The Raman vibrational frequencies in the finger print region (700-1600 cm-1) have been calculated for 2,4-
dinitrotoluene, 2,6-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT). The Raman vibrational intensities
and frequencies for these molecules have been calculated using B3LYP Density Functional Theory method with
6-311+G** and Sadlejs medium-sized polarized basis sets (Sadlej pVTZ). The normal mode assignments in the
finger print region were carried out by Normal Coordinate Analysis, where localized and de-localized coordinates
were used to facilitate an accurate description of the vibrational modes. The Raman intensities were calculated
from the Raman scattering cross sections using the ab initio calculated Raman scattering activities. Comparison
of these intensities using different basis sets indicates that the Sadlej pVTZ basis sets increase the calculated
intensities for the NO2 symmetric stretching and bending frequencies by more than 15 % relative to 6-311+G**
basis. The potential energy distribution for the symmetric and asymmetric NO2 stretches indicates that 2-NO2
and 6-NO2 couple strongly in 2,6-DNT and 2,4,6-TNT, while 2-NO2 and 4-NO2 groups couple weakly in 2,4-
DNT. These findings suggest that the coupling strength of 2-NO2, 6-NO2 and 4-NO2 groups can be used to distinguish between dinitro and trinitro toluenes.
Characterization of hexamethelene triperoxide diamine (HMTD), tetramethylene diperoxide dicarbamide (TMDD)
and tetramethylene diperoxide acetamide (TMDA) has been carried out using Direct Analysis in Real Time/Time of
Flight Mass Spectrometry (DART-TOF/MS). The study also centered in the detection of their precursors such as
hexamine and formaldehyde. Analysis of the compounds by GC-MS was also conducted. HMTD shows a clear peak
at 209 m/z that allowed its detection in standard solutions and lab made standards. TATP samples with deuterium
enrichment are being analyzed to compare results that could differentiate from HMTD and similar substances. All
samples were characterized by Raman and FT-IR to confirm the DART results. Some of the vibrations observed
were in the ν(O-O), ν(N-C), ν(N-H), ν(C-O), δ(CH3-C) and δ(C-O). Development methodology for trace detection
was compared with GC/MS and HPLC-MS results previously presented for HMTD and TATP.
To understand the fate and transport mechanisms of TNT from buried landmines is it
essential to determine the adsorption process of TNT on soil and clay minerals. In this
research, soil samples from horizons Ap and A from Jobos Series at Isabela, Puerto Rico
were studied. The clay fractions were separated from the other soil components by
centrifugation. Using the hydrometer method the particle size distribution for the soil
horizons was obtained. Physical and chemical characterization studies such as cation
exchange capacity (CEC), surface area, percent of organic matter and pH were performed
for the soil and clay samples. A complete mineralogical characterization of clay fractions
using X-ray diffraction analysis reveals the presence of kaolinite, goethite, hematite,
gibbsite and quartz. In order to obtain adsorption coefficients (Kd values) for the TNT-soil
and TNT-clay interactions high performance liquid chromatography (HPLC) was used. The
adsorption process for TNT-soil was described by the Langmuir model. A higher
adsorption was observed in the Ap horizon. The Freundlich model described the adsorption
process for TNT-clay interactions. The affinity and relative adsorption capacity of the clay
for TNT were higher in the A horizon. These results suggest that adsorption by soil organic
matter predominates over adsorption on clay minerals when significant soil organic matter
content is present. It was found that, properties like cation exchange capacity and surface
area are important factors in the adsorption of clayey soils.
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).
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.
Several military bases and monitions facilities throughout the world are contaminated with toxic explosives like 2,4,6-trinitrotoluene (TNT). This is an energetic compound and the least mobile of the military explosives. For this reason TNT gives one of the largest soil contamination problems. To understand the adsorption mechanism between TNT explosive and soil environments, the mechanical method analysis is used in our laboratory in order to obtain the soil texture classification. In these experiments, soil samples from horizons Ap and A were obtained from Jobos Series at Isabela, Puerto Rico. Based on the USDA texture triangle, the soil from the Ap horizon is classified as sandy clay. In contrast, the soil from A horizon fall in the sandy clay loam class. The clay minerals were separated from the other soil components using the mechanical method analysis. Cation exchange capacity (CEC), surface area, percent of organic matter and pH were determined for the soil and clay samples. The CEC results for soil samples were 3.62 mequiv/100 g for Ap horizon and 2.67 mequiv/100 g for A horizon while for the clay samples the CEC was 13.12 mequiv/100 g and 12.50 mequiv/100 g, for the Ap and A horizons, respectively. The results obtained for surface area analysis were 85.32 g/m2 and 51.19 g/ m2 for the two soil horizons and 189.71 g/m2 and 163.87 g/m2 for clay samples in the Ap and A horizons, respectively. These results indicate that the major adsorption could occur in the Ap horizon, specially in the clay fraction. A complete characterization of clay mineral samples using X-ray analysis reveals the present of kaolinite and quartz as main minerals. In order to obtain adsorption coefficients (Kd values), soil samples and the clay obtained from the mechanical method analysis, is being used for TNT adsorption studies by means of High Performance Liquid Chromatography (HPLC).
