Showing papers in "Applied Spectroscopy in 2008"
TL;DR: The results indicate that several cell components may contribute to the Mie scattering, and an approach was applied to a set of Fourier transform infrared absorbance spectra measured for individual lung cancer cells in order to remove unwanted interferences and to estimate ranges of important α values for each spectrum.
Abstract: We present an approach for estimating and correcting Mie scattering occurring in infrared spectra of single cells, at diffraction limited probe size, as in synchrotron based microscopy. The Mie scattering is modeled by extended multiplicative signal correction (EMSC) and subtracted from the vibrational absorption. Because the Mie scattering depends non-linearly on α, the product of the radius and the refractive index of the medium/sphere causing it, a new method was developed for estimating the Mie scattering by EMSC for unknown radius and refractive index of the Mie scatterer. The theoretically expected Mie contributions for a range of different α values were computed according to the formulae developed by Van de Hulst (1957). The many simulated spectra were then summarized by a six-dimensional subspace model by principal component analysis (PCA). This subspace model was used in EMSC to estimate and correct for Mie scattering, as well as other additive and multiplicative interference effects. The approach was applied to a set of Fourier transform infrared (FT-IR) absorbance spectra measured for individual lung cancer cells in order to remove unwanted interferences and to estimate ranges of important α values for each spectrum. The results indicate that several cell components may contribute to the Mie scattering.
175 citations
TL;DR: The preliminary results indicate that while LIBS is able to discriminate biomaterials with similar elemental compositions at standoff distances based on differences in key intensity ratios, further work is needed to reduce the number of false positives/negatives by refining the PLS-DA model to include a sufficient range of material classes and carefully selecting a detection threshold.
Abstract: Laser-induced breakdown spectroscopy (LIBS) is a promising technique for real-time chemical and biological warfare agent detection in the field. We have demonstrated the detection and discrimination of the biological warfare agent surrogates Bacillus subtilis (BG) (2% false negatives, 0% false positives) and ovalbumin (0% false negatives, 1% false positives) at 20 meters using standoff laser-induced breakdown spectroscopy (ST-LIBS) and linear correlation. Unknown interferent samples (not included in the model), samples on different substrates, and mixtures of BG and Arizona road dust have been classified with reasonable success using partial least squares discriminant analysis (PLS-DA). A few of the samples tested such as the soot (not included in the model) and the 25% BG:75% dust mixture resulted in a significant number of false positives or false negatives, respectively. Our preliminary results indicate that while LIBS is able to discriminate biomaterials with similar elemental compositions at standoff distances based on differences in key intensity ratios, further work is needed to reduce the number of false positives/negatives by refining the PLS-DA model to include a sufficient range of material classes and carefully selecting a detection threshold. In addition, we have demonstrated that LIBS can distinguish five different organophosphate nerve agent simulants at 20 meters, despite their similar stoichiometric formulas. Finally, a combined PLS-DA model for chemical, biological, and explosives detection using a single ST-LIBS sensor has been developed in order to demonstrate the potential of standoff LIBS for universal hazardous materials detection.
150 citations
TL;DR: Results indicate that the SERS-active silver nanorod array substrate is a potential analytical sensor for rapid identification of microorganisms with a minimum of sample preparation.
Abstract: Surface-enhanced Raman scattering (SERS) using novel silver nanorod array substrates has been used for the detection of pathogenic bacteria. The substrate consists of a base layer of 500 nm silver film on a glass slide and a layer of silver nanorod array with a length of ∼1 μm produced by the oblique angle deposition method at a vapor incident angle of 86°. Spectra from whole cell bacteria, Generic Escherichia coli, E. coli O157:H7, E. coli DH 5α, Staphylococcus aureus, S. epidermidis, and Salmonella typhimurium, and bacteria mixtures have been obtained. This SERS active substrate can detect spectral differences between Gram types, different species, their mixture, and strains. Principal component analysis (PCA) has been applied to classify the spectra. Viable and nonviable cells have also been examined, and significantly reduced SERS responses were observed for nonviable cells. SERS detection of bacteria at the single cell level, excited at low incident laser power (12 μW) and short collection time (10 s), has also been demonstrated. These results indicate that the SERS-active silver nanorod array substrate is a potential analytical sensor for rapid identification of microorganisms with a minimum of sample preparation.
