Trace Detection Of Explosives Research Articles

Analytical method for the estimation of transfer and detection efficiencies of solid state explosives using ion mobility spectrometry and smear matrix

Smearing method can be used as a simple and convenient sampling method for detection of trace explosives using a portable ion mobility spectrometer (IMS) in the field such as for security checks.

Rapid screening of explosives in ambient environment by aerodynamic assisted thermo desorption mass spectrometry.

Rapid, direct, and trace detection of explosives in an open environment is of particular need in homeland and/or transportation security. In this work, an aerodynamic assisted thermo desorption mass spectrometry method was developed for the direct quantitative analyses of explosives from a distance. Remote non-volatile explosive sensing was achieved for 2, 4, 6-trinitrotoluene, trinitrohexahydro-1, 3, 5-triazine, 8701 (main ingredient: RDX 98.5%), and C4 (a type of plastic explosive) with a distance of 0.65 m. Furthermore, a close to 324 cm2 effective sampling area could be achieved, and the limits of detection are in the ng range. This device can be deployed in airports and subway stations for high-throughput and automatic luggage/personnel screening of prohibited articles, such as explosives and illicit drugs. Copyright © 2016 John Wiley & Sons, Ltd.

Metal–Organic Polyhedra-Coated Si Nanowires for the Sensitive Detection of Trace Explosives

Surface-modified silicon nanowire-based field-effect transistors (SiNW-FETs) have proven to be a promising platform for molecular recognition in miniature sensors. In this work, we present a novel nanoFET device for the sensitive and selective detection of explosives based on affinity layers of metal-organic polyhedra (MOPs). The judicious selection of the geometric and electronic characteristics of the assembly units (organic ligands and unsaturated metal site) embedded within the MOP cage allowed for the formation of multiple charge-transfer (CT) interactions to facilitate the selective explosive inclusion. Meanwhile, the host-stabilized CT complex inside the cage acted as an effective molecular gating element to strongly modulate the electrical conductance of the silicon nanowires. By grafting the MOP cages onto a SiNW-FET device, the resulting sensor showed a good electrical sensing capability to various explosives, especially 2,4,6-trinitrotoluene (TNT), with a detection limit below the nanomolar level. Importantly, coupling MOPs-which have tunable structures and properties-to SiNW-based devices may open up new avenues for a wide range of sensing applications, addressing various target analytes.

Use of photoacoustic excitation and laser vibrometry to remotely detect trace explosives.

In this paper, we examine a laser-based approach to remotely initiate, measure, and differentiate acoustic and vibrational emissions from trace quantities of explosive materials against their environment. Using a pulsed ultraviolet laser (266nm), we induce a significant (>100 Pa) photoacoustic response from small quantities of military-grade explosives. The photoacoustic signal, with frequencies predominantly between 100 and 500kHz, is detected remotely via a wideband laser Doppler vibrometer. This two-laser system can be used to rapidly detect and discriminate explosives from ordinary background materials, which have significantly weaker photoacoustic response. A 100 ng/cm2 limit of detection is estimated. Photoablation is proposed as the dominant mechanism for the large photoacoustic signals generated by explosives.

Compressive Behavior of Idealized Granules for the Simulation of Composition C‐4

AbstractIn the most common approach to detect trace explosives at security checkpoints, any illicit residues must first be removed from the surface of interest (contact sampling) before they are delivered to a detector, such as an ion mobility spectrometer. Contact sampling involves applying a compressive shearing load to dislodge the residue from a surface. Optimizing this step requires an understanding of the properties of the residues and their effect on residue behavior. This study seeks to evaluate the previously unstudied effect of the binder mechanical properties and particle size distributions on the behavior of Composition C‐4. Composition C‐4 demonstrates complex granular behavior, and contains a highly viscous, non‐Newtonian binder. In addition to studies of real C‐4, simulated C‐4 was created with mechanical properties very similar to the real composite. The results with the live and simulated C‐4 indicate that – while highly important – the binder is not the only driving parameter controlling granule deformation under load; to create an effective simulant the binder must be combined with particles of an appropriate bimodal particle size distribution.