Interactions of α-RDX, with the basal siloxane surface of the clay mineral kaolinite has been perform in our laboratory using the Gaussian 03 computational package. The low energy conformation of α-RDX (89.1 kcal/mol) was used to carry out the interaction with the clay mineral using the Hartree Fock (HF), DFT, DFT//HF, MP2, and MP2//HF levels of theory in order to determine the orientation, the types of bonds reacting between the two molecules and the adsorption as well. The results point out that the nitro group in pseudo-equatorial position interacts with the siloxane surface. DFT//HF level and Basis Set Superposition Error (BSSE) corrected MP2//HF were perform to obtain the binding energies (Eb) and the contribution of dispersion interaction to the binding energies (DEb). The Eb using DFT//HF level of theory between those molecules fluctuate in a range of 38 to 51 kJ/mol once we applied the BSSE correction at different basis sets. Furthermore the results indicates a decrease in the Eb/BSSE (DFT//HF) of ~ 13 kJ/mol when polarization functions are added. The calculated binding energy of the RDX-siloxane surface complex is ~ 57 kJ/mol using MP2//HF/6-31+G (d) model chemistry. Studies of theoretical IR spectra of the interaction were obtained with DFT//HF methods and the 6-31+G(d) basis set with a small molecular model (single tetrahedra). The calculation predicted a band shift effect in the region of 1200-1800 cm-1, due to interactions of the α-RDX with the siloxane surface.
Cyclic organic peroxides are sensitive to the presence of water and other contaminants that can deactivate the substance or make it less sensitive to chock, spark or other detonating mechanism. In the case of radiation such as laser action the opposite seems to happen, making the peroxides more sensitive to laser breakdown and local burning. In recent studies, TATP has been induced to sublimate faster during Raman analysis when it had contaminants or water, however, some studies have shown that TATP does not reacts when it is wet. This study is focused on determining if the presence of water and other contaminants affects peroxide stability and the detection by current technologies, such as IMS and vibrational spectroscopy. During the study, TATP and HMTD have been synthesized by different methods using certified chemicals and common household products. The research also focused on the effect of metal salts in the syntheses and the effect of temperature in the composition of the products. Differences in the location, shape, relative intensity, and in some cases appearance of new bands possibly due to Redox and complex formation reactions were evident. Bands corresponding to ν(O-O), ν(C-O), δ(CH3-C) and δ(C-O) were located and assigned for Raman and IR spectroscopies.
Synthesis and characterization of hexamethelene triperoxide diamine (HMTD), tetramethylene diperoxide dicarbamide (TMDD) and tetramethylene diperoxide acetamide (TMDA) using GC-MS, HPLC-MS, FT-IR and Raman Microscopy has been carried out. The study also centered in the synthesis and characterization of other cyclic amine peroxides, including and different forms of caged peroxides from other diaminoalkanes. Interest also was given to the secondary products of all syntheses and the effect of temperature in the composition mixtures of the preparations. Differentiation spectroscopy and spectrometry studies were also conducted. In these studies the differences in the ν(O-O), ν(N-C), ν(N-H), ν(C-O), δ(CH3-C) and δ(C-O) bands for Raman and IR were established. For the GC/MS spectrometric studies retention times and fragmentation patterns for GC-MS and GC-FT-IR useful in amine peroxide differentiation were also established.
