Improving oxygenated ignitable liquid recovery by dual-mode heated passive headspace extraction using zeolites and activated charcoal strips
Rodgers, Corissa Leigh
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Heated passive headspace concentration is presently the most commonly utilized technique for the extraction of ignitable liquid residues from fire debris evidence. This process, introduced by William Dietz in 1991, typically involves suspending an activated charcoal strip within an airtight container such as a metal can and incubating the sample for a period of time. ASTM Standard Practice E1412-07 advises heating the sample for 2 to 24 hours at a temperature of 50 to 80° Celsius. Subsequently, the compounds are easily eluted from the adsorbent with a suitable solvent, often carbon disulfide, and analyzed using gas chromatography/mass spectrometry (GC/MS) for the potential identification of any ignitable liquid residues. It is a simple, sensitive, and nondestructive method, and can often be performed within the original sample packaging. The activated charcoal strip, which does not interact with water or nitrogen, is advantageous in its affinity for hydrocarbons and resistance to oxidation. The technique is highly efficient for recovering petroleum-based ignitable liquids, however, it has had limited success with adsorbing and concentrating oxygenated species. In an effort to improve the recovery of ignitable liquids containing oxygenated compounds, previous studies have suggested zeolites are a suitable adsorbent for the recovery of acetone through heated passive headspace concentration. Zeolites are inorganic, microcrystalline materials that have a well-defined internal structure and uniform pore size. Most frequently aluminosilicate with internally dispersed cations, zeolite particles attract small organic molecules, including alcohols and ketones. Their high thermal and chemical stability make them ideal adsorbents for heated passive headspace applications. An additional advantage to utilizing zeolites involves their well-defined pore size, which is ideal for the selective adsorption of small organic molecules. Zeolite 13X is effective for recovering analytes with molecular diameters smaller than 10 Å, such as acetone (6.3 Å). A compound with a molecular diameter greater than the zeolite pore size may not gain access to the internal channels, and thus may not be internally adsorbed. The primary aim of this study was to further optimize the conditions for implementing zeolites as a viable extraction technique within fire debris casework, as a complement to the activated charcoal strip method. Extraction time and temperature, desorption solvent, and gas chromatography parameters were all examined with the goal of providing the most efficient recovery of five oxygenated volatile compounds: ethanol, 1-propanol, 1-butanol, isopropanol, and acetone. Recovery by the use of zeolites desorbed in methanol was up to triple in amount when compared to recovery by activated charcoal strips with carbon disulfide. This is in accordance with previous studies that reported a 320% improvement in acetone recovery by utilizing zeolites. In an effort to evaluate the ability of zeolite 13X to selectively adsorb oxygenated volatile compounds, comparative recoveries of mixtures of petroleum and alcohol-based ignitable liquids were studied utilizing activated charcoal strips and zeolites, individually and in tandem. In the presence of both adsorption media within the same can, the activated charcoal strips alone recovered three major components of gasoline (toluene, 1,2,4-trimethylbenzene, and naphthalene), while the zeolites recovered the majority of oxygenated compounds. This phenomenon is attributed to the size exclusion properties, polarity, and available surface area of the zeolites. This research supports the use of both zeolites and activated charcoal strips, in what is termed a dual-mode adsorbent preparation, for the simultaneous recovery of oxygenated and petroleum-based ignitable liquids in a single fire debris extraction procedure.
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