146 citations
TL;DR: It is shown that the mineral-to-collagen ratio at the insertion site increases linearly over the distance of 120 μm from tendon to bone, rather than abruptly, as previously inferred from histological observations.
Abstract: We applied Raman spectroscopy to monitor the distribution of mineral and the degree of mineralization across the tendon-bone insertion site in the shoulders of five rats. We acquired Raman spectra from 100 to 4,000 Deltacm(-1) on individual 1 microm points across the 120 microm wide transition zone of each tissue sample and identified all the peaks detected in pure tendon and in pure bone, as well as in the transition zone. The intensity of the 960 Deltacm(-1) P-O stretch for apatite (normalized to either the 2,940 Deltacm(-1) C-H stretch or the 1,003 Deltacm(-1) C-C stretch for collagen) was used as an indicator of the abundance of mineral. We relate the observed histological morphology in the tissue thin section with the observed Raman peaks for both the organic component (mostly collagen) and the inorganic component (a carbonated form of the mineral apatite) and discuss spectroscopic issues related to peak deconvolution and quantification of overlapping Raman peaks. We show that the mineral-to-collagen ratio at the insertion site increases linearly (R(2) = 0.8 for five samples) over the distance of 120 microm from tendon to bone, rather than abruptly, as previously inferred from histological observations. In addition, narrowing of the 960 Deltacm(-1) band across the traverse indicates that the crystalline ordering within the apatite increases concomitantly with the degree of mineralization. This finding of mineral gradation has important clinical implications and may explain why the uninjured tendon-to-bone connection of the rotator cuff can sustain very high loads without failure. Our finding is also consistent with recent mechanical models and calculations developed to better understand the materials properties of this unusually strong interface.
136 citations
TL;DR: Excess infrared (IR) absorption spectroscopy is shown to be a potential method to study hydrogen bonds and to unveil new information on hydrogen bonding in condensed phases.
Abstract: Excess infrared (IR) absorption spectroscopy, a new concept brought forward by applying the idea of excess thermodynamic functions to infrared spectroscopy, is shown to be a potential method to study hydrogen bonds. It can be applied to enhance spectral resolution of complexed IR bands, to evaluate nonideality of liquid mixtures, and to estimate selective molecular interactions. The sign of the excess infrared absorption coefficient is also of importance in providing information on molecular interactions. The results demonstrate that excess infrared absorption spectroscopy can unveil new information on hydrogen bonding in condensed phases.
102 citations
TL;DR: This work validates the applicability of qualitative SER spectroscopy for analysis of bacterial species by utilizing principal component analysis (PCA) to show discrimination of biological threat simulants, based upon multivariate statistical confidence limits bounding known data clusters.
Abstract: Surface-enhanced Raman spectroscopy (SERS) can provide rapid fingerprinting of biomaterial in a nondestructive manner. The adsorption of colloidal silver to biological material suppresses native biofluorescence while providing electromagnetic surface enhancement of the normal Raman signal. This work validates the applicability of qualitative SER spectroscopy for analysis of bacterial species by utilizing principal component analysis (PCA) to show discrimination of biological threat simulants, based upon multivariate statistical confidence limits bounding known data clusters. Gram-positive Bacillus spores (Bacillus atrophaeus, Bacillus anthracis, and Bacillus thuringiensis) are investigated along with the Gram-negative bacterium Pantoea agglomerans.
101 citations
TL;DR: It is found that the vibrational features related to the peroxide bonds are strongly mixed and the spectrum is congested and highly sensitive to minor changes in the molecule.
Abstract: A comparative study of the vibrational spectroscopy of peroxide-based explosives is presented. Triacetone triperoxide (TATP) and hexamethyl-enetriperoxide-diamine (HMTD), now commonly used by terrorists, are examined as well as other peroxide-ring structures: DADP (diacetone diperoxide); TPTP [3,3,6,6,9,9-Hexaethyl-1,2,4,5,7,8-hexaoxo-nonane (tripentanone triperoxide)]; DCypDp {6,7,13,14-Tetraoxadispiro [4.2.4.2]tetradecane (dicyclopentanone diperoxide)}; TCypDp {6,7,15,16,22,23-Hexaoxatrispiro[4.2.4.2.4.2] henicosane (tricyclopentanone triperoxide)}; DCyhDp {7,8,15,16-tetraoxadispiro [5.2.5.2] hexadecane (dicyclohexanone diperoxide)}; and TCyhTp {7,8,14,15,21,22-hexaoxatrispiro [5.2.5.2.5.2] tetracosane (tricyclohexanone triperoxide)}. Both Raman and infrared (IR) spectra were measured and compared to theoretical calculations. The calculated spectra were obtained by calculation of the harmonic frequencies of the studied compounds, at the density functional theory (DFT) B3LYP/cc-pVDZ level of theory, and by the use of scaling factors. It is found that the vibrational features related to the peroxide bonds are strongly mixed. As a result, the spectrum is congested and highly sensitive to minor changes in the molecule.
