Application of Field-Portable Vapor Phase FT-IR Spectroscopy to On-Scene Fire Investigations

This research demonstrates the application of ruggedized, field-portable, commercial off-the-shelf Fourier transform infrared (FT-IR) technology for the presumptive on-scene analysis of latent ignitable liquid vapors following a fire. These controlled fire tests represent realistic fire scenarios that incorporate semi-quantitative vapor concentration assessments to guide the on-scene decisions of fire investigators about sample collection locations. A total of 32 ignitable liquid residue samples and eight comparison samples were collected across eight controlled burns and evaluated using the method described here. The results show that the FT-IR devices used in this study can reliably detect latent ignitable liquid vapors post-flashover and post-extinguishment at approximately 50 ppm isobutylene units, as determined by a commercial photoionization detector. A unique model of the FT-IR data can discriminate the measured spectra into two distinct petroleum distillate categories that align with the American Society for Testing and Materials classification of ignitable liquids for fire debris analysis. This work has significant implications for providing fire scene investigators with presumptive evidence to indicate whether specific liquids were present at a location for comparative purposes, to expedite suspect interviews, or to determine whether a criminal search warrant may be needed.