6.05 Ignitable Liquids

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Ignitable Liquids

Accelerant

Once investigators identify the origin of the flame, if they suspect the fire was intentionally set, they will look for evidence of an accelerant, which is a solid, liquid, or gas intentionally placed at the scene to start and sustain a fire.

Hydrocarbons

Most accelerants are complex mixtures of hydrocarbon molecules, including gasoline, kerosene, and diesel. The components of these mixtures will vaporize at various temperatures, therefore their composition prior to burning will be different than after.

The lighter hydrocarbons will vaporize at lower temperatures and become more concentrated in the headspace, which is the area of air located above the debris in a sealed container. The heavier components of the mixture will remain in the debris.

Recovering Ignitable Liquids

The following are different methods used to identify ignitable fluids:

• Sense of smell: According to Australian fire scene forensic examiner Tony Café, “The human sense of smell can quite correctly identify the presence of accelerants, even in trace amounts. This ability varies amongst investigators as the sense of smell is like most other senses and can become highly developed through experience, or it can become impaired both temporarily or permanently.”
  • Pros: Investigators may notice strong, unusual odors that suggest a possible ignitable liquid and quickly point out areas that should be tested.
  • Cons: Smoke and other background odors can hide or change the smell. Breathing in vapors from gasoline and other fuels can be harmful, so investigators should not rely on purposely sniffing containers or debris.
• Canines: Canines can be trained to detect signs of accelerants and ignitable liquids, even minuscule amounts which may be buried under heaps of burned debris and rubble.
  • Pros: Specially trained dogs can locate very small amounts of ignitable liquid residue, even when it is buried under debris. They can also search large areas faster than people.
  • Cons: A dog’s alert only means there might be an ignitable liquid. Fire-scene guidelines, such as NFPA 921, say that canine alerts must be confirmed by laboratory analysis before they are treated as evidence that an ignitable liquid is present.
• Electronic sniffers: Electronic “sniffers” may also be used to detect the presence of hydrocarbons. A small vacuum pulls vapors into an opening where a chemical detector identifies both the presence and concentration of ignition fluids. Photoionization detectors (PID) are tools that also recognize the presence of hydrocarbon vapors. These detection methods respond to a wide variety of compounds, including non-accelerants such as hydrocarbons from rubber-backed carpet.
  • Pros: Hydrocarbon detectors and photoionization detectors (PIDs) can sense very low levels of hydrocarbon vapors in real time. They help investigators find good spots to collect samples.
  • Cons: These devices respond to many different vapors, including some that are not accelerants. A high reading does not prove that a specific ignitable liquid is present, so results must be checked later in the lab. This is why electronic “sniffers” cannot confirm the presence of an accelerant.
• UV light: UV light can also be used to detect the presence of accelerants. Gasoline, kerosene, and other components of accelerants such as benzene will fluoresce under UV light, allowing investigators to visualize pour patterns, potentially leading to the original container.
  • Pros: Many volatile hydrocarbons, including gasoline and kerosene, glow (fluoresce) under ultraviolet light. This can make pour patterns or hidden stains easier to see and photograph.
  • Cons: Other materials, such as some oils, paints, and plastics, can also fluoresce. UV light cannot identify a specific accelerant on its own; it mainly helps investigators choose where to collect samples and record patterns

• Other Methods: Read Accelerants in Arson Cases: Identification at the Fire Scene to learn more about these and other methods used to identify ignitable fluids. Note the pros and cons of each method.
  • Kerosene is a combustible hydrocarbon liquid, which is derived from petroleum.
  • Gasoline, or petrol as it’s called in many other countries, is a common accelerant. It’s a transparent, petroleum-derived flammable liquid.
  • Mineral spirits: this petroleum-derived clear liquid is known by many names, including mineral spirits (US, Canada), white spirit (UK), mineral turpentine (AU/NZ), turpentine substitute, and petroleum spirits.
  • Traditional diesel is also a petroleum-derived fuel oil, but alternatives that are not derived from petroleum, such as biodiesel that’s derived from soybean oil, biomass to liquid (BTL) or gas to liquid (GTL) diesel.

