Samuel P. Hernandez

Improved detection of landmine components: using TEEM-GC-MS for detection of TNT and RDX in soil and other complex matrices

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.

3D Numerical simulation of the transport of chemical-signature-compounds from buried landmines

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.

Ab initio treatment of the behavior of TNT in soil

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.

A near field optical microscopy study of trinitrotoluene

Atomic force microscopy (AFM) and near field scanning optical microscopy (NSOM) are promising analytical techniques for the determination of trace amounts of explosive materials on a variety of surfaces. Information regarding the forces of interaction and explosive spectroscopic signatures, in addition to explosive morphology, can be obtained with AFM and NSOM techniques. Basic work toward the development of methodology that can enable the employment of these techniques is needed to facilitate their employment in the field and real life scenarios. In this work, we report on the use of AFM and NSOM for the determination of the morphology and spectroscopic signature of TNT particles on optically transparent substrates open to air environments. The TNT particles are about 1 mm in diameter. Transmission NSOM on the particles following 265 nm excitation reveals that the fluorescence peak is centered at 255, about 10 cm-1 lower than the excitation wavelength. The fluorescence yield is found to increase non-linearly with incident laser power, consistent with a multi photon absorption process. The results encourage future work in the area, in particular, with the use of multi-cantilevers that can increase the surface area examined in real life scenarios.

Nitroexplosives detection: from basic science to detection at a distance

In this contribution, we summarize our efforts toward detection of TNT, from traces to bulk amounts, based on the absorption fingerprint of TNT. Light absorption by TNT is broken into three regions: (1) VIS light absorption by TNT, and (2) formation and detection of NO2 upon UV irradiation of TNT and (3) formation and detection of NO following UV absorption by NO2. The absorption spectrum of TNT powder and particles has been determined from spectral analysis of backscattered VIS light in traditional optical and near field optical microscopy measurements, respectively. The smallest amount of TNT detected in the near field measurements is 7 fg. The absorption spectra of TNT are rich in structure and similar to the one measured for gas phase NO2, with lines due to rovibronic coupling of electronic excited states. Measurements of the backscattered visible light on samples, placed about 5 to 10 meters from the laser source, indicate a clear change in intensity as compared to samples containing TNT. The second light absorption region, NO2 is detected upon UV irradiation of solid TNT.

Nitro explosive detection: from basic science to detection at a distance

Standoff detection of landmines has long been central to improve quality of life in a number of countries around the world. A large body of work in the literature focuses on detection of TNT in soil as central to landmine detection. In this presentation, we summarize our efforts toward detection of TNT, from traces to bulk amounts, based on the absorption fingerprint of TNT. Light absorption by TNT is broken into three regions: (1) visible light absorption by TNT, and (2) formation and detection of NO2 upon UV irradiation of TNT and (3) formation and detection of NO following UV absorption by NO2. The absorption spectrum of TNT powder and particles has been determined from spectral analysis of backscattered visible light in traditional optical and near field optical microscopy measurements, respectively. The smallest amount of TNT detected in the near field measurements is 7 femtograms. The absorption spectra of TNT are rich in structure and similar to the one measured for gas phase NO2, with lines due to roto vibronic coupling of electronic excited states. Measurements of the backscattered visible light on samples, placed about 5 to 10 meters from the laser source, indicate a clear change in intensity as compared to samples containing TNT. Turning to the second light absorption region, NO2 is detected upon UV irradiation of solid TNT. NO can also be detected by photolysis of NO2.

FT-IR microspectroscopy of RDX in clay soils

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 clays 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 clays surface.

Time-of-flight mass spectroscopy measurements of TNT and RDX on soil surfaces

Mass spectroscopy methods are promising tools for the detection of trace amounts of explosive materials in a number of physical environments. The purpose of this study is to establish the kinetic energy distribution of NO as a product of photo-fragmentation of nitro-compounds like TNT and RDX on soil substrates surfaces using femto second laser pulses for molecular dissociation and subsequent mass spectrometry measurements as a function of time. We have successfully performed NO TOF measurements on TNT deposits photo decomposed with 130 femtosecond laser pulses with a 400 nm wavelength. The data is modeled with a modified Boltzman distribution. NO kinetic energy distributions resulting from TNT and RDX on soil substrate surfaces are compared.

Computational modeling of the adsorption of 2,4-DNT on clay

Experimental studies have shown that a key factor affecting the bioavailability and biodegradability of nitroaromatic compounds (NACs) 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.