Landmines have been a part of war technology for many years. As a result of the continued and indiscriminate use in approximately 90 countries landmines pose a severe and ever growing problem and a daily risk. Raman Spectroscopy is capable of providing rich information about the molecular structure of the sample and pinpoint detection of many chemicals, both of organic and inorganic nature. The presence of landmines in soils can be detected by Raman Spectroscopy sensing in a Point Detection modality, using characteristic vibrational signals of each explosive present in landmines. Detection of 2,4-DNT in sand and studies on how the vibrational signatures of 2,4-DNT is modified by interacting with soil particles and environmental conditions is reported. Raman Microspectrometers equipped with 514 nm and 785 nm laser excitation lines were used. The work focused in how the spectroscopic signatures of DNT in contact with Ottawa Sand are affected by the presence of humidity, pH, temperature, UV light and reaction times. Samples of mixtures of sand/2,4-DNT were analyzed by Raman Spectroscopy at 10, 50 and 100% water content and temperatures in range of 40-80 °C. Mixtures were also analyzed at different pH: 4, 7 and 10 and under ultraviolet light at 254 nm. Raman spectra were taken as a function of time in an interval from 24 to 336 hours (two weeks). Characteristic signals of 2,4-DNT were analyzed in different ranges 100-3800 cm-1, 600-1200 cm-1, 300-1700 cm-1 and 2800-3500 cm-1. The effect of these variables was measured during 45 consecutive days. It was confirmed that the decrease of characteristic vibrational signatures of 2,4-DNT can be attributed to increase of the degradation of 2,4-DNT by the simulated environmental conditions. Spectroscopic characterization of degradation products, both in contact with sand as well as airborne is under way. These results will make possible the development of highly sensitive sensors for detection of explosives materials and correlated with their degradation products in landmines.
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-1).
Experimental studies have shown that a key factor affecting the bioavailability and biodegradability of nitroaromatic compounds (NAC's) in subsurface environments is their sorption onto clay minerals. This study present the recent ab initio quantum mechanical calculations on the interaction of 2,4-DNT (DNT) with the basal siloxane site surface of kaolinite, a clay mineral. Theoretical calculations of the low energy conformation of DNT interacting with the siloxane site surface of clay minerals were performed in order to obtain their properties adsorbed on soil environments as well as the structure of the adsorbed molecule. The calculations also yielded the way of orientation and the effect of the adsorption. This study was performed using DFT//HF and MP2//HF methods taking into account the contribution of the Coulombic (CEb) and dispersion (DEb) energies, to obtain the binding energies between DNT and siloxane surface. A comparison of the CEb and DEb energies shows that the stabilization of DNT at the siloxane sites, using a small molecular model (single tetrahedra), is mainly provided by dispersion interaction energy. Considering the accuracy and cost of the computation methods the 6-31+G* basis set produced the best representation of the interaction energy (42 kJ/mol) using the MP2//HF level of theory for the DNT-Siloxane surface. These theoretical calculations give a good prediction of the interaction between the 2,4-DNT molecule with soil clay minerals. The computational results are compared with the experimental results obtained with the FT-IR microscopic technique.
KEYWORDS: Carbon, Nitrogen, Soil science, Explosives, Scanning electron microscopy, Particles, Land mines, Oxygen, Chemical analysis, Solid state electronics
Knowledge of the chemical state of explosive materials on soil surfaces is central to our understanding of the environmental effects of landmines as well as to employ existing technologies for landmine sensing. We used X ray photoelectron spectroscopy to study the surface chemistry of TNT and RDX on soil substrate surfaces as a function of soil pH. These explosives exists in many forms in the solid state. At least two forms have been reported for RDX and TNT in the solid state. Different forms are observed for both TNT and RDX adsorbed on soil substrate surfaces. These forms have a markedly different morphology than the one observed on flat surfaces and are found to be pH sensitive. The N 1s binding energy is found to be markedly different for TNT and RDX adsorbed on soil particles as compared to the one measured for these chemicals on other surfaces. The results are consistent with a change in the chemical state of the explosive upon its interaction with soil particles.
2,4,6-Trinitrotoluene is a high explosive used in manufacturing landmine, bombs, and other explosive devices. It has been the main source of contamination in groundwater, soil as a result of intentional or accidental releases at many places around the world. Crystallization of TNT in soil from TNT/methanol solutions was carried out and characterized using vibrational spectroscopy. TNT exhibits a series of characteristic bands that allow its detection when in soil. The spectroscopic signatures of neat TNT and TNT in soil samples were determined with Raman Microspectroscopy and Fourier Transform Infrared (FTIR) Microscopy. The spectroscopic signature of neat TNT is dominated by strong bands about 1380 and 2970 cm-1. The intensity and position of these bands are found remarkably different in soil samples spiked with TNT. The 1380 cm-1 band is split into a number of bands in that region. The 2970 cm-1 is reduced in intensity and new bands are observed at about 2880 cm-1. The results are consistent with a different chemical environment for TNT in soil as compared to neat TNT. Further measurements are required to fully understand the spectroscopic signature of TNT in soil samples. Its detection in soil is essential in landmine detection technology, and could address the improvement of the devices in the mentioned technology.