95 citations
TL;DR: This paper has extended and generalized the SNIP algorithm for multidimensional coincidence γ-ray spectra, and derived a set of modifications of the algorithm that allow estimation of specific shapes of background and ridges as well.
Abstract: In this paper we present sensitive nonlinear iterative peak (SNIP) clipping algorithms to estimate background in various kinds of spectra. We have proposed several improvements of the SNIP algorithm. Moreover, we have extended and generalized the SNIP algorithm for multidimensional coincidence γ-ray spectra. In addition to the continuous background, the so-called lower-order coincidences, i.e., coincidences peak-background in different dimensions, must also be included into the estimated background. Further, we have derived a set of modifications of the algorithm that allow estimation of specific shapes of background and ridges as well.
90 citations
87 citations
TL;DR: It is shown that line-scanning Raman mapping techniques are particularly suitable for the characterization of samples, which are representative of the general complexity of geological samples, and their respective advantages and disadvantages in terms of acquisition time, image quality, spatial and imaging resolutions, and signal-to-noise ratio.
Abstract: By allowing nondestructive chemical and structural imaging of heterogeneous samples with a micrometer spatial resolution, Raman mapping offers unique capabilities for assessing the spatial distribution of both mineral and organic phases within geological samples. Recently developed line-scanning Raman mapping techniques have made it possible to acquire Raman maps over large, millimeter-sized, zones of interest owing to a drastic decrease of the data acquisition time without losing spatial or spectral resolution. The synchronization of charge-coupled device (CCD) measurements with x,y motorized stage displacement has allowed dynamic line-scanning Raman mapping to be even more efficient: total acquisition time may be reduced by a factor higher than 100 compared to point-by-point mapping. Using two chemically and texturally complex geological samples, a fossil megaspore in a metamorphic rock and aragonite-garnet intergrowths in an Eclogitic marble, we compare here two recent versions of line-scanning Raman mapping systems and discuss their respective advantages and disadvantages in terms of acquisition time, image quality, spatial and imaging resolutions, and signal-to-noise ratio. We show that line-scanning Raman mapping techniques are particularly suitable for the characterization of such samples, which are representative of the general complexity of geological samples.
84 citations
TL;DR: The potential to transfer this technique to real biological samples by the detection of folic acid in human serum samples by SERS is demonstrated and the potential applications of this technique in a biological environment are highlighted.
Abstract: Surface-enhanced Raman scattering (SERS) is shown to give linear and sensitive concentration-dependent detection of folic acid using silver nanoparticles created via ethylene-diaminetetraacetic acid (EDTA) reduction. Optical detection by SERS overcomes the primary limitation of photodissociation encountered during the application of other shorter wavelength ultraviolet (UV)/near-UV techniques such as fluorescence based microscopy. The SERS approach in water-based samples was demonstrated and optimized using several longer wavelengths of excitation (514.5, 632.8, and 785 nm). Excitation in the green (514.5 nm) was found to achieve the best balance between photodissociation and SERS efficiency. Linear concentration dependence was observed in the range of 0.018 to 1 μM. The importance of folic acid in a clinical setting and the potential applications of this technique in a biological environment are highlighted. We demonstrate the potential to transfer this technique to real biological samples by the detecti...
TL;DR: It is shown that native fluorescence can be used to differentiate classes or groups of organic molecules and biological materials when excitation occurs at specific excitation wavelengths in the deep ultraviolet (UV) region.
