Ultraviolet Resonance Raman Spectroscopy of Explosives in Solution and the Solid State

Abstract

Resonance Raman cross sections of common explosives have been measured by use of excitation wavelengths in the deep-UV from 229 to 262 nm. These measurements were performed both in solution and in the native solid state for comparison. While measurements of UV Raman cross sections in solution with an internal standard are straightforward and commonly found in the literature, measurements on the solid phase are rare. This is due to the difficulty in preparing a solid sample in which the molecules of the internal standard and absorbing analyte/explosive experience the same laser intensity. This requires producing solid samples that are mixtures of strongly absorbing explosives and an internal standard transparent at the UV wavelengths used. For the solid-state measurements, it is necessary to use nanostructured mixtures of the explosive and the internal standard in order to avoid this bias due to the strong UV absorption of the explosive. In this study we used a facile spray-drying technique where the analyte of interest was codeposited with the nonresonant standard onto an aluminum-coated microscope slide. The generated resonance enhancement profiles and quantitative UV-vis absorption spectra were then used to plot the relative Raman return as a function of excitation wavelength and particle size.

Recent Advances and Remaining Challenges for the Spectroscopic Detection of Explosive Threats

“…26,27 Differences in spectra were found for HMX and RDX in solid and liquid phase and were related to the different conformations of the molecules and not by crystalline or solvent effects, while the spectra of nitroaromatic explosives, such as TNT, were similar in both the solution state and solid state. Excitation in the deep UV (wavelength , 300 nm) leads to the likely presence of resonance enhancement, giving enhanced signal levels.…”

“…These combined measurements have been used to assess the optimal excitation wavelength(s) for the detection of explosives from the deep UV to the near infrared (NIR) and determine differences in cross sections and signatures in the solid state (Table I). 26,27 In addition to Raman spectroscopy, others like Phillips and Ho at the Pacific Northwest National Laboratory 28,29 and McGill et al at the Naval Research Laboratory (NRL) 30 have been working on the detection of explosives using mid-infrared and quantum cascade lasers since 2008. Their most recent work has applied these systems for standoff detection and imaging of explosives.…”

“…Solid-state cross sections can be measured using internal standard techniques; however, nanostructured substrates must be used when using resonance excitation to avoid bias due to absorption. 27 The visible and NIR solid-state Raman cross sections of these explosives using NaNO 3 as an internal standard have been published by Emmons et al 26 In the procedure used to make those measurements, the explosive and standard were finely ground and well mixed using a ball mill to ensure a uniform relative concentration of each in the sampling volume.…”

“…A summary table of the Raman cross sections of explosives is given in Table I, including both solution and solid-phase cross sections (in parentheses) as a function of wavelength. The values listed in this table were taken from Ghosh et al 25 and Emmons et al 26,27 It is expected that the Raman spectra and cross sections of the solid phase explosives will differ from those obtained in solution due to differences in molecular conformation. For example, the most stable conformation for gas-phase RDX molecules is believed to be the chair conformation with all three nitro groups pointing parallel to the axis of the ring, the so-called AAA conformation (the conformation adopted in crystalline b-RDX).…”

A new eye-safe UV Raman spectrometer for the remote detection of energetic materials in fingerprint concentrations: Characterization by PCA and ROC analyzes

“…has greatly improved the limit of detection of the Raman instruments allowing for proximal (sample-to-device distance of some meters) or stand-off (sample-to device distance of some tenths or hundreds of meters) applications [8][9][10][11].…”