Computational algorithms have been very useful to study molecular interactions between explosives and different types of soils. In this work ab initio molecular orbital calculations were employed to study the interaction of 2,4,6-trinitrotoluene (TNT) with the basal siloxane surface of clay minerals. The intermolecular interaction energy, the vibration frequencies and efficient computational algorithms have been tested for the complex of TNT with the siloxane surface site of clay minerals. Two cluster models have been developed to represent the TNT on the siloxane surface of clay minerals. They have been employed in order to determine the changes in the spectroscopic signature of TNT. The results obtained provide information about the interaction energy of TNT on clays. The binding energy between the TNT and the basal siloxane surface was -38 kJ/mol, obtained with MP2//HF/6-31+G(d) level of theory and basis set, respectively. The calculated interaction has their minimal at separation between the two molecules of 3.5 Å. The theoretical IR spectra of the interaction was obtained with DFT//HF methods and the 6-31+G(d) basis set. The calculation predicted a shifting effect in NO2 bands, due to the interaction. The results are in excellent agreement with available experimental data. Further, result of such theoretical studies could contribute to an understanding of the interaction energy of the other kinds of explosives that may be occurring in other environments.
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-1). These results will make possible the development of highly sensitive sensors for detection of explosives materials and IDEs.
The detection of trace level of explosives is a challenging field of great importance to national security and landmine detection. Chemical signatures of buried landmines are in a very complex environment. External physical conditions that affect explosive vapors and particles in soil can affect the explosive chemical signature. The chemical spectroscopic signature of the RDX in clay soil environments has been investigated by means of reflectance FT-IR microspectroscopy. The soil obtained from the University of Puerto Rico at Mayaguez was treated using the textural mechanical method in order to separate the clay from all the other components in the soil. B3LYP/6-311G** calculations performed on the low energy conformers of RDX helped to determine its most stable conformations, their symmetry, and vibrational spectra. The FT-IR technique confirmed the existence of two different RDX solid phases, known as the α-RDX and β-RDX, which have different symmetries and revealed significant differences in their spectra. The IR microspectroscopic study showed that the RDX-Clay mineral complex and its interactions can be detected using the FT-IR technique at a low concentration of 1000 part-per-millions. Variations in the clay's pH revealed changes in the RDX-Clay complex spectroscopic signature. These results also indicate that the interaction between the RDX and the clay minerals affects mainly the ring breathing, the C-N vibrations and the NO2 groups of the explosive molecules. It is suggested that the electron donor nitrogen atoms from RDX are interacting with the electron acceptor oxygen atoms of the edge sites of the clay's surface.
Smokeless Powders are a class of propellants that were developed in the late 19th century to replace black powder; it has been used as an explosive in shotguns, rifles, firearms and many other larger caliber weapons. These propellants can be placed into one of three different classes according to the chemical composition of their primary energetic ingredients. Advance equipment have been designed and used for the detection of explosives devices and compounds potentially energetic. In this research we are developing an analytical methodology to detect different formulation of smokeless powders: Alliant-American Select, Alliant-Bullseye, and Alliant-Red Dot using the ion mobility spectrometry (IMS) technique. We used different surfaces like computer diskettes, CD’s, book covers and plastics to study their adsorption/desorption process. Using micropipettes, we delivered solutions with different amounts of Smokeless Powders from a 1000 ppm solution and deposit it on various types of filters to make a calibration curve. Several amounts of Smokeless Powder were deposited to the different surfaces and collected with filter paper. The samples were desorbed directly from the filter to the instrument inlet port. Subsequently, the percentage of explosive recovered was calculated.
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 63Ni, 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.
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, C1 and Cs, 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 Cs 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 (Eb) 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.
