SFR Guidance > Ignitable Liquids

Guidance Notes: Ignitable liquids (or accelerants)

Ignitable liquids are sometimes known as fire accelerants because, apart from their normal uses as fuels and solvents, they can be used to assist in starting and/or accelerating the progress of a fire. These liquids invariably consist of volatile ignitable organic compounds. These fall into several main groups typified by petrol, petroleum distillates, other oxygenated products such as methylated spirits and specialist solvents. These materials often comprise complex mixtures of chemical compounds; the type and proportion of which determine the overall properties and identity of the material. Some of the compounds may be common to more than one type of material. For example, paraffin contains compounds found also in diesel oil. Petrol contains a highly volatile (rapidly evaporating) mixture and diesel contains a less volatile (slowly evaporating) mixture. Lighter fuel is a blended distillate product which is highly volatile and will evaporate very rapidly after it is dispensed.

If an ignitable liquid is submitted to the laboratory for analysis, it is analysed by automated thermal desorption gas chromatography – mass spectrometry (ATD-GCMS). This is a technique that can be used to separate and identify the chemical compounds present in the ignitable liquid, allowing the forensic scientist to form a view as to the identity of the liquid.

Packaging

Items for ignitable liquid analysis should be packaged in nylon evidence bags when transported to the laboratory. This is because standard paper and polyethylene bags can allow the volatile material to evaporate during storage and their construction can interfere with the results obtained. These should be sealed by twisting the top of the bag, and folding it over into a swan-neck, which is sealed with a cable tie or string. Cellmark recommends an additional layer of polythene packaging is added to the outside of this packaging, sealed as above, which can help retain some types of ignitable liquid residue. A control sample of packaging should always be taken by the officer seizing exhibits from persons or scenes involving ignitable liquids. These are analysed to provide a record of any volatile material that was present in or on the packaging from the same batch, prior to its use to store exhibits. They also provide a record of any potential contamination which could have occurred to the items during their storage and transport to the laboratory.

Examination

If an item of clothing, or debris from a fire scene, is submitted for examination, it is generally heated within the packaging it is submitted in. This causes any ignitable liquid to vaporise and move into the gas contained in the bag. A sample of this gas is then removed using a specialist tube and is analysed using ATD-GCMS to determine if any such residue is present within the item. Thus, the integrity of the packaging of the exhibit is crucial in ensuring the ignitable liquid is retained for analysis. The method does not give an indication, except in very general terms, of the quantity of an ignitable liquid that present on an item, or the location it was splashed on the item.

Classifications

The following table provides information on common ignitable liquids and their potential uses:

Ignitable liquids on clothing

When a person uses an ignitable liquid to aid in the setting of a fire it is possible, but not inevitable, that they may spill or splash some of the liquid onto their clothing or footwear.

Any liquid deposited in such a manner will persist and remain detectable for some time afterwards; laboratory analysis of such items being able to detect and identify any ignitable liquid residues present. As the compounds in question are volatile it is not expected that they will persist on clothing for long periods of time. Hence if an ignitable liquid is detected on clothing its presence is indicative of a recent contact with a source of that ignitable liquid.
The method used to detect such ignitable liquid residues on items provides no information as to where on the item the liquid residue is. It is therefore not possible to comment on how any ignitable liquid came to be present on the item.

Depending on the time the clothing items were seized relative to the incident, it is also possible that a person could splash volatile liquid on themselves and it to have evaporated before the police have seized and properly packaged the clothing. Therefore, in most situations, a negative finding could be explained by the total evaporation of the ignitable liquid and does not indicate the person in question has not recently handled or distributed such a liquid.

Ignitable liquids in debris from a scene

If the rate of development and spread of a fire has been increased by the use of a volatile ignitable liquid, small amounts of the ignitable liquid may still be present after the fire has been extinguished. Traces of the ignitable liquid can remain in crevices or absorbed into protected material. If such material is subsequently recovered, it may be possible to detect and identify the ignitable liquid used. However, an ignitable liquid need not necessarily be used in setting a fire and any used may be consumed in the fire, or subsequently evaporate once the fire is out.
Although droplets of liquid may persist, particularly in soft surfaces, throughout a fire, it is possible that all of the traces of an ignitable liquid may be consumed during the fire, or evaporated in the time taken to recover the samples post-incident. It is also possible that the fire investigator has not sampled the correct location, therefore a negative finding should generally not be viewed as evidence that an ignitable liquid was not used to start a fire.
In cases where a Fire Service dog reacted in the vicinity of the items; that reaction could be due to traces of ignitable liquid fumes which had evaporated prior to packaging of the items, or it may be that the dog reacted to hydrocarbon pyrolysis products, rather than the true presence of an ignitable liquid. Combustion of some synthetic materials can result in volatile substances which are chemically similar to the hydrocarbons found in some petroleum products.

Samples of neat Ignitable liquids

Liquid samples can also be examined in the laboratory by the same analytical process. A small aliquot of the liquid is recovered and analysed by ATD-GCMS which enables the forensic scientist to form a view of the identity of the liquid, which will be classified according to the table above.
Samples can either be decanted by the submitting authority prior to submission or sent in their original containers where appropriate precautions for DNA or fingerprint analysis can be made (see below).

It is possible to distinguish between different types of ignitable liquids and to make some basic comments on the comparison of them however there is currently no robust analytical process which allows us to make a detailed comparison and interpretation of the comparison.

Ignitable liquids on items - Preservation for DNA and fingerprints

If the submitting authority has requested that an item is preserved for future DNA or fingerprint examinations, the item will be heated to a lower temperature, or not at all to prevent those evidence types being compromised. Appropriate anti-contamination measures are also put in place to preserve the integrity of the DNA evidence. This can reduce the efficacy of the ignitable liquid analysis so the relative importance type of the evidence types must be considered in advance.

Petrol bombs

A "petrol bomb" (also known as a Molotov cocktail) is constructed from a friable container (usually a glass bottle) filled with an ignitable liquid (typically petrol) and incorporating a means of ignition, such as a wick.

When thrown against a hard surface, the bottle shatters on impact, distributing the fuel in a cloud of droplets and vapour. The flame of the wick ignites the cloud, causing a fireball.

Petrol bombs (where specifically containing petrol, as opposed to any other ignitable liquid) have been held by the Court of Appeal in the United Kingdom to be explosive substances for the purposes of the 1883 Explosive Substances Act, as per R. v Bouch, [1982].