Abstract: We show that native fluorescence can be used to differentiate classes or groups of organic molecules and biological materials when excitation occurs at specific excitation wavelengths in the deep ultraviolet (UV) region. Native fluorescence excitation-emission maps (EEMs) of pure organic materials, microbiological samples, and environmental background materials were compared using excitation wavelengths between 200-400 nm with emission wavelengths from 270 to 500 nm. These samples included polycyclic aromatic hydrocarbons (PAHs), nitrogen- and sulfur-bearing organic heterocycles, bacterial spores, and bacterial vegetative whole cells (both Gram positive and Gram negative). Each sample was categorized into ten distinct groups based on fluorescence properties. Emission spectra at each of 40 excitation wavelengths were analyzed using principal component analysis (PCA). Optimum excitation wavelengths for differentiating groups were determined using two metrics. We show that deep UV excitation at 235 (+/-2) nm optimally separates all organic and biological groups within our dataset with >90% confidence. For the specific case of separation of bacterial spores from all other samples in the database, excitation at wavelengths less than 250 nm provides maximum separation with >6sigma confidence.
TL;DR: The results indicate that SERS can be a useful tool to study bacterial cell wall molecular differences, and indicate that a higher number of thiol residues and possible S–S bridges are present in the cell wall structure of thermophilic bacteria, providing their stability at elevated temperatures.
Abstract: Surface-enhanced Raman scattering (SERS) can provide molecular-level information about the molecules and molecular structures in the vicinity of nanostructured noble metal surfaces such as gold and silver. The three thermophilic bacteria Bacillus licheniformis, Geobacillus stearothermophilus, and Geobacillus pallidus, a Gram-negative bacterium E. coli, and a Gram-positive bacterium B. megaterium are comparatively characterized using SERS. The SERS spectra of thermophilic bacteria are similar, while they show significant differences compared to E. coli and B. megaterium. The findings indicate that a higher number of thiol residues and possible S-S bridges are present in the cell wall structure of thermophilic bacteria, providing their stability at elevated temperatures. Incubating the thermophilic bacteria with colloidal silver suspension at longer times improved the bacteria-silver nanoparticle interaction kinetics, while increased temperature does not have a pronounced effect on spectral features. A tentative assignment of the SERS bands was attempted for thermophilic bacteria. The results indicate that SERS can be a useful tool to study bacterial cell wall molecular differences.
TL;DR: This work shows how a simple analytical model can predict the surface/substrate signal ratio as a function of distance above the surface, and shows how this effect is not limited to the confocal axial profiling geometry.
Abstract: This paper considers the quantitative implications of out-of-focus regions on the lateral and depth resolution of Raman microscopy, with special regard for the surface specificity of the technique. It builds on work that has recently appeared in the literature which shows that with transparent samples, signals can originate throughout a large extended illumination volume, even though most of this region is out of focus with regard to the confocal aperture. This gives rise to weak but readily detectable spectral contributions from regions that are tens of micrometers from the point of tightest focus, an effect that is easily demonstrated if the laser is focused far above the sample surface. When we integrate the signals arising throughout this extended volume, the resulting total signal can be significant with respect to the Raman signals originating from the point of focus; this has obvious implications for surface specificity and depth resolution. Furthermore, as one moves the focal point through and above a sample surface, signals from thick transparent samples decay relatively slowly compared with thin or opaque ones, where the extended focal volume is irrelevant. This means that on moving above the surface of a thinly coated thick substrate during a confocal axial scan, the substrate-to-coating signal ratio increases dramatically, contrary to intuition. Consequently, confusing spectral artifacts arise if one focuses above the sample surface, either inadvertently when mapping an uneven sample, or deliberately in an attempt to improve surface specificity. In this work we show how a simple analytical model can predict the surface/substrate signal ratio as a function of distance above the surface. The model is validated using experimental data from monofilms and coated films. Furthermore, we show how this effect is not limited to the confocal axial profiling geometry. Similar effects are obtained when one scans laterally beyond the edge of mechanically prepared cross-sections due to an extended, out-of-focus laser field that can sample lateral regions far to the side of the optimum focus. This effect can lead to very confusing results, such as spectra from the substrate increasing in absolute intensity as one moves beyond the edge of the coating into the air. These observations, which as far as we are aware have not previously been reported, are rationalized using a simple ray-tracing description, which shows the potential for coupling light into the cross-section, which acts as a waveguide. These effects have a completely different origin than the well-known anomalies that are introduced by refraction and spherical aberration; even with a perfect, aberration-free system, the extended focal volume may cause significant degradation in depth resolution. Although the effects have been demonstrated with simple film systems, they have the potential to impact the results from Raman mapping and imaging of any samples that contain significant refractive index discontinuities, which can potentially cause refraction and waveguiding, or that have compositional depth gradients and an uneven sample surface.
