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The use of secondary ion mass spectrometry (SIMS) for the detection and spatially resolved analysis of individual high explosive particles is described. A C(8) (-) carbon cluster primary ion beam was used in a commercial SIMS instrument to analyze samples of high explosives dispersed as particles on silicon substrates. In comparison with monatomic primary ion bombardment, the carbon cluster primary ion beam was found to greatly enhance characteristic secondary ion signals from the explosive compounds while causing minimal beam-induced degradation. The resistance of these compounds to degradation under ion bombardment allows explosive particles to be analyzed under high primary ion dose bombardment (dynamic SIMS) conditions, facilitating the rapid acquisition of spatially resolved molecular information. The use of cluster SIMS combined with computer control of the sample stage position allows for the automated identification and counting of explosive particle distributions on silicon surfaces. This will be useful for characterizing the efficiency of transfer of particulates in trace explosive detection portal collectors and/or swipes utilized for ion mobility spectrometry applications.

Cluster primary ion bombardment combined with secondary ion imaging is used on an ion microscope secondary ion mass spectrometer for the spatially resolved analysis of organic particles on various surfaces. Compared to the use of monoatomic primary ion beam bombardment, the use of a cluster primary ion beam (SF 5 + or C 8 − ) provides significant improvement in molecular ion yields and a reduction in beam-induced degradation of the analyte molecules. These characteristics of cluster bombardment, along with automated sample stage control and custom image analysis software are utilized to rapidly characterize the spatial distribution of trace explosive particles, narcotics and inkjet-printed microarrays on a variety of surfaces.

Understanding the kinetics of adsorption and desorption of explosive vapors such as TNT from surfaces is important in the design of sensors. We report for the first time, the adsorption–desorption characteristics of TNT from a Si-microcantilever exposed to vapors of TNT. It was observed that TNT readily sticks to the exposed Si surface with the adsorption kinetics showing an initial exponential behavior followed by roughly linear kinetics. It was also observed that for cantilever temperatures close to room temperature, TNT desorbs spontaneously from the surface with decaying exponential kinetics. Based on the known equilibrium partial vapor pressures of TNT, the “effective” sticking coefficient for the silicon oxide surface at room temperature under the experimental conditions was calculated to be about 0.02. This information can be very useful in the design of sensors and that of vapor-delivery systems.

A cesium sputter ion source has been used to generate novel cluster and monoatomic primary ion beams for secondary ion mass spectrometry (SIMS). The source produces a variety of primary ion beam species with sufficient flux to be usable for both organic surface analysis and semiconductor depth profiling. The primary focus of this work is on the generation and use of carbon and carbon-containing cluster primary ion beams for SIMS. Stability of the sputter ion source is a few percent over 20 min, has useful lifetimes of weeks to months, and produces total primary ion beam currents for C2− ions, measured at the sample, of >1 μA at an extraction voltage of 10 kV. Larger cluster ions (Cx−x=4–10 and CsCx−x=2–8) are produced with tens of nA of beam current. Due to the divergence of the source, focused beam operation gives current densities under optimal conditions of 0.4–0.5 mA/cm2. Cluster bombardment studies of organic films using carbon clusters Cx−x=1–10 indicate that large enhancements (up to a factor of 80...

Efforts are underway in the Surface and Microanalysis Science Division at the National Institute of Standards and Technology to study trace aerodynamic sampling of contraband materials (explosives or narcotics) in non-contact trace detection systems. Trace detection systems are designed to screen people, personal items, and cargo for particles that have contaminated surfaces. In a typical implementation of people screening, a human subject walks into a confined space where they are interrogated by a series of pulsed air jets and are screened for contraband materials by a chemical analyzer. The screening process requires particle and vapor removal, transport, collection, desorption, and detection. Aerodynamic sampling is the critical front-end process for effective detection. In this paper, a number of visualization techniques are employed to study non-contact aerodynamic sampling in detail. Particle lift-off and removal is visualized using high-speed videography, transport of air and particles by laser light scattering, and desorption surface heating and cooling patterns by infrared thermography. These tools are used to identify sampling inefficiencies and may be used to study next-generation screening approaches for aerodynamic sampling of particles and vapors.