TNT and DNT are important explosives used as base charges of landmines and other explosive devices. They are often combined with RDX in specific explosive formulations. Their detection in vapor phase as well as in soil in contact with the explosives is important in landmine detection technology. The spectroscopic signatures of nitroaromatic compounds in neat forms: crystals, droplets, and recrystallized samples were determined by Raman Microspectroscopy (RS), Fourier Transform Infrared Microscopy (FTIR) and Fiber Optics Coupled - Fourier Transform Infrared Spectroscopy (FOC-FTIR) using a grazing angle (GA) probe. TNT exhibits a series of characteristic bands: vibrational signatures, which allow its detection in soil. The spectroscopic signature of neat TNT is dominated by strong bands about 1380 and 2970 cm-1. The intensity and position of these bands were found remarkably different in soil samples spiked with TNT. The 1380 cm-1 band is split into a number of bands in that region. The 2970 cm-1 band is reduced in intensity and new bands are observed about 2880 cm-1. The results are consistent with a different chemical environment of TNT in soil as compared to neat TNT. Interactions were found to be dependent on the physical source of the explosive. In the case of DNT-sand interactions, shifts in vibrational frequencies of the explosives as well as the substrates were found.
Raman Spectroscopy is a well established tool for vibrational spectroscopy analysis. Interactions of explosives with different substrates can be measured by using quantitative vibrational signal shift information of scattered Raman light associated with these interactions. A vibrational spectroscopic study has been carried out on 2,4-DNT and 2,6-DNT crystals. Raman Microscopy spectrometers equipped with 514 nm and 785 nm laser excitation lines were used. The samples were recrystallized on different solvents (water, methanol and acetonitrile) and allowed to interact with soil samples. The interaction with sand and soil samples doped with the nitroaromatic compounds showed significant shifts in its peaks. The above information was used to detect DNT in soil using Raman Microscopy. These results will make possible the development of highly sensitive sensors for detection of explosives materials.
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 (Eb) 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.
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.
2,4,6-Trinitrotoluene, commonly known as TNT, is an explosive used in military shells, bombs, landmines, grenades, demolition operations, and underwater blasting. It is produced in the United States only at military facilities. Accidental releases of TNT and residues in battle fields have contaminated groundwater, soil, and sand at numerous sites around the world. TNT exists in two physical forms at room temperature: droplets and crystals. The spectroscopic information conveyed depends on its physical form and the substrate on which it is deposited. Vibrational spectroscopy is a powerful tool that can be used to characterize TNT in its diverse forms. Crystallization of TNT from different solvents (acetonitrile, methanol, and water) was carried out to subsequently measure the vibrational spectra. The important nitroaromatic compound exhibits a series of unique characteristic bands that allow its detection and spectroscopic characterization. The spectroscopic signatures of neat TNT samples were determined with Raman Microspectroscopy and Fourier Transform Infrared (FTIR) Microscopy. The Raman spectra of neat TNT are dominated by strong bands at about 1365 and 2956 cm-1. The intensity and even the presence of these bands are found to be remarkably dependent on TNT form and source.
The photochemistry of TNT in toluene, water and methanol has been studied with femtosecond laser spectroscopy, surface reflection Fourier transform infrared absorption spectroscopy and ultraviolet-visible absorption spectroscopy measurements. Aqueous and alcoholic TNT solutions change from colorless to yellow or red upon irradiation with ultrafast 266.7 nm laser pulses. Irradiated aqueous or alcoholic TNT solutions exhibit increased absorption of light above 300 nm. Surface reflection FTIR measurements of dry deposits of irradiated samples are consistent with the formation of amines or alcohols in the photochemistry of TNT in aqueous or alcoholic solutions. In contrast, no evidence is observed in post irradiation UV-visible absorption or surface reflection FTIR measurements of TNT in toluene samples exposed to 266.7 nm femtosecond laser pulses. The results suggest that the hydroxyl group is involved in the formation of photoproducts of TNT photolysis. In addition, the results suggest that femtosecond laser photolysis is suitable for TNT detection in wet media.
The chemical spectroscopic signature of the RDX-clay mineral complex has been investigated by means of reflectance FT-IR micro spectroscopy. The mechanical analysis method was used to separate the clay from the other soil components. The soil was obtained from the University of Puerto Rico at Mayagüez (UPRM) campus backyard. B3LYP/6-311G** calculations performed on RDX helped to determine the most stable conformations, their symmetry, and vibrational spectra. The FTIR technique confirmed the existence of two different RDX solid phases, known as the α-RDX and β-RDX, which have different symmetries and revealed significant differences in their spectra. The IR microspectroscopic study showed that the RDX-clay mineral complex and its interactions can be detected using the FTIR technique at a low concentration of 1000 part-per-millions. The results also suggest that the vibrational modes presenting changes in the different vibrational spectra correspond to the C-N and NO2 groups. In comparison with α-RDX spectrum, the complex exhibits three bands at 740, 754 and 792 cm-1. A 12 cm-1 red shift is observed in this last band assign to the C-N stretching and NO2 scissoring vibrations in the equatorial position. Differences in the spectra were also seen in the shifted bands at 942 and 953 cm-1. These vibrational modes are assigned to the ring breathing and N-N stretching vibration in the axial position for the -phase. Comparison of the spectra of the α-RDX, the β-RDX and the RDX mixed with clay in the range from 1190 to 1700 cm-1 clearly indicated that the FTIR technique can be used to study the interaction between RDX and clay. The results also indicate that the interaction between the RDX and the clay minerals affects mainly the NO2 groups of the explosive molecules. It is suggested that the electron donor nitrogen atoms from RDX are interacting with the electron acceptor oxygen atoms of the siloxane surface that is present in the majority of clays.
