What is accelerant – in forensic science

In arson cases, an accelerant is the flammable material that is used to start the fire. Accelerants can be solids, liquids, or gases, with gasoline being the most commonly used. Solid accelerants include paper, fireworks, highway flares, and black powder. Butane (cigarette lighter fuel), propane, and natural gas are examples of gaseous accelerants, which do not leave any residue at a fire scene. However, gases must be contained and transported, so severed gas lines or spent containers serve as critical physical evidence in such cases.

Liquid accelerants fall into two broad categories: petroleum distillates, which include gasoline and other petroleum products; and nonpetroleum products such as methanol, acetone (used in nail polish remover), and turpentine.

Petroleum distillates are derived from crude oil and are also called hydrocarbons or petroleum hydrocarbons. In crude oil, volatility of the individual components range from extremely volatile substances such as propane (a gas at room temperature) to asphalt, which remains solid even at high temperatures.

Petroleum distillates such as gasoline and kerosene are not single hydrocarbons but mixtures of different components with similar volatilities. The volatility of an accelerant is an important consideration in the combustion process, determining how much residue will be left and how quickly it will evaporate after the fire is out. Related to volatility is the flash point, defined as the temperature at which a liquid will give off enough vapor to form an ignitable mixture. For gasoline, the flash point is -50°F (-45.56°C). The National Fire Protection Association (NFPA) defines a flammable liquid as one with a flash point of less than 140°F (60°C).

Based on volatility and molecular structure, petroleum distillates are often divided into the following categories:

Category Example

Light petroleum distillates (LPD) Propane, butane

Medium (MPD) Paint thinner

Heavy (HPD) Kerosene, diesel fuel

Aromatics Benzene, toluene, xylenes

Mixtures Gasoline

Aromatic hydrocarbons have unique molecular structures and were originally named based on their distinctive smell. Aromatics such as benzene and toluene are found in gasoline.

At a fire scene, the presence of accelerants can be determined using several procedures including trained dogs, chemical color tests, and portable instruments and sensors. Materials such as wood and carpet absorb liquid accelerants, so samples of these materials can harbor valuable evidence. It is important for the investigator to collect control samples since carpets and other synthetic materials can interfere with or lead to false positives during laboratory analysis. Evidence from the scene is usually collected in glass jars or metallic paint cans that are tightly sealed to prevent vapors from escaping. As shown in the figure on the previous page, once the can is sealed, any volatile accelerants present will continue to evaporate into the headspace above the debris.

The primary tool used to detect and identify liquid accelerants is gas chromatography (gc) coupled to either a flame ionization detector (fiD) or a mass spectrometer (ms). Analysis using either instrument produces an output that is distinctive for most common petroleum distillates. Patterns are identified by comparison to standards of known composition. The patterns obtained from evidence can be influenced by weathering and by microbial activity, particularly if the sample is on soil or vegetation. Weathering occurs as lighter (more volatile) components of the accelerant evaporate, and the longer the sample sits before collection, the more severe the weathering effects.

Samples are prepared for introduction into the GC using several methods:

1. Cold headspace: The can is punctured and a syringe is used to withdraw a headspace sample that is injected into the GC.

2. Heated headspace: Prior to syringe introduction, the can is heated.

3. Extraction: The accelerant is extracted from the sample using a solvent such as carbon disulfide or steam. Small portions of the extract are injected into the GC.

4. Purge-and-trap: Inlet and outlet holes are put in the can lid. A stream of filtered air is pumped in through the inlet and a charcoal trap is placed on the outlet. The can may be heated, and vapors are trapped on the charcoal. The trapped compounds can be removed using heat (thermal desorption) or solvent extraction.

5. Charcoal strip/soud phase microextraction (spme): A charcoal strip or other adsorptive material is lowered into the can or placed on an inlet drilled into the can. A vacuum can be used to draw sample through the trap or a stream of filtered air can be pumped into the can to force headspace to flow out through the trap. The can may be heated, with a thermometer inserted in the can to monitor temperature.

In some cases, the presence of a flammable material in a given area is to be expected and may not be associated with arson. For example, if a fire is started in a garage where a car is parked and gasoline powered equipment such as a snow blower or lawn mower is kept, the gasoline associated with the tools or car is considered to be an incidental accelerant that would normally be present in the area.