigation.

Explosive
It is defined as any chemical compound, mixture, or device, the primary or
common purpose of which is to function by explosion, i.e., with
substantially instantaneous release of gas and heat

Classification of Explosives

Low explosive (LE): It is an explosive material that can be caused to burn

when unconfined.

 Burn rapidly but do not detonate (subsonic combustion).

 Deflagration process with reaction rates below the speed of
sound.
 Pressure wave is mild and sustained.

Examples: Gunpowder, smokeless powder, fireworks.

High explosive (HE): It is an type of explosive material that undergoes

detonation, a rapid chemical reaction that propagates through the material
at a supersonic speed via a shockwave. This reaction releases a large
amount of energy in the form of heat, gas, and pressure within
microseconds. Examples: TNT, RDX, PETN.

High explosives can further be classified as

 Primary Explosives: Extremely sensitive to heat, friction, and impact (e.g.,
lead azide, mercury fulminate).
 Secondary Explosives: Less sensitive but more stable; require a detonator (e.g.,
TNT, RDX, PETN).
 Tertiary Explosives: Require extreme conditions to detonate (e.g., ANFO).

Synthesis and Characteristics of TNT, PETN, and RDX

1. Trinitrotoluene (TNT): The process can be carried out in three
steps or continuously. The Bofors-Norell process is one method
 that involves continuous nitration of toluene or mononitrotoluene
(MNT) to TNT, and continuous crystallization from dilute nitric
acid.
The synthesis of trinitrotoluene (TNT) is a multi-step process that
involves the nitration of toluene with nitric acid and sulfuric acid:
 Mononitrotoluene (MNT): Toluene is nitrated with a mixture
of nitric and sulfuric acid to produce MNT.
 Dinitrotoluene (DNT): MNT is separated and then renitrated
to produce DNT.
 TNT: DNT is nitrated with an anhydrous mixture of nitric
acid and oleum to produce TNT.

TNT is a yellow, odorless, explosive substance that's used in military and

industrial applications. It's used in shells, grenades, bombs, and for
underwater and industrial blasting.

2. Pentaerythritol Tetranitrate (PETN): PETN is a well-known

chemical compound that is often used in making explosives.
PETN is prepared by the reaction of nitric acid with
 pentaerythritol (C5H12O4). Pentaerythritol is a commonly used
alcohol in varnishes and paints. The reaction takes place in
chilled conditions so that PETN can precipitate out.After its
precipitation, PETN is filtered, washed, dried, and undergoes
recrystallization resulting in a colorless crystalline material.
PETN is stored in a mixture of alcohol and water.

Pentaerythritol PETN

PETN is a white crystalline powder with a melting point of 141 °C. PETN is
used in many commercial and military explosives, as the primary
ingredient in detonating fuses, and as a component in some plastic
explosives. It is also used to treat chronic cardiac insufficiency.

3. RDX

The full form of RDX is Royal Demolition eXplosive. It is synthesized

through the nitration of hexamine under controlled conditions.
Hexamine is treated with concentrated nitric acid (HNO3), sometimes
in the presence of acetic anhydride as a dehydrating agent. The
nitration process converts the hexamine into RDX through the
substitution of nitro groups (-NO2).
It is a white crystalline solid having melting point ~204°C. RDX is used as
a base charge for detonators, or mixed with other explosives to create
bursting charges for aerial bombs, mines, and torpedoes. RDX is also used
as an ingredient in plastic explosives. It can be used as a propellant or
gunpowder.

Mechanism of Explosion Process

An explosion occurs when a material undergoes a rapid chemical or

physical reaction, releasing a large amount of energy in the form of heat,
gas, and pressure. This process is typically initiated by an external energy
source (e.g., a detonator) or a catalytic reaction.

Steps in the Explosion Process:

1. Initiation: It requires an energy source (e.g., a detonator or a spark)

that initiates the explosive reaction. In high explosives, the reaction
begins with the formation of a shockwave that propagates through the
explosive material.
2. Propagation: The explosive material undergoes rapid decomposition or
detonation after initiation. The detonation front travels at supersonic
speed, breaking molecular bonds, releasing energy, and forming high-
pressure gases.
3. Detonation: Detonation velocity is the speed at which the chemical
reaction front moves through the explosive material, usually in excess
of the speed of sound (greater than 1,000 m/s). This creates a
shockwave that can cause significant destruction due to the rapid
release of energy.
4. Expansion: The hot gases produced during the explosion expand
rapidly, causing a rise in pressure and a sudden release of energy in
the form of a blast wave. The energy dissipates in the surrounding
area, causing the destruction of objects and structures.
5. Completion: The explosive reaction completes, leaving behind
fragments, gases, and other byproducts such as unburned explosives
or residues.

