Detection of vapors of explosives and explosive-related compounds by ultraviolet cavity ringdown spectroscopy

Analytical Tools and Methods for Explosive Analysis in Forensics: A Critical Review

This review summarizes (i) compositions and types of improvised explosive devices; (ii) the process of collection, extraction and analysis of explosive evidence encountered in explosive and related cases; (iii) inter-comparison of analytical techniques; (iv) the challenges and prospects of explosive detection technology. The highlights of this study include extensive information regarding the National & International standards specified by USEPA, ASTM, and so on, for explosives detection. The holistic development of analytical tools for explosive analysis ranging from conventional methods to advanced analytical tools is also covered in this article. The most important aspect of this review is to make forensic scientists familiar with the challenges during explosive analysis and the steps to avoid them. The problems during analysis can be analyte-based, that is, interferences due to matrix or added molding/stabilizing agents, trace amount of parent explosives in post-blast samples and many more. Others are techniques-based challenges viz. specificity, selectivity, and sensitivity of the technique. Thus, it has become a primary concern to adopt rapid, field deployable, and highly sensitive techniques.

Comprehensive Infrared Study of Tetryl, Dinitrotoluene, and Trinitrotoluene Compounds

The present work describes an experimental and theoretical study of energetic materials used for detecting explosives in order to prevent terrorist actions, as well as for de-mining projects. Particular attention was devoted to examining the infrared absorption spectroscopy of classic explosives in order to create a useful mobile apparatus for on-field detection of explosives. This paper reports the vibrational absorption spectra of tetryl, dinitrotoluene, and trinitrotoluene molecules approached using two different spectroscopic techniques, Fourier transform infrared spectroscopy (FT-IR) and laser photoacoustic spectroscopy (LPAS). Diffuse reflectance Fourier transform infrared spectra of all samples were analyzed in a very wide spectral range (400-7500 cm(-1)) showing for the first time the existence of weak absorption bands attributable to overtones or combination bands, while laser photoacoustic spectroscopy spectra have been investigated in the fingerprint region of organic compounds that share the CO2 laser emission range (~920-1100 cm(-1)). The Fourier transform infrared spectra of both matrix isolated dinitrotoluenes have been also investigated. The theoretical treatment of tetryl is reported for the first time.

"Fooling fido"--chemical and behavioral studies of pseudo-explosive canine training aids

Genuine explosive materials are traditionally employed in the training and testing of explosive-detecting canines so that they will respond reliably to these substances. However, challenges arising from the acquisition, storage, handling, and transportation of explosives have given rise to the development of "pseudo-explosive" training aids. These products attempt to emulate the odor of real explosives while remaining inert. Therefore, a canine trained on a pseudo-explosive should respond to its real-life analog. Similarly, a canine trained on an actual explosive should respond to the pseudo-explosive as if it was real. This research tested those assumptions with a focus on three explosives: single-base smokeless powder, 2,4,6-trinitrotoluene (TNT), and a RDX-based plastic explosive (Composition C-4). Using gas chromatography-mass spectrometry with solid phase microextraction as a pre-concentration technique, we determined that the volatile compounds given off by pseudo-explosive products consisted of various solvents, known additives from explosive formulations, and common impurities present in authentic explosives. For example, simulated smokeless powders emitted terpenes, 2,4-dinitrotoluene, diphenylamine, and ethyl centralite. Simulated TNT products emitted 2,4- and 2,6-dinitrotoluene. Simulated C-4 products emitted cyclohexanone, 2-ethyl-1-hexanol, and dimethyldinitrobutane. We also conducted tests to determine whether canines trained on pseudo-explosives are capable of alerting to genuine explosives and vice versa. The results show that canines trained on pseudo-explosives performed poorly at detecting all but the pseudo-explosives they are trained on. Similarly, canines trained on actual explosives performed poorly at detecting all but the actual explosives on which they were trained.

Cavitand-based solid-phase microextraction coating for the selective detection of nitroaromatic explosives in air and soil

A selective cavitand-based solid-phase microextraction coating was synthesized for the determination of nitroaromatic explosives and explosive taggants at trace levels in air and soil. A quinoxaline cavitand functionalized with a carboxylic group at the upper rim was used to enhance selectivity toward analytes containing nitro groups. The fibers were characterized in terms of film thickness, morphology, thermal stability, and pH resistance. An average coating thickness of 50 (±4) μm, a thermal stability until 400 °C, and an excellent fiber-to-fiber and batch to batch repeatability with RSD lower than 4% were obtained. The capabilities of the developed coating for the selective sampling of nitroaromatic explosives were proved achieving LOD values in the low ppbv and ng kg(-1) range, respectively, for air and soil samples.

