Stand-off Hyperspectral Raman Imaging and Random Decision Forest Classification: A Potent Duo for the Fast, Remote Identification of Explosives.

In this study, we present a stand-off hyperspectral Raman imager (HSRI) for the fast detection and classification of different explosives at a distance of 15 m. The hyperspectral image cube is created by using a liquid crystal tunable filter (LCTF) to select a specific Raman shift and sequentially imaging spectral images onto an intensified CCD camera. The laser beam is expanded to illuminate the field of view of the HSRI and thereby improves large area scanning of suspicious surfaces. The collected hyperspectral image cube (HSI) is evaluated and classified using a random decision forest (RDF) algorithm. The RDF is trained with a training set of mg-amounts of different explosives, i.e., TNT, RDX, PETN, NaClO3, and NH4NO3, on an artificial aluminum substrate. The resulting classification is validated, and variable importance is used to optimize the RDF using spectral descriptors, effectively reducing the dimensionality of the data set. Using the gained information, a faster acquisition and calculation mode can be designed, giving improved results in classification at a much higher repetition rate.

Fourier Transform Infrared (FT-IR) and Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) Imaging of Cerebral Ischemia: Combined Analysis of Rat Brain Thin Cuts Toward Improved Tissue Classification.

Microspectroscopic techniques are widely used to complement histological studies. Due to recent developments in the field of chemical imaging, combined chemical analysis has become attractive. This technique facilitates a deepened analysis compared to single techniques or side-by-side analysis. In this study, rat brains harvested one week after induction of photothrombotic stroke were investigated. Adjacent thin cuts from rats' brains were imaged using Fourier transform infrared (FT-IR) microspectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The LA-ICP-MS data were normalized using an internal standard (a thin gold layer). The acquired hyperspectral data cubes were fused and subjected to multivariate analysis. Brain regions affected by stroke as well as unaffected gray and white matter were identified and classified using a model based on either partial least squares discriminant analysis (PLS-DA) or random decision forest (RDF) algorithms. The RDF algorithm demonstrated the best results for classification. Improved classification was observed in the case of fused data in comparison to individual data sets (either FT-IR or LA-ICP-MS). Variable importance analysis demonstrated that both molecular and elemental content contribute to the improved RDF classification. Univariate spectral analysis identified biochemical properties of the assigned tissue types. Classification of multisensor hyperspectral data sets using an RDF algorithm allows access to a novel and in-depth understanding of biochemical processes and solid chemical allocation of different brain regions.

Toward a Noninvasive, Label-Free Screening Method for Determining Spore Inoculum Quality of Penicillium chrysogenum Using Raman Spectroscopy.

We report on a label-free, noninvasive method for determination of spore inoculum quality of Penicillium chrysogenum prior to cultivation/germination. Raman microspectroscopy providing direct, molecule-specific information was used to extract information on the viability state of spores sampled directly from the spore inoculum. Based on the recorded Raman spectra, a supervised classification method was established for classification between living and dead spores and thus determining spore inoculum quality for optimized process control. A fast and simple sample preparation method consisting of one single dilution step was employed to eliminate interfering signals from the matrix and to achieve isolation of single spores on the sample carrier (CaF2). Aiming to avoid any influence of the killing procedure in the Raman spectrum of the spore, spores were considered naturally dead after more than one year of storage time. Fluorescence staining was used as reference method. A partial least squares discriminant analysis classifier was trained with Raman spectra of 258 living and dead spores (178 spectra for calibration, 80 spectra for validation). The classifier showed good performance when being applied to a 1 µL droplet taken from a 1:1 mixture of living and dead spores. Of 135 recorded spectra, 51% were assigned to living spores while 49% were identified as dead spores by the classifier. The results obtained in this work are a fundamental step towards developing an automated, label-free, and noninvasive screening method for assessing spore inoculum quality.

Picomolar Traces of Americium(III) Introduce Drastic Changes in the Structural Chemistry of Terbium(III): A Break in the "Gadolinium Break".