Validation of a Miniaturized Spectrometer for Trace Detection of Explosives by Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) measurements of some common military explosives were performed with a table-top micro-Raman system integrated with a Serstech R785 miniaturized device, comprising a spectrometer and detector for near-infrared (NIR) laser excitation (785 nm). R785 was tested as the main component of a miniaturized SERS detector, designed for in situ and stand-alone sensing of molecules released at low concentrations, as could happen in the case of traces of explosives found in an illegal bomb factory, where solid microparticles of explosives could be released in the air and then collected on the sensor’s surface, if placed near the factory, as a consequence of bomb preparation. SERS spectra were obtained, exciting samples in picogram quantities on specific substrates, starting from standard commercial solutions. The main vibrational features of each substance were clearly identified also in low quantities. The amount of the sampled substance was determined through the analysis of scanning electron microscope images, while the spectral resolution and the detector sensitivity were sufficiently high to clearly distinguish spectra belonging to different samples with an exposure time of 10 s. A principal component analysis procedure was applied to the experimental data to understand which are the main factors affecting spectra variation across different samples. The score plots for the first three principal components show that the examined explosive materials can be clearly classified on the basis of their SERS spectra.

Photo-vibrational spectroscopy of solid and liquid chemicals using laser Doppler vibrometer.

Photoacoustic/photothermal spectroscopy is an established technique for trace detection of chemicals and explosives. However, prior sample preparation is required and the analysis is conducted in a sealed space with a high-sensitivity microphone or a piezo sensor coupled with a lock-in amplifier, limiting the technique to applications in a laboratory environment. Due to the aforementioned requirements, traditionally this technique may not be suitable for defense and security applications where the detection of explosives or hazardous chemicals is required in an open environment at a safe standoff distance. In this study, chemicals in various forms (membrane, powder and liquid) were excited by an intensity-modulated quantum cascade laser (QCL), while a laser Doppler vibrometer (LDV) based on the Mach-Zehnder interferometer was applied to detect the vibration signal resulting from the photocoustic/photothermal effect. The photo-vibrational spectrum obtained by scanning the QCL's wavelength in MIR range, coincides well with the corresponding spectrum obtained using typical FTIR equipment. The experiment demonstrated that the LDV is a capable sensor for applications in photoacoustic/photothermal spectroscopy, with potential to enable the detection of chemicals in open environment at safe standoff distance.

Recent Advances and Applications of External Cavity-QCLs towards Hyperspectral Imaging for Standoff Detection and Real-Time Spectroscopic Sensing of Chemicals

External-cavity quantum cascade lasers (EC-QCL) are now established as versatile wavelength-tunable light sources for analytical spectroscopy in the mid-infrared (MIR) spectral range. We report on the realization of rapid broadband spectral tuning with kHz scan rates by combining a QCL chip with a broad gain spectrum and a resonantly driven micro-opto-electro-mechanical (MOEMS) scanner with an integrated diffraction grating in Littrow configuration. The capability for real-time spectroscopic sensing based on MOEMS EC-QCLs is demonstrated by transmission measurements performed on polystyrene reference absorber sheets, as well as on hazardous substances, such as explosives. Furthermore, different applications for the EC-QCL technology in spectroscopic sensing are presented. These include the fields of process analysis with on- or even inline capability and imaging backscattering spectroscopy for contactless identification of solid and liquid contaminations on surfaces. Recent progress in trace detection of explosives and related precursors in relevant environments as well as advances in food quality monitoring by discriminating fresh and mold contaminated peanuts based on their MIR backscattering spectrum is shown.

