The detection and identification of flammables
Introduction
The combustion is a physico-chemical process. It is an oxidation reaction which flammable
substances react at high speed with an oxidizing agent to produce heat and light. It is an exothermic
reaction. In order to combust the presence of several factors is required:
(i) solid matter (wood, paper, textile),
(ii) liquid substance (petrol, thinner)
(iii) gaseous substance (for example natural gas, acethylene)
(iv) oxidizing agent (oxidant, oxygen)
(v) initiator of combustion (flame, electrical discharge, high temperature).
The burning process usually required all 3 components. Flammability of matters depends on their
affinity for free oxygen and also partly chemically bound in compounds. Flammables can be in
terms of chemical composition divided into compounds containing only combustible elements
(separate elements, chemical compounds composed only of combustible elements and mixtures of
these elements) and materials containing combustible and incombustible components. For these
substances the flammability depends on number and weight of constituting flammable elements.
Flammable liquids
Each liquid evaporates due to the influence of the environment. The flash point is the temperature at
which the vapors above the liquid are so concentrated that they start to burn. This temperature is
characteristic for each flammable liquid and according to this value liquids are filed into the hazard
classes.
The Ignition is characteristic of substances at high temperatures to release volatile, combustible
products which ignite in mixture with an oxidizing agent. The ignition occurs only due to heat, not
because of the source of ignition or exhibition of flame burning. It may also cause a phenomenon
called the spontaneous combustion.
One of the basic conditions of fire spread is the amount of flammable substances and its location.
This will consequently determine the direction of fire spread. It is obvious that if substances with a
higher combustibility occur at the site of a fire, the speed of fire propagation would be higher. If we
take into account the physical properties of flammable substances, the state of matter has the
greatest influence on propagation. Generally speaking, the highest rate of spread is for burning of
gaseous substances, followed by liquid substances and smallest propagation velocity is for burning
of solids. This is because the combustion is generally in the gas phase and the solids must be first
prepared into this state.
Flammable liquids divided into four hazard classes according to their temperature of ignition:
I. hazard class – temperature of ignition under 21 °C
II. hazard class - temperature of ignition from 21 °C to 55 °C
III. hazard class - temperature of ignition from 55 °C to 100 °C
IV hazard class - temperature of ignition from 100 °C to 250 °C
The most dangerous are flammable liquids I. and II. hazard class. The I. hazard class includes
benzene, acetone, alcohol and some solvents. Among flammable liquids of II. hazard class belong
for example thinners, paint and some types of diesel.
In many cases, when an unknown liquid is found at a crime scene or accidental fire (eg. Illegal
warehouses, pyrotechnic charge or industrial areas) is necessary to identify flammable liquid and to
assessment whether a particular substance and the amount could lead to endanger life or property.
In this case, the rapid and sensitive methods play the important role. Spectral methods (eg. infrared
spectroscopy, raman spectroscopy) are suitable in case of the analysis of major constituents of the
sample, which allows precise identification of the main components of a sample without consuming
adjustments. Furthermore, there are special database of spectra, so that identification can occur
automatically without comparison to the standard.
In the case of complex multicomponent mixtures the flammables are often analyzed using GC-MS.
This technique has several advantages:
(i) suitable for the analysis of volatile and flammable liquids,
(ii) allows the separation and identification of the mixture composition of liquids without necessity
to analyze standard,
(iii) allows to perform repeated analyzes of the sample, even in cases when the sample is only very
little available.
GC-MS is the most widely used analytical technique in fire laboratories, where not only
flammables, but also burning accelerants to discover the cause of the fire are identified and
determined.
In the framework of the causes of fires is also carried out to identify possible burning accelerants in
samples from fire residues (ash). Burning Accelerant is the material used to initiate the process and
support the spread of fire. This is a flammable liquid, usually oil fractions or any organic solvent of
paints (possible content of alcohols, esters, ketones, cycloalkanes etc.), used by arsonist to
deliberately setting of fire. A routine methodology of analysis of combustion enhancers uses solid
phase microextraction (SPME) technique for vapor collecting from head-space container. The
carboxen/polydimethylsiloxane fiber is the most commonly used for it. General scheme of SPME
fiber is shown in Fig. 1.
Fig. 1: The scheme of SPME fiber
The identification of burning accelerants depends on sampling and finding of the corresponding
stop at the fire site. These tracks are subject of so-called dynamic inspection of the place of fire,
which is revealed at the outbreak sites of forensic mechanical stripping of logs, ash or charred
structures and seek for possible initiators or their remnants. The used flammables are surveyed.
The proper collecting of evidence is the first key step in the analysis of debris after fire. The main
goal is to provide evidence which can be properly analyzed in the laboratory, and also to avoid the
risk of contamination or cross-contamination between fluids to expansion or to intensification of
burning.
Task: Identify the components of flammables in the material found at the scene of a fire (cloth
with remnants of the soil, apparently soaked with volatile organic compounds).
Tools:
20 mL vials with screw caps and Teflon-silicone septum, automatic pipette 10 - 100 uL, tips for
automatic pipette.
Chemicals: aviation gasoline, kerosene, motor gasoline NATURAL 95, diesel fuel, gasoline.
Workflow:
1. Cut 3 samples with scissors from different locations of submitted material. Accurately weigh
each sample and adjust its size so that its mass would be about 10 g.
2. Place each weighed sample by tweezers into headspace vials for gas chromatograph, and
capped the vials with a septum.
3. Pipette 10 uL of standards of aviation gasoline, kerosene, gasoline NATURAL 95 and benzine
to other vials.
4. Place headspace vials into GC-MS thermostated carousel with tutor. Then place SPME fiber
inside over the weighed sample and let it to temper and adsorb for 15 minutes at 60 °C.
5. After 15 minutes put the SPME fiber into a heated GC-MS dosing space and perform the
desorption of adsorbed analytes from the SPME fiber to the column placed in the gas
chromatograph.
6. Perform with the help of tutor the identification of individual components and determine the
composition of the volatile liquid that was present in the sample with the accompanying tables
and the results of the analysis of standards of volatile compounds.
Conditions of determination by GC-MS
Separation and analysis of materials was performed under the following conditions:
• Column: HP-5 MS (5% phenylmethoxysilane), length 30 m, Ø 0,32 mm, phase 1 µm.
• Carrier gas: Helium, flow 1 mL/min
• Scan range: 30-400 amu.
• GC program: 40°C–2min, from 40°C to 280°C - dT/dt 10°C/min, 280°C – 10min.