A comparison of techniques for extracting ignitable liquid residue from concrete

Abstract

Determining that a fire is the result of arson is a challenging task. Since many arsonists use some form of ignitable liquid, such as petrol, investigators may attempt to recover ignitable liquid residue (ILR) from a fire scene to help indicate if the fire was deliberately lit. Analysing debris for ILR can become difficult when debris cannot be transported to a laboratory for testing, such as when ILR is on a fixed concrete surface. Therefore, other techniques that are suitable for fieldwork must be considered. In this paper, four techniques for recovering ILR from a concrete surface are compared. The first three techniques involve covering the surface with an absorbent material: cat litter, absorbent matting or cotton pads. Each of these conditions requires an additional, laboratory-based extraction stage before it can be analysed for ILR. The fourth technique uses a new device that can perform the extraction procedure within the crime scene, thereby providing samples that are ready for analysis. This new device is known as the Passive Headspace Residue Extraction Device (PHRED). Extractions obtained using each technique were analysed via a gas-chromatograph mass-spectrometer to determine if it was capable of detecting ILR post-fire. The presence of certain chemical compounds in each sample indicated it had successfully absorbed ILR. The sensitivity of each technique was calculated by comparing the abundance of the ILR-specific compounds in a sample to the presence of additional compounds that produced ‘noise’. The results of this experiment indicated that cat litter and the PHRED are capable of extracting ILR from a concrete surface in an open-air environment one hour after a fire has self-extinguished. The cotton padding and absorbent matting did not detect any of the ILR compounds. Additional calculations of sensitivity indicate that the PHRED is more sensitive than the cat litter to compounds found in ILR.