Detection and mapping of trace explosives on surfaces under ambient conditions using multiphoton electron extraction spectroscopy (MEES)

Multiphoton electron extraction spectroscopy (MEES) is an analytical method in which UV laser pulses are utilized for extracting electrons from solid surfaces in multiphoton processes under ambient conditions. Counting the emitted electrons as a function of laser wavelength results in detailed spectral features, which can be used for material identification. The method has been applied to detection of trace explosives on a variety of surfaces. Detection was possible on dusty swabs spiked with explosives and also in the standard dry-transfer contamination procedure. Plastic explosives could also be detected. The analytical limits of detection (LODs) are in the sub pmole range, which indicates that MEES is one of the most sensitive detection methods for solid surface under ambient conditions. Scanning the surface with the laser allows for its imaging, such that explosives (as well as other materials) can be located. The imaging mode is also useful in forensic applications, such as detection of explosives in human fingerprints.

Sensitive and selective detection of adsorbed explosive molecules using opto-calorimetric infrared spectroscopy and micro-differential thermal analysis

Multi-modal explosive sensors based on microelectromechanical systems (MEMS) have been developed. Opto-calorimetric infrared (IR) spectroscopy, capable of obtaining molecular signatures of extremely small quantities of adsorbed explosive molecules, has been realized with the use of microheater/thermometer devices and a widely tunable quantum cascade laser. These devices respond to the heat generated by non-radiative decay process when the adsorbed explosive molecules are resonantly excited with IR light. Monitoring the variation in microthermometer signal as a function of illuminating IR wavelength corresponds to the conventional IR absorption spectrum of the adsorbed molecules. Moreover, the mass of the adsorbed molecules is determined by measuring the resonance frequency shift of the device for the quantitative analysis. In addition, micro-differential thermal analysis, which can be used to differentiate exothermic or endothermic reaction of heated molecules, has been performed with the same devices to provide additional orthogonal signal for trace explosive detection and sensor surface regeneration. We have demonstrated successful detection, differentiation, and quantification of trace amounts of explosive molecules (cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN)) and their mixtures using three orthogonal sensing signals.

Review of explosive detection methodologies and the emergence of standoff deep UV resonance Raman

Due to terrorist use of explosive devices, intense interest has been directed towards the development of techniques and instrumentation to detect explosives. A number of analytical techniques are currently under investigation for trace explosive detection. Several methods have been successfully commercialized including colorimetric kits, fluorescence quenching sensors, and ion mobility spectrometers that see widespread field use. A key limitation of all currently utilized field techniques is that they require close proximity or physical interaction with the object being analyzed. The ideal trace explosive detection technique would be able to detect trace amounts of explosives from a distance to ensure personnel safety. Laser‐based spectroscopies are the only currently viable techniques that can be utilized to sense trace amounts of explosives at standoff distances. This review compares currently used explosive detection techniques with recently developed standoff methods. We emphasize the development of UV resonance Raman spectroscopy for standoff explosive detection. This work has recently been enabled by advances in laser and spectrometer design. Trace explosive detection has also been advanced by the recently developed understanding of deep UV photochemistry for several explosive molecules. Copyright © 2016 John Wiley & Sons, Ltd.

Improved explosive collection and detection with rationally assembled surface sampling materials

Inorganic sampling cloth chemically modified with phenyl-functional groups for improving the collection and detection of trace explosives.

Direct analysis in real time mass spectrometry of potential by-products from homemade nitrate ester explosive synthesis

