This paper reports on work done to better define the causes of chromatographic profile distortion of hydrocarbons that have been sampled by adsorption onto activated charcoal, and it proposes a potential method for avoiding chromatographic distortion in some cases by using a subsampling technique.
In a study of the performance of commercial containers for the storage of evidence from fire debris, the passive headspace concentration of ignitable liquids was monitored by adsorption onto activated charcoal, following the ASTM E1412 standard practice, which is the commonly used method for separating ignitable liquid residues from fire debris. The current study was conducted to obtain a better understanding of the system dynamics, which is necessary in order to develop an experimental design for investigating the performance of commercial containers. Examination of the dynamics of the passive headspace concentration system involved the characterization of the hydrocarbon vapor behavior within a closed system. The study determined the concentration of hydrocarbons in the system, the distribution of the hydrocarbon vapors within the system, and the hydrocarbon adsorption behavior on the activated charcoal in order to assess their influence on the chromatographic profile of the ignitable liquid. In a one-quart container, hydrocarbon volumes as small as 24 ml (liquid) were sufficient to saturate the surface area available for adsorption on a 99 mm square of activated charcoal, resulting in significant distortions in the molar ratio and the chromatographic profile of the recovered hydrocarbons. Passive headspace sampling of a similarly small volume of unweathered gasoline spiked onto carpet padding resulted in a significant distortion of the chromatographic profile. The chromatograhic profile of the recovered hydrocarbons closely resembled 75-percent weathered gasoline. Heating the container spiked with unweathered gasoline to evenly distribute the components and then removing a subsample of the carpet padding to a second container for passive headspace analysis greatly reduced the amount of distortion in the resulting chromatogram. 1 table, 10 figures, and 14 references