The highly energetic material (HEM) hexahydro-1,3,5-trinitro-s-triazine, also known as RDX, has two stable conformational polymorphs at room temperature: α-RDX (molecular conformation of −NO2 groups: axial–axial–equatorial) and β-RDX (molecular conformation of −NO2 groups: axial–axial–axial). Both polymorphs can be formed by deposition on stainless steel substrates using spin coating methodology. α-RDX is the most stable crystal form at room temperature and ambient pressure. However, β-RDX, which has been reported to be difficult to obtain in bulk form at room temperature, was readily formed. Reflection–absorption infrared spectroscopy measurements for RDX-coated stainless steel substrates provided spectral markers that were used to distinguish between the conformational polymorphs on large surface areas of the substrates. Raman microspectroscopy was employed to examine small areas where the intensity was proportional to the height of the structures of RDX. Spectral features were interpreted and classified by using principal component analysis (PCA). The results from these spectral analyses provided good correlation with the values reported in the literature. Conditions to generate predominantly β-RDX crystalline films as a function of the spin coating rotational speed on these substrates were obtained. PCA was also applied to predict percentages of polymorphs present in experimental samples. Applications of the results obtained suggest the modification of existing vibrational spectroscopy based spectral libraries for defense and security applications. Understanding the effects of polymorphism in HEMs will result in the attainment of higher confidence limits in the detection and identification of explosives, especially at trace or near trace levels.

Mid-infrared (MIR) laser spectroscopy was used to detect the presence of residues of high explosives (HEs) on fabrics. The discrimination of the vibrational signals of HEs from a highly MIR-absorbing substrate was achieved by a simple and fast spectral evaluation without preparation of standards using the classical least squares (CLS) algorithm. Classical least squares focuses on minimizing the differences between the spectral features of the actual spectra acquired using MIR spectroscopy and the spectral features of calculated spectra modeled from linear combinations of the spectra of neat components: HEs, fabrics, and bias. Samples in several combinations of cotton fabrics/HEs were used to validate the methodology. Several experiments were performed focusing on binary, ternary, and quaternary mixtures of TNT, RDX, PETN, and fabrics. The parameters obtained from linear combinations of the calculated spectra were used to perform discrimination analyses and to determine the sensitivity and selectivity of HEs with respect to the substrates and to each other. However, discrimination analysis was not necessary to achieve successful detection of HEs on cotton fabric substrates. The RDX signals ( m > 0.02 mg) on cotton were used to calculate the limit of detection (LOD). The signal-to-noise ratios (S/N) calculated from the spectra of cotton dosed with decreasing masses of RDX until S/N ≈ 3 resulted in a LOD of 15-33 µg, depending on the vibrational band used. Linear fits generated by comparing the mass dosed RDX with the fraction predicted were also used to calculate the LOD based on the uncertainty of the blank and the slope. This procedure resulted in a LOD of 58 µg. Probably the most representative value of the method LOD was calculated using an interpolation of a threshold determined using the predicted average value for the blank plus 3.28 times the standard deviations ( p-value threshold) for low surface dosages of RDX (LOD = 40 µg). The contribution demonstrates that to achieve HE detection on fabrics using the proposed algorithm, i.e., determining the presence/absence of HEs on the substrates, the library must contain the spectra of HEs, substrates, and potential interferents or that these spectra be added to the models in the field. If the model does not contain the spectra of the fabric components, there is a high probability of finding false positives for clean samples (no HEs) and a low probability for failed detection in samples with HEs. More work will be required to demonstrate that these new approaches to HE detection work on real-world samples and when contaminating materials are present in the samples.