Pulsed quantum cascade laser based hypertemporal real-time headspace measurements

Optical cavity enhancement is a highly desirable process to make sensitive direct-absorption spectroscopic measurements of unknown substances, such as explosives, illicit material, or other species of interest. This paper reports advancements in the development of real-time cavity ringdown spectroscopy over a wide-bandwidth, with the aim to make headspace measurements of molecules at trace levels. We report results of two pulsed quantum cascade systems operating between (1200 to 1320)cm(-1) and (1316 to 1613)cm(-1) that measure the headspace of nitromethane, acetonitrile, acetone, and nitroglycerin, where the spectra are obtained in less than four seconds and contain at least 150,000 spectral wavelength datapoints.

Production of the NO photofragment in the desorption of RDX and HMX from surfaces

A promising scheme for the remote detection of nitrate-based explosives, which have low vapor pressure, involves two lasers: the first to desorb, vaporize, and photofragment the explosive molecule and the second to create laser-induced fluorescence in the NO fragment. It is desirable to use for the first a powerful 532 nm frequency-doubled Nd:YAG laser. In this study, we investigate the degree of photofragmentation into NO resulting from the irradiation of the explosives RDX and HMX coated on a variety of surfaces. The desorption step is followed by femtosecond laser ionization and time-of-flight mass spectrometry to reveal the fragments produced in the first step. We find that modest laser power of 532 nm desorbs the explosive and produces adequate amounts of NO.

Precise wavelength calibration in continuous-wave cavity ringdown spectroscopy based on the HITRAN database

We describe the wavelength calibration method of a narrowband laser diode in continuous-wave (CW) cavity ringdown spectroscopy (CRDS). The method uses known spectral lines as wavelength markers to calibrate and refine the wavelength-current relation of laser diodes, and their spectral positions are taken directly from the HITRAN 2004 database. We built a compact CW CRDS apparatus with a 1.517 microm (approximately 6594 cm-1) distributed feedback (DFB) laser diode as the light source and a 25 cm long glass ceramic as the cavity in which to demonstrate the method. A wavelength precision of approximately 0.8 x 10(-3) cm-1 was obtained by comparing the HITRAN 2004 database, which was approximately four times more precise than that of the conventional method.

Optofluidic ring resonator sensors for rapid DNT vapor detection

We demonstrated rapid 2,4-dinitrotoluene (DNT) vapor detection at room temperature based on an optofluidic ring resonator (OFRR) sensor. With the unique on-column separation and detection features of OFRR vapor sensors, DNT can be identified from other interferences coexisting in the analyte sample mixture, which is especially useful in the detection of explosives from practical complicated vapor samples usually containing more volatile analytes. The DNT detection limit is approximately 200 pg, which corresponds to a solid phase microextraction (SPME) sampling time of only 1 second at room temperature from equilibrium headspace. A theoretical analysis was also performed to account for the experimental results. Our study shows that the OFRR vapor sensor is a promising platform for the development of a rapid, low-cost, and portable analytical device for explosive detection and monitoring.

Identification of explosives with two-dimensional ultraviolet resonance Raman spectroscopy

The first two-dimensional (2D) resonance Raman spectra of TNT, RDX, HMX, and PETN are measured with an instrument that sequentially and rapidly switches between laser wavelengths, illuminating these explosives with forty wavelengths between 210 nm and 280 nm. Two-dimensional spectra reflect variations in resonance Raman scatter with illumination wavelength, adding information not available from single or few one-dimensional spectra, thereby increasing the number of variables available for use in identification, which is especially useful in environments with contaminants and interferents. We have recently shown that 2D resonance Raman spectra can identify bacteria. Thus, a single device that identifies the presence of explosives, bacteria, and other chemicals in complex backgrounds may be feasible.

Effect of photochemistry on molecular detection by cavity ringdown spectroscopy: case study of an explosive-related compound

Explosives and explosive-related compounds usually have dissociative excited electronic states. We consider the effect of excited-state dissociation upon an absorption event on the UV cavity ringdown spectroscopy (CRDS) detection of these molecules. A change in the photon decay lifetime with increasing laser energy is demonstrated with vapors of 2,6-dinitrotoluene in the open atmosphere. The magnitude of the effect is modeled with coupled equations describing the time-dependent light intensity and molecular concentration within the cavity. The light intensities required within this model to explain the observed changes in the photon decay lifetimes are consistent with the light intensities expected within the cavity under our experimental conditions. It was also found that the slow diffusion of the molecules in static air can magnify the effect of photochemistry on UV CRDS trace detection of molecules with dissociative excited states.