The DoD Center for Chemical Sensors Development at the University of Puerto Rico-Mayagüez has worked in
developing sensors for threat agents for over 8 years. Work has continued under the ALERT DHS Center of
Excellence. The approaches for sensing have covered many types of threat chemicals and some types of biological
simulants, including high energetic materials, homemade explosives, mixtures and formulations, chemical agents
simulants, toxic industrial chemicals and spore forming microorganisms. Sensing in the far field has been based in
vibrational spectroscopy: Raman and infrared. Near field detection has been mainly based on nanotechnology
enabled sensing platforms for Surface Enhanced Raman Scattering. Initial use of colloidal suspensions of silver and
gold nanospheres eventually evolved to metallic and metal oxide nanorods and to particle immobilization, including
sample smearing on substrates and drop-on-demand thermal inkjet printing of nanoparticles. Chemical reduction of
metal ions has been substituted by clean photonic physical reduction that leaves the nanoactive surface highly
exposed and overcomes the physico-chemical problem of double electrical layers posed by colloidal suspensions of
nanoparticles. New avenues have open wide research endeavors by using laser techniques to form nanoprisms and
interference based metallic nano-images and micro-images. UV based metal reduction on top of metal oxides
nanostructures promises to provide the selectivity and sensitivity expected for the last 30-40 years. Various
applications and experimental setups will be discussed.
The transport of Explosive Related Compounds (ERCs) has been studied as part of a research program aiming to the
development of chemical sensors for detecting landmines. TNT and its degradation products typically make up the
explosive charge in buried mines. The spatial and temporal distribution of concentrations of ERCs depends primarily on
the mobility of the water phase since the main chemicals are transported through the liquid phase of the soil (water). This
work presents an analytical approach to the description of the transport process. The model is based on the conservation
equations applied to the vadose zone and predicts the concentration profiles of water and ERCs as a function of time.
Techniques, such as linearization, variable transformations, and perturbation analysis are used in the development of the
model. Results agree with experiments and numerical simulations previously reported.
As part of a large research program aiming to the development of chemical sensor for detecting landmines, we
have studied the fate and transport of TNT subject to different ambient parameters. The space and temporal
concentration profiles of TNT, and its degradation compounds have been measured using soil tanks. The following
ambient parameters were controlled to emulate environmental factors: water content, temperature, relative humidity,
and UV-VIS radiation. A series of soil tanks were kept under controlled conditions for longer than a year and
sampled periodically at the surface. After several months, all tanks were sampled vertically and disposed of.
Chromatography (GC-&mgr;ECD) with direct injection was used for the analysis of the samples. Of particular interest is
the presence of several degradation compounds, as time evolves, responding to the ambient parameters imposed.
The vertical concentration profiles of the several chemicals found, gives an interesting view of the degradation
process as well as of the transport mechanisms. The results agreed with our computer simulations, and are used to
validate previous numerical analyses.
2,4,6-Trinitrotoluene (TNT) has a number of specific properties that make it a nearly ideal explosive for military applications. It is relatively stable with respect to non desired detonation, easy to store and handle and has a high explosive power. A broad variety of landmines contain TNT as the main explosive charge. There are several methods currently used to detect buried landmines, both physically and chemically. The goal of this work is develop new methods for detecting TNT in contact with soil, based on Chemical Point Detection methodologies. FT-IR spectroscopy is used to provide information about identity and composition of compounds in very small samples or small heterogeneities in large samples. The main objective of this work is to study the vibrational behavior of TNT when in contact with soil that contains argillaceous minerals, specifically of the group of the smectites. Literature indicates that clays of this group present certain characteristics leading to affinity to nitroaromatic compounds, such as TNT. The clay used in this investigation was saturated with potassium cations to increase the adsorption of TNT on clay. The study includes the exposure of Clay/TNT mixtures to a series of environmental variables, which include: variation of alkalinity and acid content of the mixtures, variation of temperature, addition of water and explosive mass fraction in the mixture. Visible changes of color in the K-clay-TNT or Na-clay-TNT mixtures were observed but without displaying vibrational changes in highly basic clays.
