Sample preparation for the chemical analysis of debris in suspect arson cases

Abroad review of current methods for the recovery of liquid residual accelerants in fire debris is given. A summary of current practices of sample preparation methods in U.S. laboratories is presented. Sampling of heated headspace is useful as a screening method. It is also applicable for samples that contain a high concentration of accelerant. Distillation-based methods are effective only when large amounts of accelerants are present but they permit the physical isolation of relatively pure liquids, including some water-soluble materials such as alcohols. Solvent extraction is particularly advantageous for accelerants of low volatility such as diesel fuel or highly evaporated petroleum-based products, but it is very susceptible to interferences from the matrix. Dynamic enrichment by adsorption/elution is the most popular and widely used recovery method for liquid accelerants. It offers high enrichment factors but shows weaknesses with respect to polar and low-volatility liquids. No single method is universally effective for all types of accelerants that are commonly encountered in practice. It appears that currently applied recovery methods are particularly weak for water-soluble organics such as alcohols. Sample preparation methods have to be viewed in conjunction with the method to be used to characterize and identify suspect accelerants. Gas chromatography is used almost exclusively for separation and measurement. Selective detection of components that are diagnostic of accelerants allows important trade-offs between sensitivity, chromatographic efficiency and the level of interferences that can be tolerated. Pattern recognition with comparison of chromatographic profiles is greatly simplified if interferences from the matrix can be suppressed. Application of capillary columns capable of high resolution facilitates the recognition of diagnostically important components. Petroleum-based fluids and products are ubiquitous in the human environment and it is therefore particularly important to pay attention to petroleum-based fluids that are derived from such sources. Many common inconspicuous household materials contain petroleum-based solvents and are thus a part of the natural background. The term sensitivity as it relates to the effectiveness of a recovery method must be critically viewed in this light.