Blast Waves

A blast wave is a high-pressure wave that propagates outward from the

point of an explosion. It consists of a shock front that travels at
supersonic speed and is followed by a less intense negative pressure
phase.

Searching the Scene of Explosion

Post-explosion investigations are critical to understand the cause of the
explosion and gather evidence. The process of searching the scene of an
explosion involves several careful and systematic steps.

1. Safety First: Ensure that the area is secure and free from further
hazards (e.g., secondary explosions, collapsing structures). Protective gear
(gloves, masks, and suits) should be worn by investigators to avoid
contamination or exposure to dangerous residues.

2. Initial Survey: Conduct a preliminary survey to assess the scale of the

blast and identify any immediate hazards (e.g., fire, unexploded ordnance,
toxic gases). Take general photographs of the scene, noting key features
like the blast center, debris patterns, and visible damage.

3. Identify Blast Effects: Look for physical indicators of the blast, such
as the presence of craters, burn marks, and scattered debris. Record the
direction of the blast wave based on the damage pattern and the location
of the explosion’s epicenter.

4. Secure the Scene: Isolate the area and prevent contamination of

evidence. Ensure that all individuals involved in the investigation are
authorized and trained to handle explosive evidence.

5. Systematic Search: Perform a detailed search for specific evidence,

including remnants of explosive devices, residues, and materials that
could explain the cause of the explosion. Look for items like
detonators, bomb fragments, or unexploded ordnance.

Post-Blast Residue Collection and Analysis

Collecting and analyzing residues after an explosion is crucial for

identifying the explosive used, determining the source of the explosion,
and conducting forensic investigations.
1. Collection of Residues:

 Handling of Samples: Use clean tools (e.g., tongs, brushes) to

collect samples from the scene. Prevent contamination by
wearing gloves and using sterile containers.
 Collection Areas: Focus on areas near the explosion epicenter,
surfaces exposed to the blast, and any nearby unburned material
or fragments.
 Types of Samples: Collect dust, soil, debris, and any remaining
traces of explosives from walls, floors, or nearby objects.

2. Residue Analysis:

 Chemical Analysis:
o Use techniques like Gas Chromatography-Mass
Spectrometry (GC-MS), Liquid Chromatography-Mass
Spectrometry (LC-MS), and Ion Chromatography to detect
explosive residues such as TNT, PETN, or RDX.
 Spectroscopy:
o Fourier Transform Infrared Spectroscopy (FTIR) and
Raman Spectroscopy are used to identify specific explosive
molecules and their chemical composition.
 Microscopic Examination:
o Scanning Electron Microscopy (SEM) can reveal detailed
traces of explosives and their physical properties.
 X-ray Diffraction (XRD):
o Identify crystalline residues of explosives like TNT or other
high-energy compounds.

3. DNA/Trace Evidence:

 Search for fingerprints, clothing fibers, or other trace evidence

that might link a suspect to the explosion.

4. Documentation and Chain of Custody:

 Document all steps of residue collection and analysis in detail to

maintain an unbroken chain of custody for legal and forensic
purposes.
The investigation and analysis of an explosion scene are crucial for understanding
the cause, reconstructing events, and gathering evidence to identify those responsible.
A thorough scene investigation helps pinpoint the type of explosive used, its
placement, and the mechanism of initiation, providing valuable insights into whether
the explosion was accidental or intentional. Proper analysis of blast effects, debris
patterns, and residue aids in determining the direction and intensity of the explosion,
this is vital for understanding its impact and preventing similar incidents in the future.
Additionally, forensic evidence collected at the scene, such as explosive residues,
device fragments, or biological material, can link suspects to the event and support
legal proceedings. Effective scene investigation also ensures the safety of responders,
protects evidence from contamination, and upholds the integrity of the investigation,
ultimately contributing to public safety and justice.

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