Lesson 2 - Investigating Arson Fires

Analysis of Fire Scene Evidence

Analysis of Fire Scene Evidence

After evidence from a suspicious fire scene has been collected, forensic experts analyze it in laboratories.

In most cases, arson investigators try to confirm if an accelerant was used and the type of accelerant used. Specific identification of accelerants used in arson fires is a challenging task. Often the accelerant can be detected and identified. However, at times an accelerant cannot be detected because all traces of accelerant were destroyed by the fire.

Gas Chromatography:  Gas chromatography can be used to confirm the presence of an accelerant and to identify the type of accelerant. Several devices use the principles of gas chromatography; some are portable, but others operate only in laboratory settings. No matter the type, each device functions similarly.

Gas chromatography separates mixtures of compounds based on the differences in their physical or chemical properties. A stream of nitrogen or helium gas moves a mixture of unknown gaseous materials through a long tube coated with an absorbent separating compound. Depending on how strongly the unknown gaseous materials adhere to the separating compound, the length of time required for the components of the mixture to exit the column will vary. The unknown mixture is then introduced into the detector where there is a small flame. An electrical charge is produced and detected each time the flame meets a different compound within the unknown mixture. The identity of the component is determined by comparing the chromatogram that is produced to a databank of chromatograms of various accelerants and matching up the amount of time the component required to pass through the separating compound.

In some cases, a criminal profiler is brought into an arson investigation to create a profile of the suspected arsonist. The profiler will use information and evidence from the arson scene and any available witness reports. The profile is used to identify or rule out potential suspects.

Mass Spectrometry: In mass spectrometry, the ionized components of unknown molecules are separated. This is done by passing the charged ions through either a magnetic or an electrical field. Each type of ion is then detected and used to identify the unknown compound. The disadvantage of mass spectrometry is that it can identify only one compound at a time. Mass spectrometry cannot be used to identify mixtures. However, gas chromatography is excellent at simultaneously separating several compounds in an unknown mixture. Most accelerant residues found in fire debris involve complex mixtures. Therefore, using both gas chromatography and mass spectrometry is currently the best way to identify accelerants found at arson scenes.

Metal Oxide Sensors (MOS): Semi-conductor or metal oxide sensors (MOS) are the oldest and least expensive measurement technologies used in arson investigations. Gas fumes from an arson scene are exposed to the MOS, which consist of a thin film of a metal oxide on a substrate. The substrate is heated, which causes a chemical reaction between the gas and the metal oxide. This reaction changes the electrical conductivity of the metal oxide. The resistance of the metal oxide film is measured using electrodes. Although MOS sensors can detect a broad range of chemical contaminants very quickly, they may also respond to moisture or carbon dioxide, which results in false measurements.

Fire protection engineering is the application of science and engineering principles to protect people and their property from the destructive effects of fire and smoke. Fire protection engineers identify risks and design safeguards that help prevent, control, and minimize the effects of fire. Fire protection engineers assist architects with building safety evaluations and often function as fire investigators. For example, fire protection engineers investigated the collapse of the World Trade Center.