Extraction of Ignitable Liquid Residues by Dynamic Capillary Headspace Sampling and Comparison to the Carbon Strip Method

Detecting ignitable liquid (IL) residues on fire debris requires extracting and concentrating IL compounds prior to separation and analysis, typically by gas chromatography with mass spectrometry. Multiple sampling approaches exist; however, activated carbon strip (ACS) static headspace sampling is by far most common in US forensic labs. The ACS method requires 16–24 hours of collection time and ACSs must be eluted with carbon disulfide, known to be highly toxic. Porous layer open tubular (PLOT) dynamic capillary headspace sampling was developed at NIST to collect low-volatility compounds from the headspace of explosive materials. The method utilizes a length of alumina-coated capillary column, chilled to freezing temperatures, to capture headspace, and acetone as an elution solvent. PLOT sampling has been applied to a variety of samples, including clandestine gravesoil, spoiled food, and fuels. Preliminary experiments with laboratory-generated fire debris samples suggest that the PLOT method offers advantages in both safety and speed; however, experimental variables such as temperature, flow rate, and total collected volume have not been optimized for IL residue collection, and the method has not been challenged with real fire debris samples which consist of diverse matrices and may contain water. PLOTs may offer an additional advantage due to the possibility of eliminating breakthrough and the efficiency of desorption: they may collect a larger suite of IL compounds. The proposed work will systematically tune the PLOT method and compare its performance to the ACS method using nominally identical simulated fire debris samples and real fire debris samples. We will apply an in situ nuclear magnetic resonance method developed at NIST to probe IL compounds that are differentially extracted by PLOTs and ACSs and study differences driven by competitive adsorption or elution efficiency. The proposed work will identify whether PLOT sampling is advantageous for fire debris analysis and lay the necessary groundwork for future research, standardization, and implementation in forensic practice.