Knowledge Check #1

True or False: Electronic "sniffers" can confirm the presence of an accelerant.

1. True
2. False

Answer: b. False

Accelerant Evidence Collection

Many arsonists employ similar methods when using an ignitable liquid to set a fire. Some important tips on collecting residue samples follow.

If evidence of ignitable liquid accelerant use is identified, always search for the container. Latent fingerprints can often be developed even on scalded or sooty containers. Containers are often found:

• at the end of a pour pattern,
• thrown back into the trail,
• onto the roof above the exit,
• into a rubbish disposal,
• in nearby vegetation along the escape route, or
• may be found in the suspect's vehicle or house

Many arsonists "trail" an ignitable liquid across a floor toward a secluded building exit or interior barrier to make ignition and escape safer, therefore:

• Begin your search for evidence by looking for objects that do not seem to belong.
• Concentrate the search for accelerants beginning where any suspected accelerant container was found, or from any possible escape point and leading back towards the areas of greatest damage.
• Concentrate the search for remains of the ignition device (matchbook, etc.) at or near the most probable escape point.
• The best possible ignitable liquid residue samples are often found around the point of origin; the best physical evidence proving a forcible entry is usually at the point of entry itself.

To Collect or Not To Collect?

One major fire investigator skill is knowing what to collect and what not to collect. Ignitable liquids used as accelerants burn better than most of the surfaces onto which they are poured. Expect to find better, stronger samples in protected areas and inside absorbent materials within the pour pattern.

Reference (Comparison) Samples

When investigators collect debris that might contain an ignitable liquid, they may also collect a reference, or comparison, sample. A comparison sample is the same type of material (such as carpet, wood, or padding) taken from an area that is not believed to contain an ignitable liquid.

In the laboratory, scientists test both the debris sample and the comparison sample. The comparison sample shows which chemicals come from the material itself. Any extra peaks that appear only in the debris sample are more likely to be from an ignitable liquid that was added to the scene. Collecting comparison samples helps analysts avoid confusing normal building materials or household products with an accelerant.

Most and Least Desirable Collection Areas

Most Desirable Collection Areas	Least Desirable Collection Areas
Lowest areas and insulated areas within the pattern	Deeply charred wood
Samples taken from porous plastic or manmade fibers	Gray ash
Cloth, paper, cardboard in direct contact with the pattern	Edge of a hole burned through a floor
Inside seams, tears, cracks	Samples from absolutely nonporous surfaces
The edges of burn patterns	The center or any burn pattern
Floor drains, bases, of load-bearing columns or walls	In general, areas exposed to greatest fire hose streams

Knowledge Check #2

Where should investigators concentrate their search for an accelerant container?

1. around the origin of the fire
2. around the probable exit point of the arsonist
3. around garbage cans within the home
4. around sinks within the home

Answer: b. around the probable exit point of the arsonist

Knowledge Check #3

Which of the following is most desirable for collecting accelerant evidence?

1. nonporous surfaces
2. the center of burn patterns
3. porous plastic
4. deeply charred wood

Answer: c. porous plastic

Volatile Substances

During evidence collection, all potentially volatile substances, or substances that can easily change to a gas at room temperature, should be stored in airtight containers (metal containers, glass jars, or impervious plastic bags). These samples should be stored and submitted separately.

Volatile samples are usually analyzed using a technique called headspace analysis. During headspace analysis, a piece of activated charcoal or a similar adsorbent material is stored in an airtight container with the volatile sample. Adsorbent materials allow a gas to form a thin layer on its surface. Volatile compounds are drawn into the material either passively or dynamically. After, the volatile material is desorbed, or heated to release, for analysis ("Fire Investigation").