New analytical methods have been developed and existing methods have been improved for the detection of explosives and their degradation products by increasing their sensitivity and selectivity. Some of the analytical methods available for detection of explosives and degradation products are gas chromatography, mass spectrometry, high performance liquid chromatography, and gas chromatography with mass spectrometry. This work presents the design and development of the experiments for the detection of the spectroscopic signature of TNT buried in sand and its degradation products. These experiments are conducted using a series of soil tanks with controlled environmental conditions such as: temperature, soil moisture content, relative humidity and radiation (UV and VIS). Gas chromatography and solid-liquid extraction with acetonitrile were used for the analysis of explosives. Sampling of tanks was performed in three points on the surface. The results show that TNT and 2,4-DNT are the main explosives that reach the surface of tanks. Temperature and water content play a most important role in the degradation and diffusion of TNT. Finally, the tanks were disassembled and sampling in deep with the objective to obtain a concentration profile. The results demonstrated that the highest concentration was located at 5 cm from surface.
Infrared Spectroscopy is a well established tool for standoff detection of chemical agents in military applications. Vibrational IR spectroscopic analysis can also be used in Chemical Point Detection mode and to the arena of explosives identification and detection when energetic compounds are in contact with soil. PETN is an important nitroaliphatic explosive for military applications. Due to its intrinsic explosive power, it can be used in laminar form or mixed with RDX to manufacture Semtex plastic explosive and in the fabrication of Improvised Explosive Devices (IEDs). This investigation focused on the study of spectroscopic signatures of PETN in contact with soil. For this study, clay was mixed in different proportions with PETN. Detection of the vibrational signatures of PETN constitutes the central part of the investigation. The mixtures were submitted to the effect of water, acid and alkaline solutions, heat and deep UV light (234 nm) in order to establish the effect on these environmental parameters on the vibrational signatures of the explosive in the mixtures. The results reveal that the characteristic bands of PETN are highly persisted, degraded only by extreme conditions of UV radiation and exposure to high temperature for prolonged time. These results could be used in the development of sensitive sensors for detection of landmines, and improvised explosives devices (IDEs).
The transport of the chemical signature compounds from buried landmines in a three-dimensional (3D) array has been numerically modeled using the finite-volume technique. Compounds such as trinitrotoluene, dinitrotoluene, and their degradation products, are semi volatile and somewhat soluble in water. Furthermore, they can strongly adsorb to the soil and undergo chemical and biological degradation. Consequently, the spatial and temporal concentration distributions of such chemicals depend on the mobility of the water and gaseous phases, their molecular and mechanical diffusion, adsorption characteristics, soil water content, compaction, and environmental factors. A 3D framework is required since two-dimensional (2D) symmetry may easily fade due to terrain topography: non-flat surfaces, soil heterogeneity, or underground fractures. The spatial and temporal distribution of the chemical-signature-compounds, in an inclined grid has been obtained. The fact that the chemicals may migrate horizontally, giving higher surface concentrations at positions not directly on top of the objects, emphasizes the need for understanding the transport mechanism when a chemical detector is used. Deformation in the concentration contours after rainfall is observed in the inclined surface and is attributed to both: the advective flux, and to the water flux at the surface caused by the slope. The analysis of the displacements in the position of the maximum concentrations at the surface, respect to the actual location of the mine, in an inclined system, is presented.
The detection of trace amount of explosives is of utmost importance in many day-to-day military operations. Moreover, the detection of landmines is a complex and urgent worldwide problem, which needs specific, rapid and cost effective solutions. The most commonly used explosive in landmines is 2,4,6-trinitrotoluene (TNT). Almost 80% of the types of mines manufactured worldwide contain TNT. This contribution describes the use of Immersion Mode Solid-Phase Microextraction (I-SPME) for extraction of TNT and their degradation products from surface soil samples for subsequent analysis by either GC with <sup>63</sup>Ni micro cell Electron Capture Detector or gas chromatograph-mass spectrometer coupled to a Tunable Electron Energy Monochromator. A pretreatment step was introduced for the soil samples which extracted the target compounds into an aqueous phase. The experimental results demonstrated the effects of controllable variables. Parameters studied include the chemical properties of the fiber coating, extraction and desorption times, fiber extraction and matrix effect. Surface soil samples containing TNT were evaluated to study the detection of the nitroaromatic explosives and its degradations products using different environmental conditions such as sample temperature, sample contact time and water content.