TL;DR: Laser-induced breakdown spectroscopy has been used to determine the period of time that a shooter will test positive for gunshot residue (GSR) after firing a revolver.
Abstract: Laser-induced breakdown spectroscopy (LIBS) has been used to determine the period of time that a shooter will test positive for gunshot residue (GSR) after firing a revolver. Multiple rounds of primer were fired and samples collected at multiple hour intervals using an adhesive tape pressed against the skin. Samples were analyzed directly using a commercially available laser-induced breakdown spectrometer where barium emission (originating from barium nitrate in the primer) was observed. Population statistics were used to compare suspected GSR to a library of blank samples from which a threshold value was established. Statistically significant results, positive for GSR, are obtained 5.27 days after a firearm discharge using these techniques.
TL;DR: A correction based on the Kubelka–Munk theory is proposed to evaluate the extent of the spectral distortion and is assessed on modern replicas of wall paintings of known composition, and qualitative distinctions between real and apparent emissions are achieved.
Abstract: Ultraviolet-induced fluorescence spectroscopy is a commonly used technique for the characterization and identification of painting materials, such as organic binders and colorants. Its interpretation is strictly connected to both the experimental setup and an understanding of the physical and chemical interactions among materials in paint layers, which are commonly composed of a fluorescent organic binder and a pigment. When irradiated with ultraviolet radiation, the light emitted by fluorophores present in the organic binder undergoes several types of interactions, in particular scattering and absorption by neighboring pigmented particles and auto-absorption. As a result of scattering and absorption phenomena, the emission spectrum is deformed according to the physical properties of the surrounding pigmented particles. This can lead to shifts of the emission maxima and/or to the formation of apparent new emission bands. The extent of the modifications to the emission spectra, caused by auto-absorption and selective absorption phenomena, may lead to the erroneous characterization or identification of the fluorescent materials. As a consequence, the interpretation of the emission signal can be greatly compromised. A correction based on the Kubelka-Munk theory is proposed to evaluate the extent of the spectral distortion and is assessed on modern replicas of wall paintings of known composition. Although the model cannot be applied to all cases, qualitative distinctions between real and apparent emissions are achieved.
TL;DR: The results indicate that hydrated aluminum hydroxide controls the distribution of organic matter in the different grain sizes, and the formation of metal carboxylate bonds indicate the possible interaction between heavy metals and other organic structures, mainly humic substances.
Abstract: The greater Cairo region is the most populated area in Egypt. The aquatic environment of the Nile River in this area is being affected by industrial activities. The study of the molecular structure of sediment may provide a good trace for such changes. Both Fourier transform infrared spectroscopy (FT-IR) and density functional theory (DFT) were used to study the effect of industrial waste disposal south of Cairo on the molecular structure of Nile River sediment. Four seasonal samples were collected from six sites covering 75 km along the Nile River. Grain sizes of 200 microm, 125 microm, 65 microm, and 32 microm, respectively, were examined. The results indicate that hydrated aluminum hydroxide controls the distribution of organic matter in the different grain sizes. Furthermore, the hydration of phenol may take place in grain sizes lower than 200 microm, which is indicated by the OH stretching at 3550 cm(-1) and verified by the obtained model. The formation of metal carboxylate bonds at 1638 cm(-1) (asymmetric) and 1382 cm(-1) (symmetric) indicate the possible interaction between heavy metals and other organic structures, mainly humic substances.
TL;DR: The technique of waveguide THz-TDS has been recently developed, resulting in sharper spectral features associated with the vibrational modes of the molecule, and has been demonstrated with aspirin and its precursors, benzoic acid and salicylic acid, as well as the salicyolic acid isomers 3- and 4-hydroxybenzoic Acid.
Abstract: Low frequency vibrational modes of pharmaceutical molecules are dependent on the molecule as a whole and can be used for identification purposes. However, conventional Fourier transform far-infrared spectroscopy (FT-IR) and terahertz time-domain spectroscopy (THz-TDS) often result in broad, overlapping features that are difficult to distinguish. The technique of waveguide THz-TDS has been recently developed, resulting in sharper spectral features. Waveguide THz-TDS consists of forming an ordered polycrystalline film on a metal plate and incorporating that plate in a parallel-plate waveguide, where the film is probed by THz radiation. The planar order of the film on the metal surface strongly reduces the inhomogeneous broadening, while cooling the waveguide to 77 K reduces the homogeneous broadening. This combination results in sharper absorption lines associated with the vibrational modes of the molecule. Here, this technique has been demonstrated with aspirin and its precursors, benzoic acid and salicylic acid, as well as the salicylic acid isomers 3- and 4-hydroxybenzoic acid. Linewidths as narrow as 20 GHz have been observed, rivaling single crystal measurements.