A pillared layer network containing amide functional groups (Cu­(pzdc)­(pia); pzdc = pyrazine-2,3-dicarboxylate; pia = N-(4-pyridyl)­isonicotinamide) was used to test a postsynthesis metalation rationale to insert lithium and create a porous surface with enhanced CO2 adsorption capacity. Synchrotron powder X-ray diffraction (XRD) was used to determine variations after lithiation in long-range and textural properties. CO2 adsorption measurements at room temperature showed a concave up isotherm shape with an increasing adsorption at high pressures, surpassing by 1 order of magnitude the values previously reported for the unmodified material. There was significant hysteresis upon desorption, which suggests structural variations consequent to different or stronger adsorption sites. Results from elemental, thermal gravimetric, and crystal refinement analyses indicate that the lithium content is ca. 3 Li atoms per asymmetric unit. Raman scattering showed N–Li and Li–O stretching bands, a shift of pia amide- and pyridyl-related bands, and other significant skeletal vibrations associated with nitrogen and oxygen lone pair variations. In situ XRD and CO2 adsorption observations at up to 50 bar at ambient temperature were consistent with the anticipated structural dynamic variation. The lattice changes observed at pressures below 10 bar following lithiation may be directly related to an enhancement in the CO2 adsorption amount.

A simple optical layout for a grazing-angle probe (GAP) mount for coupling to a midinfrared (MIR) quantum cascade laser (QCL) spectrometer is described.This assembly enables reflectance measurements at high incident angles. In the case of optically thin films and deposits on MIR reflective substrates, a double-pass effect occurs, which is accompanied by the absorption of deposited samples in a reflection-absorption infrared spectroscopy modality. The optical system allows MIR light to pass through the sample twice.Applications to cleaning validation and detection of traces of explosives using the QCL-GAP is reported. Principal component analysis and partial least squares multivariate chemometrics methods were employed to analyze MIR spectra to evaluate an analytical methodology for confirming the presence of residues of pharmaceutically active ingredients (irbesartan) and of traces of explosives (cyclotrimethylenetrinitramine [RDX]) that have been deposited on metallic substrates. The performance of spectral preprocessing via fast Fourier transform (FFT) analysis was evaluated for the ability to extract more powerful and accurate information from the obtained reflectance spectra.According to the figures of merit of this new technique, FFT with chemometric routines can obtain sensitivity and specificity values of 1.000. The limits of detection that were obtained for irbesartan and RDX were 26 and 8 ng/cm 2 , respectively. The experimental results demonstrate that the proposed system, when used together with proper chemometrics routines, constitutes a powerful tool for the development of methodologies that have lower detection limits for a range of applications that involve detecting traces of analytes that reside on substrates as contaminants. KEYWORDS fast Fourier transform (FFT), grazing angle MIR laser spectroscopy, irbesartan/RDX, partial least squares (PLS), principal component analysis (PCA)

Owing to scientific advances in the field of materials sciences and engineering, researchers have developed new energy sources used for spectroscopic applications and measurements of properties resulting from the interaction of matter and electromagnetic radiation in the mid-infrared (MIR) region. MIR lasers, such as quantum cascade lasers (QCLs), used for spectroscopy have quickly found numerous applications in a wide cadre of IR techniques. This provides the opportunity to study properties of highly energetic materials (HEM), among many other applications. MIR laser spectroscopy based detection experiments of HEMs were carried out using a QCL optically coupled to compact grazing angle probe mount (QCL-GAP) enabling reflection-absorption infrared spectroscopy (RAIRS) measurements of thin films of HEMs. A saturated solution of RDX in acetone was prepared, and aliquots of subsequent dilutions of the stock solutions were transferred to test surfaces for QCL-GAP back-reflectance measurements. RDX reflectance signals were monitored as function as the decreasing surface concentration until the signal/noise was ∼ 3. Stainless steel (SS) plates were used as reflective substrates, and anodized aluminum (AN-Al), cardboard, and Teflon were used as non-reflective (matte) substrates. Using generated calibration curves a low limit of detection (LOD) of 1.7 ng/cm2 for RDX/SS and 95 μg/cm2 for RDX/AN-Al were found. Based on the area of laser spot (0.3 cm2) we conclude the minimum masses detected were 490 pg (RDX/SS) and 28 μg (RDX/AN-Al).