Landmines have become a problem and a daily risk in approximately 70 countries. There exists a broad variety of types of mines in which trinitrotoluene (TNT) is mostly used as the main explosive charge. TNT has a number of specific properties that make it a nearly ideal explosive for military applications. There are several methods currently used to detect buried landmines. The goal of this work is develop new methods for detecting TNT in contact with soil and sand. Raman microscopy is used to provide information about identity, composition, molecular orientation or crystal formation in very small samples or small heterogeneities in large samples. The possible interactions of the energetic material with sand particles have been studied by quantitative vibrational spectroscopy. Ambient conditions that may affect the spectroscopic signature of the explosive in contact with soil were also studied. Among the parameters investigated were: Sand-TNT mass ratio, temperature, pH of soil, incidence of UV light and water content. The characteristic bands of TNT are not significantly shifted, but rather appear constant with respect of the characteristic band of Si-O in sand (~464 cm<sup>-1</sup>).
The detection of explosive materials is not only important as an issue in landmines but also for global security reasons, unexploded ordnance, and Improvised Explosive Devices detection. In such areas, explosives detection has played a central role in ensuring the safety of the lives of citizens in many countries. Raman Spectroscopy is a well established tool for vibrational spectroscopic analysis and can be applied to the field of explosives identification and detection. The analysis of PETN is important because it is used in laminar form or mixed with RDX to manufacture Semtex plastic explosive and in the fabrication of Improvised Explosive Devices (IEDs). Our investigation is focused on the study of spectroscopic signatures of PETN in contact with soil. Ottawa sand mixed in different proportions with PETN together with the study of the influence of pH, temperature, humidity, and UV light on the vibrational signatures of the mixtures constitute the core of the investigation. The results reveal that the characteristic bands of PETN are not significantly shifted but rather appear constant with respect of the ubiquitous band of sand (~463 cm<sup>-1</sup>). These results will make possible the development of highly sensitive sensors for detection of explosives materials and IDEs.
Detection and removal of antipersonnel and antitank landmines is a great challenge and a worldwide enviromental and humanitarian problem. Sensors tuned on the spectroscopic signature of the chemicals released from mines are a potential solution. Enviromental factors (temperature, relative humidity, rainfall precipitation, wind, sun irradiation, pressure, etc.) as well as soil characteristics (water content, compaction, porosity, chemical composition, particle size distribution, topography, vegetation, etc), have a direct impact on the fate and transport of the chemicals released from landmines. Chemicals such as TNT, DNT and their degradation products, are semi-volatile, and somewhat soluble in water. Also, they may adsorb strongly to soil particles, and are susceptible to degradation by microorganisms, light, or chemical agents. Here we show an experimental procedure to quantify the effect of the above variables on the spectroscopic signature. A number of soil tanks under controlled conditions are used to study the effect of temperature, water content, relative humidity and light radiation.
Among the many different signature compounds emitted from a landmine in the vapor phase, 2,4-dinitrotoluene (2,4-DNT) is the most common nitroaromatic compound in terms of detecting buried landmines, although it is a byproduct in the synthesis of TNT. 2,4-DNT is used as an ingredient in mining explosives and also prevalent on the soil surface but is somewhat seasonally dependent. The B3LYP hybrid functional was used to obtain the lowest-energy structure of both 2,4 and 2,6-DNT. Increasing basis sets from the 3-21G up to the 6-31++G (d, p) are used to predict structural parameters, vibrational frequencies, IR intensities and Raman activities for the explosives molecules. The calculated energies show that the 2,4-dinitrotoluene isomer is more stable than 2,6-dinitrotoluene isomer due to the lesser levels of steric effects between the nitro groups and the methyl group. The optimized structures were interacted with the siloxane site of clay minerals, using the density functional level of theory and the basis sets used to optimize the geometry of the DNT molecules. The binding energy (E<sub>b</sub>) between the optimized molecules and the basal siloxane site surface of clay minerals was calculated at distances in a range between 2.5 to 8.5 Å.
