Raman scattering detection of high explosives on human hair

Abstract

High explosives (HE) represent a high risk to the safety and health of the general population. Therefore, there is an ongoing demand for methods of analysis with limits of detection at trace levels for these hazardous chemicals. Since human hair is one of the main physical attributes of our bodies, the interaction of hair surfaces with HE can be of critical importance in forensic and security applications. Noninvasive in situ approaches such as spectroscopic methods are preferred for elucidating these interactions. Among the spectroscopy-based methodologies, those based on Raman scattering are often favored because the sharpness of the vibrational signals facilitates the precise location of the peak maxima and enables more precise determination of the band shifts due to intermolecular interactions. This preliminary study is based on the detection of the HEs 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), and pentaerythritol tetranitrate (PETN) on human hair strands by RS. Raman spectral libraries obtained with a 660-nm laser excitation line were generated for black, bleached, and natural gray hair using direct deposition of PETN, TNT, and RDX. Results obtained were confirmed using portable systems operating at 785 and 1064 nm. Spectral data were preprocessed to correct a high fluorescence background exhibited by the samples due to the indole groups and melanin present in hair. Despite the high fluorescence levels that characterized all the samples, the vibrational signatures that identify the presence of the HEs studied could be detected once the optimum acquisition parameters were established. Among the three hair types studied, gray hair was the best substrate to observe the vibrations of HE crystals on hair. A method limit of detection of 2 ± 1 ng was estimated from image analysis of the HE crystals observed and confirmed as ∼3 ± 1 ng using spectroscopic analysis.