This work demonstrates the coupling of direct analysis in real time (DART) ionization with time-of-flight mass spectrometry (MS) in an off-axis configuration for the trace detection and analysis of potential partially nitrated and dimerized by-products of homemade nitrate ester explosive synthesis. Five compounds relating to the synthesis of nitroglycerin (NG) and pentaerythritol tetranitrate (PETN) were examined. Deprotonated ions and adducts with molecular oxygen, nitrite, and nitrate were observed in the mass spectral responses of these compounds. A global optimum temperature of 350°C for the by-products investigated here enabled single nanogram to sub nanogram trace detection. Matrix effects were examined through a series of mixtures containing one or more compounds (sugar alcohol precursors, by-products, and/or explosives) across a range of mass loadings. The explosives MS responses experienced competitive ionization in the presence of all by-products. The magnitude of this influence corresponded to both the degree of by-product nitration and the relative mass loading of the by-product to the explosive. This work provides a characterization of potential by-products from homemade nitrate ester synthesis, including matrix effects and potential challenges that might arise from the trace detection of homemade explosives (HMEs) containing impurities. Detection and understanding of HME impurities and complex mixtures may provide valuable information for the screening and sourcing of homemade nitrate ester explosives.

F127/conjugated polymers fluorescent micelles for trace detection of nitroaromatic explosives

The cost-effective and rapid detection of nitroaromatic explosives is highly desired for growing con cern about the environmental pollution and terror threats. Herein, a facile sensing system for fast and trace detection of nitroaromatic explosives (e.g. picric acid (PA) and 2,4-dinitrotoluene (DNT)) in aqueous solution is developed. In this system, the self-assembled nanomicelles of commercial available Pluronic F127 are selected as vesicles and fluorescent conjugated polymers poly(9,9′-dioctylfluorene) (PFO) and poly(2,7-(9,9-hexylfluorene)-alt-4,4′-phenylether) (PFPE) are used as sensing materials. Through the microextraction effect of F127 nanomicelles, the hydrophobic conjugated polymers can be well dispersed in aqueous media and effectively quenched by trace amount of PA or DNT based on electron transfer mechanism. In compare with the conjugated polymers dissolved in organic solvents, the conjugated polymers encapsulated in F127 nanomicelles show dramatically enhanced sensing selectivity and sensitivity. The current nitroaromatic explosives sensing system is versatile. These results suggest that the F127/conjugated polymers system is a feasible method for detecting nitroaromatic explosives in water.

Quantifying Trace 2,4,6‐Trinitrotoluene (TNT) in Polymer Microspheres

AbstractWell characterized test materials are essential for validating the performance of current trace explosive detection systems. Explosive encapsulated microspheres have proven to be a valuable test material for trace explosive detection because of their precise size, shape, and composition. Presented herein is the quantification of explosives in the polymer microspheres by high performance liquid chromatography with UV/Vis detection (HPLC‐UV/Vis). A size exclusion separation is employed to quantify the amount of explosive encapsulated in cured microspheres. Complete quantification was achieved by simultaneously separating and quantifying the explosive and polymer components. Results indicate that approximately 30 % of the TNT is lost in the manufacturing of the microspheres and subsequent loss from the cured microspheres is minimal if stored at 4 °C.

Metal–Organic Polyhedra Cages Immobilized on a Plasmonic Substrate for Sensitive Detection of Trace Explosives

A novel strategy for highly sensitive detection and discrimination of explosives is developed based on the metal–organic polyhedra (MOP)‐decorated plasmonic substrate. It is found that the careful selection of the geometric and electronic characteristics of the assembly units (organic ligands and unsaturated metals sites) embedded within the MOP cage allows for the integration of multiple weak molecular interactions in a controllable fashion and thus the MOP cage can serve as an excellent receptor for selective uptake and binding of explosives. By further grafting of the MOP cage onto a plasmonic substrate with good surface‐enhanced Raman scattering enhancement factor, the resulting sensor shows a good sensing capability to various groups of ultratrace explosives, especially the challenging aliphatic nitro‐organics.