The crystallization of terbium 5,5'-azobis[1H-tetrazol-1-ide] (ZT) in the presence of trace amounts (ca. 50 Bq, ca. 1.6 pmol) of americium results in 1) the accumulation of the americium tracer in the crystalline solid and 2) a material that adopts a different crystal structure to that formed in the absence of americium. Americium-doped [Tb(Am)(H2 O)7 ZT]2 ZT⋅10 H2 O is isostructural to light lanthanide (Ce-Gd) 5,5'-azobis[1H-tetrazol-1-ide] compounds, rather than to the heavy lanthanide (Tb-Lu) 5,5'-azobis[1H-tetrazol-1-ide] (e.g., [Tb(H2 O)8 ]2 ZT3 ⋅6 H2 O) derivatives. Traces of Am seem to force the Tb compound into a structure normally preferred by the lighter lanthanides, despite a 108 -fold Tb excess. The americium-doped material was studied by single-crystal X-ray diffraction, vibrational spectroscopy, radiochemical neutron activation analysis, and scanning electron microcopy. In addition, the inclusion properties of terbium 5,5'-azobis[1H-tetrazol-1-ide] towards americium were quantified, and a model for the crystallization process is proposed.

Image-Based Chemical Structure Determination.

Chemical imaging is a powerful tool for understanding the chemical composition and nature of heterogeneous samples. Recent developments in elemental, vibrational, and mass-spectrometric chemical imaging with high spatial resolution (50-200 nm) and reasonable timescale (a few hours) are capable of providing complementary chemical information about various samples. However, a single technique is insufficient to provide a comprehensive understanding of chemically complex materials. For bulk samples, the combination of different analytical methods and the application of statistical methods for extracting correlated information across different techniques is a well-established and powerful concept. However, combined multivariate analytics of chemical images obtained via different imaging techniques is still in its infancy, hampered by a lack of analytical methodologies for data fusion and analysis. This study demonstrates the application of multivariate statistics to chemical images taken from the same sample via various methods to assist in chemical structure determination.

Implementation of a quantum cascade laser-based gas sensor prototype for sub-ppmv H2S measurements in a petrochemical process gas stream.

The implementation of a sensitive and selective as well as industrial fit gas sensor prototype based on wavelength modulation spectroscopy with second harmonic detection (2f-WMS) employing an 8-µm continuous-wave distributed feedback quantum cascade laser (CW-DFB-QCL) for monitoring hydrogen sulfide (H2S) at sub-ppm levels is reported. Regarding the applicability for analytical and industrial process purposes aimed at petrochemical environments, a synthetic methane (CH4) matrix of up to 1000 ppmv together with a varying H2S content was chosen as the model environment for the laboratory-based performance evaluation performed at TU Wien. A noise-equivalent absorption sensitivity (NEAS) for H2S targeting the absorption line at 1247.2 cm-1 was found to be 8.419 × 10-10 cm-1 Hz-1/2, and a limit of detection (LOD) of 150 ppbv H2S could be achieved. The sensor prototype was then deployed for on-site measurements at the petrochemical research hydrogenation platform of the industrial partner OMV AG. In order to meet the company's on-site safety regulations, the H2S sensor platform was installed in an industry rack and equipped with the required safety infrastructure for protected operation in hazardous and explosive environments. The work reports the suitability of the sensor prototype for simultaneous monitoring of H2S and CH4 content in the process streams of a research hydrodesulfurization (HDS) unit. Concentration readings were obtained every 15 s and revealed process dynamics not observed previously.

On-line monitoring of methanol and methyl formate in the exhaust gas of an industrial formaldehyde production plant by a mid-IR gas sensor based on tunable Fabry-Pérot filter technology.

On-line monitoring of key chemicals in an industrial production plant ensures economic operation, guarantees the desired product quality, and provides additional in-depth information on the involved chemical processes. For that purpose, rapid, rugged, and flexible measurement systems at reasonable cost are required. Here, we present the application of a flexible mid-IR filtometer for industrial gas sensing. The developed prototype consists of a modulated thermal infrared source, a temperature-controlled gas cell for absorption measurement and an integrated device consisting of a Fabry-Pérot interferometer and a pyroelectric mid-IR detector. The prototype was calibrated in the research laboratory at TU Wien for measuring methanol and methyl formate in the concentration ranges from 660 to 4390 and 747 to 4610 ppmV. Subsequently, the prototype was transferred and installed at the project partner Metadynea Austria GmbH and linked to their Process Control System via a dedicated micro-controller and used for on-line monitoring of the process off-gas. Up to five process streams were sequentially monitored in a fully automated manner. The obtained readings for methanol and methyl formate concentrations provided useful information on the efficiency and correct functioning of the process plant. Of special interest for industry is the now added capability to monitor the start-up phase and process irregularities with high time resolution (5 s).

Tip-Enhanced Raman Spectroscopy of Atmospherically Relevant Aerosol Nanoparticles.

