Terahertz Spectroscopy of the Explosive Taggant 2,3-Dimethyl-2,3-Dinitrobutane

Advances in explosives analysis--part II: photon and neutron methods

The number and capability of explosives detection and analysis methods have increased dramatically since publication of the Analytical and Bioanalytical Chemistry special issue devoted to Explosives Analysis [Moore DS, Goodpaster JV, Anal Bioanal Chem 395:245-246, 2009]. Here we review and critically evaluate the latest (the past five years) important advances in explosives detection, with details of the improvements over previous methods, and suggest possible avenues towards further advances in, e.g., stand-off distance, detection limit, selectivity, and penetration through camouflage or packaging. The review consists of two parts. Part I discussed methods based on animals, chemicals (including colorimetry, molecularly imprinted polymers, electrochemistry, and immunochemistry), ions (both ion-mobility spectrometry and mass spectrometry), and mechanical devices. This part, Part II, will review methods based on photons, from very energetic photons including X-rays and gamma rays down to the terahertz range, and neutrons.

Terahertz spectroscopy of 2,4-dinitrotoluene over a wide temperature range (7-245 K)

Previous THz spectroscopy of the TNT explosive precursor, 2,4-dinitrotoluene (DNT), has been restricted to room temperature (apart from one set of data at 11 K). Here, for the first time, we investigate the spectrum as the temperature is systematically varied, from 7 to 245 K. Many new features appear in the spectrum on cooling below room temperature. As well as the five absorption lines observed previously, we observe five additional lines. In addition, a new room-temperature line at 8.52 THz (281 cm(-1)) is observed. Six of the lines red-shift with temperature and four of them blue-shift. The blue shift is explained by interplay between intramolecular and intermolecular hydrogen bonds. The variation in line width and line intensity with temperature is not systematic, although a conspicuous decrease in line intensity with temperature is observed in all cases. Modeling with hybrid PBE0 and TPSSh functionals helps identify absorption modes.

Dielectric properties of ethanol and gasoline mixtures by terahertz spectroscopy and an effective method for determination of ethanol content of gasoline

Investigation of frequency dependent permittivity of mixture solutions provides information on the role of intermolecular interactions on relaxation processes of solvent and solute molecules. In this study the dielectric properties of ethanol/gasoline mixtures in the terahertz spectral region are investigated. Frequency dependent absorption coefficients, refractive indices, and complex permittivities of pure ethanol and gasoline, and their mixtures at varying ethanol volume percentages (v/v %) are reported. As the mixing ratio changes, meaningful shifts are observed in the frequency dependent refractive index and absorption coefficients associated with the dominant component, ethanol. The relaxation dynamics of the pure gasoline and ethanol are successfully modeled with the Debye model using the ultrafast nature of the terahertz transients, and those of mixture solutions are investigated by an additive model with an assumption of minimum interaction due to the significant differences in their molecular natures; polar and nonpolar. Successful modeling of the mixtures confirms the weak interaction assumption and enables us to accurately determine the ethanol content. Among five ethanol/gasoline blends, except for one mixture, the estimated percent ethanol in gasoline is predicted with an accuracy of ca. 1% with respect to the actual ethanol percentage. In addition, the results show that free OH contribution to the macroscopic polarization is significantly higher at low concentrations (5-20%) and lower at 50% compared to the case of pure ethanol. The measurements and analysis presented here show that time domain terahertz studies can offer invaluable insight into development of new models for polar/nonpolar complex mixture solutions.