Detection of High-Explosive Materials within Fingerprints by Means of Optical-Photothermal Infrared Spectromicroscopy

As we live under a constant threat of global terrorism, the effective detection of highly energetic materials is one of the critical procedures needed at a variety of locations, including airports, border checkpoints, and entrances to high-security buildings. In this work, the application of optical-photothermal infrared (O-PTIR) spectromicroscopy for the detection of highly explosive materials within fingerprints is described. High-explosive (HE) materials (e.g., PETN, RDX, C-4, or TNT) were used to prepare contaminated fingerprints. These were subsequently deposited on various objects, including microscopic glass slides, a table, a mug, etc. Samples deposited on glass slides were directly sent for analyses; for other samples, adhesive tapes were used to lift off fingermarks. In cases of difficulty in locating fingerprints, additional powders were used to enhance their visibility. Experiments were performed with a mIRage IR microscope working in a noncontact, far-field reflection mode, offering submicron IR spectroscopy and imaging. Fast imaging (several characteristic absorbances were selected for every substance of interest) was used to locate "suspicious" particles among various residues present in fingerprints. Subsequently, spectra were collected for those particles. Reflection mode O-PTIR spectra taken from powdered and nonenhanced fingerprints were of comparable quality to transmission mode FTIR spectra collected for pure HEs. On the basis of the performed experiments, we consider O-PTIR spectromicroscopy to open a new avenue for the nondestructive, efficient, and reliable analysis of exogenous substances deposited within fingerprints. The real significance of O-PTIR is in its ability to deliver high-quality, spatially resolved FTIR transmission-like spectra below the diffraction limit of infrared wavelengths, doing so in an easy-to-use reflection (far-field) mode. Collected spectra are also searchable and interpretable in both commercial and institutional IR databases without mathematical modeling.