Infrared photothermal imaging spectroscopy for detection of trace explosives on surfaces

We are developing a technique for the standoff detection of trace explosives on relevant substrate surfaces using photothermal infrared (IR) imaging spectroscopy (PT-IRIS). This approach leverages one or more compact IR quantum cascade lasers, which are tuned to strong absorption bands in the analytes and directed to illuminate an area on a surface of interest. An IR focal plane array is used to image the surface and detect increases in thermal emission upon laser illumination. The PT-IRIS signal is processed as a hyperspectral image cube comprised of spatial, spectral, and temporal dimensions as vectors within a detection algorithm. The ability to detect trace analytes at standoff on relevant substrates is critical for security applications but is complicated by the optical and thermal analyte/substrate interactions. This manuscript describes a series of PT-IRIS experimental results and analysis for traces of RDX, TNT, ammonium nitrate, and sucrose on steel, polyethylene, glass, and painted steel panels. We demonstrate detection at surface mass loadings comparable with fingerprint depositions ( 10μg/cm2 to 100μg/cm2) from an area corresponding to a single pixel within the thermal image.

In-source collision induced dissociation of inorganic explosives for mass spectrometric signature detection and chemical imaging

The trace detection, bulk quantification, and chemical imaging of inorganic explosives and components was demonstrated utilizing in-source collision induced dissociation (CID) coupled with laser desorption/ionization mass spectrometry (LDI-MS). The incorporation of in-source CID provided direct control over the extent of adduct and cluster fragmentation as well as organic noise reduction for the enhanced detection of both the elemental and molecular ion signatures of fuel-oxidizer mixtures and other inorganic components of explosive devices. Investigation of oxidizer molecular anions, specifically, nitrates, chlorates, and perchlorates, identified that the optimal in-source CID existed at the transition between fragmentation of the ionic salt bonds and molecular anion bonds. The chemical imaging of oxidizer particles from latent fingerprints was demonstrated, including both cation and anion components in positive and negative mode mass spectrometry, respectively. This investigation demonstrated LDI-MS with in-source CID as a versatile tool for security fields, as well as environmental monitoring and nuclear safeguards, facilitating the detection of elemental and molecular inorganic compounds at nanogram levels.

Chemical ionization mass spectrometry using carbon nanotube field emission electron sources.

A novel chemical ionization (CI) source has been developed based on a carbon nanotube (CNT) field emission electron source. The CNT-based electron source was evaluated and compared with a standard filament thermionic electron source in a commercial explosives trace detection desktop mass spectrometer. This work demonstrates the first reported use of a CNT-based ion source capable of collecting CI mass spectra. Both positive and negative modes were investigated. Spectra were collected for a standard mass spectrometer calibration compound, perfluorotributylamine (PFTBA), as well as trace explosives including trinitrotoluene (TNT), Research Department explosive (RDX), and pentaerythritol tetranitrate (PETN). The electrical characteristics, lifetime at operating pressure, and power requirements of the CNT-based electron source are reported. The CNT field emission electron sources demonstrated an average lifetime of 320 h when operated in constant emission mode under elevated CI pressures. The ability of the CNT field emission source to cycle on and off can provide enhanced lifetime and reduced power consumption without sacrificing performance and detection capabilities. Graphical Abstract ᅟ.

Development and use of a thermal desorption unit and proton transfer reaction mass spectrometry for trace explosive detection: Determination of the instrumental limits of detection and an investigation of memory effects

A novel thermal desorption unit (TDU) has been developed and specifically designed for the detection of trace quantities of explosives using a proton transfer reaction mass spectrometer (PTR-MS). For the first time details on recovery times and instrumental limits of detection for the screening of explosives with this TDU/PTR-MS system are reported. We demonstrate that traces (nanograms or less) of explosives deposited on swabs are desorbed within less than a second upon insertion into the TDU. For a short period of time (seconds) a concentration “pulse” of an explosive enters the drift (reaction) tube of the PTR-MS. This temporal concentration pulse of material is monitored in real-time by recording the product ion intensities for a given explosive as a function of time. By changing the reduced electric field in the drift tube region of the PTR-MS, we demonstrate how selectivity can be improved. This study demonstrates that the TDU/PTR-MS instrument meets security application criteria in terms of sensitivity, selectivity and recovery times.