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Investigators can prove that a fire was set intentionally by finding an accelerant at the scene of a fire. An accelerant is a chemical fuel that causes a fire to burn hotter, spread more quickly than usual, or be unusually difficult to extinguish. The presence of an accelerant in fire debris can be used as evidence of arson.

The most commonly used accelerants are gasoline, kerosene, turpentine, and diesel fuel. These are all organic compounds containing mixtures of hydrocarbon molecules. As accelerants evaporate, the hydrocarbons more into the air above the fire debris, which is called the "headspace."

Various techniques exist to detect accelerants at a fire scene. These range from an experienced fire investigator or a specially trained "sniffer" dog using their sense of smell to detect the characteristic odor of various accelerants in the surrounding air, to more complex laboratory methods.

One of the most advanced techniques for detecting accelerants in fire debris is called headspace gas chromatography. Gas chromatography involves separating mixtures of gases into their individual components based on the different boiling points of their hydrocarbons. Each gas in the mixture can then be identified, because each produces a distinct chemical fingerprint called a chromatogram.

In headspace gas chromatography, solid debris taken from the suspected point of origin of the fire is placed in an airtight vial to prevent any accelerants from evaporating. The vial is then heated, releasing the accelerant's hydrocarbons into the trapped headspace above the debris. A needle is inserted through the cap of the vial to remove a sample of the hydrocarbons and inject them into an instrument called a gas chromatogram for separation and analysis.

If an accelerant is used to start a fire, a small amount will likely still be present in the charred debris. Identification of the accelerant can serve as physical evidence to support a charge of arson.