Introduction

Purpose

Perform burn susceptibility tests, analyze known accelerants, and determine the identity of an unknown ignitable liquid residue.

Fire Evidence

Accelerants are substances used to aid in the spread of fire. You may use them to jump start a camp fire or charcoal grill or fuel your car. Many substances can be used as accelerants outside of those marketed for that purpose, though – acetone, mineral spirits, isopropanol, high proof liquors, , some household cleaners, and more can all play a part in accidental or intentional fires.

Arson is the term we use to describe the intentional use of fire to destroy property with criminal or fraudulent intent. In arson cases, many aspects of the crime scene are investigated to try to piece together a sequence of events like in any other crime scene. Arson scenes, though, are much more complex due to the damage done by the fire, evidence damaged or lost while putting out the fire, and possible safety risks making part or all of the scene inaccessible.

One way chemists can help the investigation is to confirm the presence and identity of ignitable liquid residues (ILRs) from the scene. ILR refers to unburned, ignitable liquids left behind after a fire has occurred. Confirming the use of an accelerant in a fire can make the difference between classifying a scene as arson or accidental, though it is important to remember that the presence of ILRs does not necessarily indicate arson. Investigators will need to determine how it may have been used or why it may have been there incidentally before attempting to designate a fire as arson. That’s why all factors must work together to put scientific analysis and results into context with other evidence and investigators’ expertise.

A Few Types of Accelerant Testing

Burn Susceptibility Testing

Burn susceptibility testing is sometimes done by investigators at the scene or lab personnel on a small fragment of evidence from a fire scene. It’s fairly simple to perform, though expertise and knowledge of how a variety of accelerants burn is necessary to make it a particularly useful practice.

When assessing a burn test, descriptive observations should be noted in real time describing how readily the item ignited, how long it burned, did the flames spread quickly, color of the flames, presence/color of any smoke, presence/color of any residues, and any other noises, reactions, movements, etc.

Hydrocarbon “Sniffers” / Gas Detectors

These are handheld devices that can detect the presence of hydrocarbons or other gases depending on the model and application. They can be made with a variety of technologies such as electrochemical sensors, photoionization detectors, infrared, laser diode spectroscopy, and so on. They are typically not specific, though some may be able to classify the gases into broad classes (light, medium, heavy gases, for example).

GC Analysis

While portable tools to detect presence and maybe broad classification of hydrocarbons are available, portable confirmatory instrumentation is not yet common (though researchers are definitely working on making it easier, better, and more appealing!). So when we need to confirm, compare, and/or identify an ILR from a scene, investigators will seal the evidence in an unlined metal can (like a paint can) or other airtight container. This preserves the volatile compounds we’re interested in analyzing until it gets to the lab. GC is typically used to analyze ILRs, most often in conjunction with mass spectrometry or perhaps an FID.

Gas Phase Sampling

Headspace sampling is one of the most common analytical methods to test accelerants and may be performed by placing a small portion of the evidence in a headspace vial, heating the vial, and using a gas-tight syringe to extract gas phase substances and inject onto the GC column. This is just like how we sample blood samples for alcohol content.

Activated charcoal strips are also sometimes used – they are placed in a container with the evidence at a specified temperature for a specified period of time and will adsorb ILRs. The strips can be subsequently analyzed by washing with a solvent and injecting that solution onto the GC or direct analysis with newer MS ionization methods.

Similar to the charcoal strips, SPME (solid phase microextraction) could be useful in this application – this is a type of headspace sampling that uses a thin fiber coated with a stationary phase (sort of like the filter coating in SPE you used for urine toxicology) to adsorb certain types of gas phase molecules rather than aspirate a full sample of the headspace. This method can result in cleaner data depending on the coating used, as it may exhibit some increased specificity for some types of compounds and avoid other volatile contaminants that would be drawn in by a gas-tight syringe.

Solvent Wash

A solvent wash means applying a organic solvent to the evidence either in a clean container like the image below or carefully pouring/pipetting the solvent over the object into a clean container. This might be used for more large-chain hydrocarbons that do not evaporate as readily or for very large pieces of evidence. It is susceptible to increased matrix effects, over-diluting trace amounts of analytes, and is generally less desirable than most other methods when it is not necessary.

Materials and Methods

Burn Susceptibility Tests

Supplies

1. 3-5 accelerants
2. 3-5 sample materials
3. large watch glasses or ceramic evaporating dishes
4. glass pipets
5. lighter/torch

Procedure

It is VERY important that you stay focused and pay close attention while performing this exercise. You will perform test burns of a variety of materials with no accelerant first so you have a frame of reference of how that material burns on its own. Then, you will perform test burns on those same materials with a variety of accelerants.

Preliminary Observations and Prep

1. Take note of the characteristics of each sample material
2. Take note of the characteristics of each accelerant
3. Make sure you have enough 1-2 cm² pieces of each material to burn
   • 1 without accelerant and 1 for each accelerant you are testing

Prep for Burn Test Without Accelerant

1. Add a small piece of sample material, no larger than 1-2 cm² to the dish provided
2. Place the watch glass in a hood

STOP

Ensure everyone is paying attention before proceeding and that no one is doing something in another part of the hood.

Once safety is assured, proceed.

Perform Burn Test Without Accelerant

1. Carefully light the material and remove all hands from the hood to observe and take notes
2. If it is still burning after 1-2 minutes and has a flame small enough that you feel comfortable, you may use a second watch glass to cover the sample and smother the flame
3. Allow any remaining smoke to dissipate before proceeding

Prep for Burn Test With Accelerant

1. Add a small piece of the first material, no larger than 1-2 cm² to the dish provided
2. Place the watch glass in a hood
3. Using a glass pipet, carefully add ~1 mL of the first accelerant to the center of the material
4. Allow this to sit for ~5 minutes

STOP

Ensure everyone is paying attention before proceeding and that no one is doing something in another part of the hood.

Once safety is assured, proceed.

Perform Burn Test With Accelerant

1. Carefully light the material and remove all hands from the hood to observe and take notes
2. If it is still burning after 1-2 minutes and has a flame small enough that you feel comfortable, you may use a second watch glass to cover the sample and smother the flame
3. Allow any remaining smoke to dissipate before proceeding

Repeat both sets of instructions for every sample material and accelerant combination provided.

Analysis of Unknown

In this exercise, you’ll be given an unknown sample that has been recovered from an arson scene and an unburned portion of the same material. Many materials contain volatile substances that may also be present in the headspace of your unknown.

Supplies

1. Unknown arson evidence sample
2. Reference sample of unburned evidence
3. Reference samples of accelerants
4. Headspace-GC-MS or GC-FID

Procedure

1. Perform an inspection of the burned sample and note any remarkable characteristics compared to the unburned sample (visual observations, smells, etc.)
2. Place your reference samples and unknowns into headspace vials and into the headspace autosampler
   • If no autosampler is available, place vials in an oven, heating block, or bead bath at 55-65 degrees C for 10 minutes
3. Run your samples
4. Go over the data with your TA or the lab coordinator to ensure that you achieved good separation of multiple components of each accelerant
5. Obtain raw data for a representative chromatogram of each sample or type of sample