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Chemical Analysis of Fire Debris
By Wolfgang Bertsch

Losses as a direct result of fire are estimated at about $6 billion annually, and about 20 of all fires are believed to be of incendiary origin. The National Fire Protection Association states that in 1994, some 107,800 arson-related fires were reported, and 550 people lost their lives in these fires (1). Fifty-five percent of arrests for arson involved juveniles-33% under the age of 15 and 7% under the age of 10. Combating arson involves individuals from a wide range of disciplines, from finance to the legal profession. Forensic chemists work with physical evidence to determine whether there are residual materials in the fire debris indicating that the fire was deliberately set and/or accelerated.

An arson investigation usually begins right after a fire has been brought under control. The fire chief makes a preliminary determination of the cause and origin of the fire by assessing the factors that contributed to the fire's behavior. A fire that appears to have multiple origins, suspicious burn patterns, an unusually high rate of spreading, or remnants of an ignition device is a primary candidate for arson. Arson investigators often have backgrounds in law enforcement, electrical engineering, or some other area of science and have had formal training in fire investigation. They interrogate victims, suspected perpetrators, and others who may have pertinent knowledge about the source of the fire.

The fire scene is photographed, and all items removed from it are documented. Physical evidence such as charred material is collected, and a strict chain of custody is established. The fire investigator weighs all of these factors and determines whether a fire originated from natural causes or appears to be of an incendiary nature. If the physical evidence points toward a fire that has been set deliberately, the investigator assembles as much information as possible to thoroughly document the case. Indirect evidence, such as motive, opportunity, and past history of the beneficiary of the fire, is then weighed to determine whether a crime has been committed and if an insurance claim should be settled or denied.

The nature of the beast
Conventional fires can be characterized as a complex interaction between fuel, air, and heat. A source of ignition of sufficient energy is required to start the chain reaction in which a fuel is converted into gaseous products, mostly water and CO2. Fire behavior can be predicted and modeled fairly accurately once the starting conditions have been established. The nature of the fuel and the size, dimensions, and ventilation of the afflicted structure are factors in determining the velocity, duration, and intensity of fire development.

Reactions are complex but distinctively different for normal fires and those that have been accelerated by an artificial source of fuel, such as gasoline. In a normal fire, energy in the form of hot gases moves upward in a room, and the fire spreads vertically if fuel is available. A room may fill with hot gases at the ceiling, which in turn may create other sources of ignition. These gases, soot, and pyrolysis products ignite once the temperature increases sufficiently, and the fire may spread across the top of the room and into adjacent spaces if there are no physical barriers. There the layer of hot gases may initiate further thermal decomposition of other fuel sources, resulting in a flashover. At this point, temperatures rise rapidly and also spread downward, distributing heat more evenly. Further pyrolysis takes place when the oxygen content in a confined space drops rapidly.

In contrast, accelerated fires produce a large amount of heat from the readily available fuel vapors within a short period of time and at a specific location. In extreme cases, gases can expand so rapidly that windows or doors are blown out with explosive force. If liquid accelerants are used, the available oxygen may be quickly consumed, leaving an excess of fuel. The flames are intense, but the amount of heat and the rate at which it is transferred into the matrix may be insufficient to cause pyrolysis at a level necessary to sustain combustion.

In most cases that involve arson, petroleum-based distillates such as gasoline, paint thinners, charcoal lighter, and kerosene are poured along the corners of the walls and floors of a structure. Information is scarce on alcohol-based accelerants; it is unclear whether this lack of information indicates a potential weakness in analysis methodology or whether the use of alcohol as an accelerant is simply underreported. Solid accelerants that involve mixtures of oxidizers (chlorates or nitrates) and combustible compounds (sugar or starch) require analysis procedures that are entirely different from those used for liquids.

The analytical problem
The basic goal of chemical analysis of fire debris is to establish whether materials are present in the remnants of a fire that could have helped to start or accelerate it. At first it seems that a highly volatile fuel such as gasoline would "go up in smoke" along with the structure. (This is certainly the expectation of the arsonist.) However, traces of the fuel often remain even after an intense fire, and successful collection and analysis of these residual accelerant samples is as much an art as it is a science.

Sample preparation
The first step is to determine the location from which a debris sample should be taken. The human nose is the most widely used primary detector, but it has obvious shortcomings. Although several canine accelerant detection programs have been used across the country, dogs respond to minute traces of hydrocarbon vapor but cannot distinguish between residual accelerant vapors and those generated by burned furniture, building materials, or plastics. Mechanical devices, so-called "sniffers," can signal the presence of volatiles but also cannot distinguish between pyrolysis products generated by the fire and residual accelerants.

Serious problems can also arise from the composition of the debris matrix. For example, fire investigators frequently sample carpet or carpet padding because these materials tend to retain liquid accelerants. Although these matrices are a logical choice from the investigator's point of view, the synthetic polymers used in these materials also tend to generate copious amounts of pyrolysate containing volatile hydrocarbons.

Isolating target volatiles in a reasonable quantity is another challenge. Several sample preparation procedures can be used to enrich and isolate the analyte from the matrix; the final choice of method depends largely on the properties and concentration of the expected accelerant. If destructive testing is used, part of the sample must be saved in case additional testing is necessary. Although it would appear that these factors present significant hurdles, they are actually quite manageable in practice.

Analysis
Although the physical properties of petroleum distillates (such as ignition temperature and evaporation rate) vary widely, chemical composition is more uniform, and accelerants can be classified according to their carbon number and dominant components.

Because of their volatility, accelerants are particularly amenable to GC. Surprisingly, it may not be difficult to correctly classify a distillate, even after as much as 90 evaporation. For example, kerosene maintains many of its characteristic chromatographic features following evaporation, whereas gasoline profiles after evaporation show little resemblance to those before evaporation. Recognition of weathered accelerants thus requires a library of standards and a keen eye.

Bertsch, W.(September 1996). Chemical Analysis of Fire Debris. Analytical Chemistry 1996, 68, 541A-545A.

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