Landmine detection is an important task for military operations and for humanitarian demining. Conventional methods for landmine detection involve measurements of physical properties. Several of these methods fail on the detection of modern mines with plastic enclosures. Methods based on the detection signature explosives chemicals such as TNT and DNT are specific to landmines and explosive devices. However, such methods involve the measurements of the vapor trace, which can be deceiving of the actual mine location because of the complex transport phenomena that occur in the soil neighboring the buried landmine. We report on the results of the study of the explosives subject to similar environmental conditions as the actual mines. Soil samples containing TNT were used to study the effects of aging, temperature and moisture under controlled conditions. The soil used in the investigation was Ottawa sand. A JEOL GCMate II gas chromatograph ñ mass spectrometer coupled to a Tunable Electron Energy Monochromator (TEEM-GC/MS) was used to develop the method of analysis of explosives under enhanced detection conditions. Simultaneously, a GC with micro cell <sup>63</sup>Ni, Electron Capture Detector (μECD) was used for analysis of TNT in sand. Both techniques were coupled with Solid-Phase Micro Extraction (SPME) methodology to collect TNT doped sand samples. The experiments were done in both, headspace and immersion modes of SPME for sampling of explosives. In the headspace experiments it was possible to detect appreciable TNT vapors as early as 1 hour after of preparing the samples, even at room temperature (20 °C). In the immersion experiments, I-SPME technique allowed for the detection of concentrations as low as 0.010 mg of explosive per kilogram of soil.
The transport of the chemical signature compounds from buried landmines in a three-dimensional minefield array has been numerically modeled using the finite-volume technique. Compounds such as trinitrotoluene and dinitrotoluene are semi-volatile and somewhat soluble in water; furthermore, they can strongly adsorb to the soil and undergo chemical and biological degradation. Consequently, the spatial and temporal distributions of such chemicals depend on the mobility of the water and gaseous phases, their molecular and mechanical diffusion, adsorption characteristics, soil water content and compaction, and environmental factors. Surface concentrations decrease, when precipitation occurs due to advective flux around the object. Deformation in the concentrations contours after rainfall is observed in the inclined surface case and it is attributed to both: the advective flux, and to the water flux at the surface caused by the inclination. The LaGrit code developed at Los Alamos National Laboratory (LANL) was used to generate the 3D grid array and to place several landmines at different underground positions. The simulations were performed by using the Finite-Element Heat and Mass-transfer code also developed originally at LANL.
2,4,6-trinitrotoluene (TNT) is the most used explosive as main charge in landmines. There have been found contamination of soil and groundwater with munitions residues of TNT due to buried landmines. We are investigating the molecular structure, vibration behavior and the binding energy of TNT with the siloxane surface site of clay minerals in order to determine the spectroscopic signature of TNT in soil. Two different molecular symmetry structures were found with density functional theory (DFT) B3LYP method with 6-31G, 6-31G*, 6-311G, 6-311G*, and 6-311+G** basis sets from the Gaussian 98 systems of programs. Different deformations of the phenyl ring and distortions of the nitro and methyl groups with the ring were observed. In both structures, C<sub>1</sub> and C<sub>s</sub>, the nitro groups in positions 2 and 6 are out of plane with the phenyl ring due to steric interaction with the methyl group while the nitro group in position 4 is planar to the phenyl ring. The difference between the two structures is the internal rotation of the methyl group and 2, 6-nitro groups. Comparison of the calculated energies of the two structures in several basis sets reveals that the lowest-energy geometry for the TNT structure corresponds to C<sub>s</sub> symmetry with B3LYP/6-311+G**. FTIR spectra of TNT are presented and assigned assisted by B3LYP/6-311+G** result. The lowest-energy molecular structure of TNT was interacted with the basal siloxane surface of clay minerals to determine the binding energy (E<sub>b</sub>) between them. The binding energy was obtained by optimizing the vertical distance, the rotational and inclination angles between TNT and siloxane surface using the B3LYP hybrid functional with different basis sets.
We report on scanning electron microscopy and energy disperse X ray fluorescence measurements of TNT deposits on dry, wet and basic Ottawa sand particles. On clean Ottawa sand particles, TNT deposits form elongated crystals that change the morphology with time. The surfaces of the crystals acquire roughness features in one month old deposits and are no longer observed in two month deposits. On wet surfaces, fresh TNT deposits form assembles that resemble wire meshes. One month old TNT deposits on wet Ottawa sand appear to cover the particles surfaces and are no longer observed in structures that resemble the crystals observed on dry deposits. Fresh TNT deposits on Ottawa sand pre treated with sodium hydroxide appear amorphous. The deposits appear to cover the particle surfaces after a month and break into thin fibers in two month old deposits.