Non nitrogen containing, organic peroxides explosives Triacetone triperoxide and diacetone diperoxide have been prepared in the laboratory in order to study various aspects of their synthesis and their experimental and theoretical spectroscopic characteristics. By using different proportions of acetone/hydrogen peroxide (Ac/H2O2), sulfuric, hydrochloric and methanosulfuric acids as catalyzers, it was possible to obtain both compounds in a rapid and simple form. Raman, IR spectroscopy, and GC-MS were used in order to determine the precursors, intermediates and final analytes. Experiments and theoretical studies using density functional theory (DFT) have been used in the elucidation step of the mechanism of the synthesis of the so called "transparent" explosives. The B3LYP functional with the 6-31G** basis set was used to carry out the electronic structure calculation of the intermediates and internal rotations and vibrations of TATP. Raman spectra of solid TATP and FTIR spectra of gas TATP, were recorded in order to assign the experimental spectra. Although full agreement with experiment was not obtained, spectral features of the main TATP bands were assigned.
Nitrogen-rich compounds have a large cross section for resonance electron capture at very low incident electron energies. Although this fact has been known for a number of years, full benefit of this ubiquitous property of NOX compounds for explosives detection studies has not been fully implemented. Here we report detection of picogram to femtogram levels of TNT, 2,4-DNT and RDX in soil samples and other complex matrices. Toluene extracts as well as thermally desorbed GC-MS analyses were conducted using a JEOL GCmate II coupled to a Tunable-Energy Electron Monochromator (TEEM). Use of TEEM-GC/MS permitted rapid sweeping of electron energy and tuning of the electron monochromator and ion source while monitoring the electron capture resonance in real time. In addition, Solid-Phase Micro-Extraction (SPME) was used to selectively preconcentrate analytes prior conventional GC/MS analysis. The SPME protocol was able to screen explosives in spiked water, in concentrations below the reported detection limits. Standard solutions of TNT were prepared in the range of interest (0.5-10 ppm) and analyzed using a GC/MSD direct injection. Potential use of developed methodology in landmine environmental studies and sensors development will be discussed.
RDX, a high power explosive used as the main charge in some landmines, was investigated in our laboratory in order to determine the spectroscopic signature to be used in its identification by means of ion mobility spectrometry (IMS) and FTIR. Density functional theory (DFT) was also used to predict structural parameters and vibrational frequencies. It was confirmed that RDX has two conformers known as the β and α-RDX phases. There are several conformers depending on the position of the NO2 groups with respect to the triazine ring. This is important in order to determine whether RDX will have affinity for soil and/or the different materials in the ground or will be carried out by water once it starts to leak from the container holding the explosives. Different amounts of RDX were deposited on soil, aluminum plates, glass, and vinyl polymeric films. For IMS studies, the surfaces were rubbed with filter paper and the RDX was desorbed directly from the filter to the instrument inlet port. In the case of the FT-IR studies the samples were examined using an ATR coupled FTIR system. The FTIR spectra showed significant differences between the α and β phases of RDX.
Trace explosive detection is a major technological challenge. Spectroscopic characterization of explosive traces is a major step toward explosive detection strategies and sensor development. We report here on white light imaging measurements and Raman microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM) and energy dispersed X ray analysis (EDX) for the characterization RDX nanoparticles deposited on glass substrates surfaces. The RDX nanoparticles were prepared by exposure of glass substrate surfaces to an aerosol jet containing RDX. An average RDX particle size of 300 nm is determined from the SEM measurements. The spectroscopic signature of the RDX nanoparticles between 750 and 950 cm-1 is dominated by the ring breathing mode centered at about 877 cm-1. The smallest particle characterized with vibrational spectroscopy measurements are about 750 nm in size.
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