Atmospheric aerosol nanoparticles play a major role in many atmospheric processes and in particular in the global climate system. Understanding their formation by homogeneous or heterogeneous nucleation as well as their photochemical aging and atmospheric transformation is of utmost importance to evaluate their impact on atmospheric phenomena. Single particle analysis like tip-enhanced Raman spectroscopy (TERS) opens access to a deeper understanding of these nanoparticles. Atmospherically relevant nanoparticles, formed above a simulated salt lake inside an aerosol smog-chamber, were analyzed using TERS. TERS spectra of 11 nanoparticles were studied in detail. First results of TERS on atmospherically relevant aerosol nanoparticles reveal the presence of inorganic seed particles, a chemical diversity of equally sized particles in the nucleation mode, and chemical transformation during photochemical aging. Therefore, single particle analysis by optical near-field spectroscopy such as TERS of atmospheric nanoparticles will significantly contribute to elucidate atmospheric nucleation, photochemical aging, and chemical transformation processes by uncovering single particle based properties.

Application of a ring cavity surface emitting quantum cascade laser (RCSE-QCL) on the measurement of H2S in a CH4 matrix for process analytics.

The present work reports on the first application of a ring-cavity-surface-emitting quantum-cascade laser (RCSE-QCL) for sensitive gas measurements. RCSE-QCLs are promising candidates for optical gas-sensing due to their single-mode, mode-hop-free and narrow-band emission characteristics along with their broad spectral coverage. The time resolved down-chirp of the RCSE-QCL in the 1227-1236 cm-1 (8.15-8.09 µm) spectral range was investigated using a step-scan FT-IR spectrometer (Bruker Vertex 80v) with 2 ns time and 0.1 cm-1 spectral resolution. The pulse repetition rate was set between 20 and 200 kHz and the laser device was cooled to 15-17°C. Employing 300 ns pulses a spectrum of ~1.5 cm-1 could be recorded. Under these laser operation conditions and a gas pressure of 1000 mbar a limit of detection (3σ) of 1.5 ppmv for hydrogen sulfide (H2S) in nitrogen was achieved using a 100 m Herriott cell and a thermoelectric cooled MCT detector for absorption measurements. Using 3 µs long pulses enabled to further extend the spectral bandwidth to 8.5 cm-1. Based on this increased spectral coverage and employing reduced pressure conditions (50 mbar) multiple peaks of the target analyte H2S as well as methane (CH4) could be examined within one single pulse.

In situ formation of reduced graphene oxide structures in ceria by combined sol-gel and solvothermal processing.

Raman and IR investigations indicated the presence of reduced graphene oxide (rGO)-like residues on ceria nanoparticles after solvothermal treatment in ethanol. The appearance of such structures is closely related to cerium tert-butoxide as precursor and ethanol as solvothermal solvent. The rGO-like residues improve the catalytic CO oxidation activity. This was also confirmed by introduction of "external" graphene oxide during sol-gel processing, by which the rGO structures and the catalytic activity were enhanced.

Chemometric analysis of multisensor hyperspectral images of precipitated atmospheric particulate matter.

The chemometric analysis of multisensor hyperspectral data allows a comprehensive image-based analysis of precipitated atmospheric particles. Atmospheric particulate matter was precipitated on aluminum foils and analyzed by Raman microspectroscopy and subsequently by electron microscopy and energy dispersive X-ray spectroscopy. All obtained images were of the same spot of an area of 100 × 100 µm(2). The two hyperspectral data sets and the high-resolution scanning electron microscope images were fused into a combined multisensor hyperspectral data set. This multisensor data cube was analyzed using principal component analysis, hierarchical cluster analysis, k-means clustering, and vertex component analysis. The detailed chemometric analysis of the multisensor data allowed an extensive chemical interpretation of the precipitated particles, and their structure and composition led to a comprehensive understanding of atmospheric particulate matter.

Quasi-simultaneous in-line flue gas monitoring of NO and NO2 emissions at a caloric power plant employing mid-IR laser spectroscopy.