Explosives detection is a very important task for National Security. The formidable task includes development of new probes and methods for detection of concealed explosives which is of utmost priority to Homeland Security and other security enforcing federal agencies. Here we report on the detection of triacetone triperoxide (TATP) on metallic surfaces using a Fiber Optic Coupled FTIR method. FT-IR spectroscopy is well suited to be used outside the confinement of the sample compartment, provided the excitation source and the reflected light can be transported to the interferometer. Fiber optic cables that transmit in the Mid-IR range have made this possible by allowing the development of a range of spectroscopic probes for in situ analysis. In our study we used a specially designed sampling probe that operates at the grazing-angle to detect and to quantify μg/cm2 levels of explosives on stainless steel. Calibration curves were prepared using stainless steel plates, 3 inches wide x 6 inches long. The samples were deposited on the surface using a smearing method. To carry out the experiments, TATP was synthesized in the laboratory. For the calibration curves TATP was dissolved in dichloromethane. The standard solutions (20) μL were transferred on the plates resulting in surface mass concentrations of TATP that ranged from 8 to 150 μg/cm2. The data was analyzed using Chemometrics routines and Discriminant Analysis algorithms. In particular, multivariate Partial Least Squares (PLS) was used to determine the most significant peak for the analysis. In other experiments done with stainless steel plates coated with 150 μg/cm2 TATP, spectra were recorded every 27 seconds. For this concentration TATP sublimates to surface concentrations below detection in 800 s.

We report on the effect of trinitrotoluene (TNT) in the fluroescence emission of CdSe quantum dots covered with a ZnS core shell. Pulses of 100 femtoseconds are employed to excite the fluorescence of the dots off resonance. We found evidence for a strong quenching by TNT on the fluorescence emission of the quantum dots. The fluorescence emission of the QD experiences a blue or red shift that depends on the CdSe particle size and amount of TNT. The results are explained in terms of a charge transfer mechanism or a modification of the traps involved in electron relaxation in QD. The results show that QD are a promising tool for TNT sensing.

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.

Atomic force microscopy (AFM), scanning electron microscopy (SEM), white light imaging measurements, and Raman microscopy were employed for the characterization of hexahydro-1,3,5-trinitro-1,3,5-s-triazine (RDX) nanoparticles deposited on glass substrate surfaces. The RDX nanoparticles were prepared by exposure of glass substrate surfaces to an aerosol jet containing RDX. The spectroscopic signature of RDX particles and the two known forms of the material, β and α RDX, are compared. Raman measurements reveal that RDX nanoparticles and β deposits have similar spectroscopic signatures between 750 and 1000 cm-1.

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 and -RDX phases. There are several conformers depending on the position of the NO 2 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 and phases of RDX. Keywords: RDX chemical signature, explosives, FTIR, IMS, RDX interactions, RDX in soil. 1.

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.

We are evaluating the use of polyatomic and cluster primary ion beams for characterization of semiconductor materials by secondary ion mass spectrometry using both magnetic sector and time-of-flight SIMS instruments. Primary ion beams of SF5+, C8− and CsC6− have been used to analyze low energy arsenic implants in silicon, boron delta-doped structures, thin gate oxides, metal multilayers, organic surface contamination and photoresist thin films. Compared to monoatomic bombardment under the same conditions, cluster ion beams offer improved depth resolution for silicon depth profiling and a reduction in sputter-induced topography for metals. For organic materials, the use of a cluster ion beam can give large improvements in yield for characteristic secondary ions and can minimize beam-induced degradation in some materials.

The primary ion column of a secondary ion mass spectrometer (Cameca IMS 3f) has been used as an ion implanter to prepare calibrated standards, In situ for quantitative SIMS analysis, with an accuracy better than 10%. The technique has been used to determine oxygen concentrations in contaminated TiSi2 films by implanting a reference level of 18O into a portion of the film.