Testing Vapors

Once the accelerants are released, samples of the vapors are collected for testing by Gas Chromatography (GC) and Mass Spectrometry (MS), which will provide details about the compounds within the vapor including their molecular mass and relative abundance. These methods can detect as little as 1/1000 of a drop of accelerant on a piece of evidence.

Gas Chromatography

Chromatography is a method of separating the components of a mixture based on their properties. Previously, we saw how paper chromatography was used to separate the components of black ink.

Testing Vapors

Gas chromatography (also known as GC) works in a similar way to separate the components of a mixture of gases. The gas molecules will move through the column at different speeds based on their attraction to either the mobile phase (the carrier gas) or the stationary phase (the column which contains small particles). As the gases exit the column they are detected, and a graph is created.

Below are two examples of diagrams showing information about gas chromatography:

Results

The graph produced shows the time it took for the mixture to exit the column on the x-axis and the signal strength on the y-axis. The signal strength relates to the relative abundance of that compound within the mixtures. Look at the graphs below for different mixtures of octane and nonane.

• In mixture 1, the peak for nonane was higher.That means there was more nonane in the mixture than octane.
• In mixture 2, the peak for octane was higher.That means there was more octane in the mixture than nonane.
• In mixture 3, both peaks are about the same size. Octane and nonane have similar abundance within the mixture.

If you look along the x-axis, you see the time it took for the gas molecules to leave the column. Octane has a peak around 5 minutes, while nonane has a peak around 15 minutes. Because nonane took longer to exit the column, it has a greater attraction to the stationary phase of the column.

Knowledge Check #4

Which substance showed greater attraction to the stationary phase?

1. Octane because it exited the column more quickly.
2. Nonane because it exited the column more quickly.
3. Octane because it took longer to exit the column.
4. Nonane because it took longer to exit the column.

Answer: d. Nonane because it took longer to exit the column.

Knowledge Check #5

In which mixture was the concentration of nonane greater than the concentration of octane?

1. Mixture 1
2. Mixture 2
3. Mixture 3
4. All mixtures have an equal concentration of nonane.

Answer: a. Mixture 1

Mass Spectrometry

Mass spectrometry is often combined with gas chromatography. This method separates atoms and molecules according to their molecular mass. The sample is loaded into the mass spectrometer where it undergoes vaporization and ionization, which means it is given a charge by the removal of one or more electrons. As the sample passes through a magnetic field, the amount which it curves reveals its mass, with large compounds deflecting less than smaller compounds.

Spectrometry

Open Spectrometry (FuseSchool) in a new window

Note: The presentation may take a moment to load.

Knowledge Check #6

Arrange the steps of mass spectrometry into the correct order.

1. Deflected by a magnetic field.
2. Ionized by bombarding it with electrons.
3. Detected and displayed as a mass spectrum.
4. Accelerated in an electric field.
5. Vaporized by heating.

Answer: The correct order is

1. Vaporized by heating.
2. Ionized by bombarding it with electrons.
3. Accelerated in an electric field.
4. Deflected by a magnetic field.
5. Detected and displayed as a mass spectrum.

Knowledge Check #7

The height of a given peak tells the ________ of the substance.

1. mass
2. size
3. abundance
4. charge

Answer: c. abundance

Known Accelerants

These analysis methods create a chemical "fingerprint," which is compared to patterns of known accelerants that are stored on a database.

Accelerants are classified based on their chemical composition that is revealed using GC and MS.

Review the Ignitable Liquid Classification System chart from the ATF (scroll down to the bottom of page 3).

Presence of Accelerants

Does the presence of a known accelerant confirm that a fire was intentionally set?

Open Forensic Chemist Describes the Challenges of Investigating a Fire Scene from Chemical and Engineering News to hear from forensic chemist Jose Almirall. Scroll down to the section "Do any cases stick in your mind where this was particularly challenging?" and read Almirall's response. Then, reflect on the question above.