Two pulsed thermoelectrically cooled mid-infrared distributed feedback quantum cascade lasers (QCLs) were used for the quasi-simultaneous in-line determination of NO and NO2 at the caloric power plant Dürnrohr (Austria). The QCL beams were combined using a bifurcated hollow fiber, sent through the flue tube (inside diameter: 5.5 m), reflected by a retro-reflector and recorded using a fast thermoelectrically cooled mercury-cadmium-telluride detector. The thermal chirp during 300 ns pulses was about 1.2 cm(-1) and allowed scanning of rotational vibrational doublets of the analytes. On the basis of the thermal chirp and the temporal resolution of data acquisition, a spectral resolution of approximately 0.02 cm(-1) was achieved. The recorded rotational vibrational absorption lines were centered at 1900 cm(-1) for NO and 1630 cm(-1) for NO2. Despite water content in the range of 152-235 g/m(3) and an average particle load of 15.8 mg/m(3) in the flue gas, in-line measurements were possible achieving limits of detection of 73 ppb for NO and 91 ppb for NO2 while optimizing for a single analyte. Quasi-simultaneous measurements resulted in limits of detection of 219 ppb for NO and 164 ppb for NO2, respectively. Influences of temperature and pressure on the data evaluation are discussed, and results are compared to an established reference method based on the extractive measurements presented.

Halogen-induced organic aerosol (XOA): a study on ultra-fine particle formation and time-resolved chemical characterization.

The concurrent presence of high values of organic SOA precursors and reactive halogen species (RHS) at very low ozone concentrations allows the formation of halogen-induced organic aerosol, so-called XOA, in maritime areas where high concentrations of RHS are present, especially at sunrise. The present study combines aerosol smog-chamber and aerosol flow-reactor experiments for the characterization of XOA. XOA formation yields from alpha-pinene at low and high concentrations of chlorine as reactive halogen species (RHS) were determined using a 700 L aerosol smog-chamber with a solar simulator. The chemical transformation of the organic precursor during the aerosol formation process and chemical aging was studied using an aerosol flow-reactor coupled to an FTIR spectrometer. The FTIR dataset was analysed using 2D correlation spectroscopy. Chlorine induced homogeneous XOA formation takes place at even 2.5 ppb of molecular chlorine, which was photolysed by the solar simulator. The chemical pathway of XOA formation is characterized by the addition of chlorine and abstraction of hydrogen atoms, causing simultaneous carbon-chlorine bond formation. During further steps of the formation process, carboxylic acids are formed, which cause a SOA-like appearance of XOA. During the ozone-free formation of secondary organic aerosol with RHS a special kind of particulate matter (XOA) is formed, which is afterwards transformed to SOA by atmospheric aging or degradation pathways.

Circular multireflection cell for optical spectroscopy.

We constructed a circular multireflection (CMR) cell, allowing multireflection around the center of the cell. This is caused by a skewed adjustment of the entering beam (equivalent to a simple parallel shift/offset), avoiding the center of the cell, thus leading to multiple reflections. The experimental setup with a cell with an inner diameter of 6 cm showed up to 17.5 beam passes on polished aluminum and attained path lengths up to 105 cm, demonstrated by Fourier transform infrared measurements of CO(2) gas between 2283 and 2400 cm(-1). The circular concept, i.e., the centering of the reflections, is useful for absorption spectroscopy on trace gases and aerosols. The optical alignment of the cell can completely be performed from outside the experimental setup, e.g., an aerosol flow reactor or a vacuum system. The variation of the path length is easily possible by adjusting the position of the cell with respect to the entering light beam.

Formation of binary and ternary colloids and dissolved complexes of organic matter, Fe and As.

Natural organic matter can change As speciation via redox reactions and complexation influencing its mobility and toxicity. Here we show that binary and ternary colloids and dissolved complexes of As(V), Fe and organic matter (OM) form at environmentally relevant conditions and analyzed these colloids/complexes using ATR-FTIR- and Mossbauer-spectroscopy. Dissolved Fe-OM complexes and ferrihydrite-OM colloids were formed by reacting OM with ferrihydrite (Fe(OH)(3)). Mossbauer-spectroscopy showed that 95% of the Fe in the Fe-OM fraction were present as ferrihydrite-OM colloids while the remaining 5% were in the dissolved fraction. In As(V) plus Fe-OM systems (containing both dissolved and colloidal Fe-OM), 3.5-8 microg As(V)/mg OC was bound to the Fe-OM complexes/colloids compared to <0.015 microg As(V)/mg OC in As-OM systems (without Fe). Upon filtration of As-Fe-OM complexes/colloids with a 3 kDa filter, approximately 6% As was found in the dissolved fraction and approximately 94% As in colloidal Fe-OM. This suggests that As(V) is associated with Fe-OM mainly via ferrihydrite-OM colloids but to a small extent also in dissolved Fe-OM complexes via Fe-bridging. Since As-contaminated soils and aquifers contain Fe(III) minerals and OM, colloids of As with OM-loaded ferrihydrite and complexes of As with dissolved Fe-OM have to be considered when studying As transport.