Detection of high explosives samples deposited on reflective and matte substrates using mid-infrared laser spectroscopy at the grazing angle of incidence assisted by multivariate analysis

Abstract

Mid-infrared (MIR) laser spectroscopy measurements coupled with multivariate analysis (MVA) routines were applied to detection and classification of high explosives (HEs) deposited on aluminum and acrylonitrile butadiene styrene substrates. An MIR quantum cascade laser (QCL) was optically coupled to a grazing angle probe (QCL-GAP) mount to operate in diffuse reflectance mode at an incidence angle of 82 deg from the surface normal. The experimental conditions enabled reflection–absorption infrared reflectance spectroscopy of the laser light by the surface deposits of HEs and allowed obtaining spectra with high signal-to-noise ratios in a very short time. This optical arrangement permitted collecting data for identifying the vibrational signatures of the HEs to be detectable even at low surface concentrations. The HEs used included pentaerythritol tetranitrate, 1,3,5-trinitroperhydro-1,3,5-triazine, and 2,4,6-trinitrophenylmethylnitramine. Using principal components analysis, 100% of the HE samples deposited on the reflective and nonreflective substrate were correctly classified, avoiding false-positive responses in the analyses. The combination of QCL-GAP spectroscopy with MVA presented will also allow obtaining highly confident detections/identification of hazardous materials and other chemical/biological agents residing on both reflective and nonreflective surfaces. The methodology can easily be extended to other types of applications.