TL;DR: It was concluded that the aging phenomenon in wood is due to the decrease of adsorption sites on hemicellulose and amorphous cellulose.
Abstract: We investigated the adsorption/desorption mechanism of water and the variation of water adsorption for modern and archaeological wood using near-infrared spectroscopy. A mixture model of water was used to decompose the near-infrared difference spectra into three components (free water molecules (S0), those with one OH group engaged in hydrogen bonding (S1), and those with two OH groups engaged in hydrogen bonding (S2)) based on a principal component analysis. The variations of each water component with relative humidity could be explained by proposing a model that describes water absorption in three stages. It was concluded that the aging phenomenon in wood is due to the decrease of adsorption sites on hemicellulose and amorphous cellulose.
TL;DR: The potential of synchronous front-face fluorescence spectroscopy in the characterization at the molecular level of milk changes during mild heating from 4 to 50 °C and acidification in the pH range of 6.8 to 5.1 is reported.
Abstract: This paper reports the potential of synchronous front-face fluorescence spectroscopy in the characterization at the molecular level of milk changes during mild heating from 4 to 50 °C and acidification in the pH range of 6.8 to 5.1. Synchronous fluorescence spectra were collected in the 250–550 nm excitation wavelength range using offsets of 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, and 240 nm between excitation and emission monochromators. The potential of parallel factor (PARAFAC) analysis in the decomposition of the whole synchronous fluorescence data set into the contribution of each of the fluorescent compounds present in milk has been investigated for heating and acidification data sets. Models were fitted from 1 to 7 components. Considering the core consistency values, PARAFAC models with three components have been considered. The first three components explained 94.43% and 94.13% of the total variance for heating and acidification data sets, respectively. The loading profiles of the first and second components derived from PARAFAC analysis performed on heating and acidification data sets corresponded quite well with the characteristics of tryptophan and vitamin A fluorescence spectra, respectively. The third component corresponded to the riboflavin fluorescence spectrum. Considering the heating experiment, the profile of the concentration mode for the second component showed large variations according to the temperature, which were assigned to the melting of triglycerides between 4 and 50 °C. For the acidification experiment, drastic changes in the concentration modes of the three components were observed for pH below 5.6, in agreement with structural changes in casein micelles.
TL;DR: This work has developed a filter based on the widely used and conceptually simple moving average method or zero-order Savitzky–Golay filter and its iterative relative, the Kolmogorov–Zurbenko filter that requires no parameter specification or parameter optimization.
Abstract: The automated processing of data from high-throughput and real-time collection procedures is becoming a pressing problem. Currently the focus is shifting to automated smoothing techniques where, unlike background subtraction techniques, very few methods exist. We have developed a filter based on the widely used and conceptually simple moving average method or zero-order Savitzky–Golay filter and its iterative relative, the Kolmogorov–Zurbenko filter. A crucial difference, however, between these filters and our implementation is that our fully automated smoothing filter requires no parameter specification or parameter optimization. Results are comparable to, or better than, Savitzky–Golay filters with optimized parameters and superior to the automated iterative median filter. Our approach, because it is based on the highly familiar moving average concept, is intuitive, fast, and straightforward to implement and should therefore be of immediate and considerable practical use in a wide variety of spectroscopy applications.
TL;DR: A theoretical approach is given on how to extract the true VCD spectrum out of superimposed spectra of anisotropic thin solid samples, which leads to the expected result, i.e., a zero line for achiral polymers.
Abstract: Vibrational circular dichroism (VCD) spectra of anisotropic thin solid samples are often superimposed with large contributions of linear birefringence and linear dichroism. In this study a theoretical approach is given on how to extract the true VCD spectrum out of such superimposed spectra. To verify this approach, the VCD spectra of achiral polymer films were examined. The polymers are supposed to give a zero line as VCD spectrum after eliminating the linear contributions. Applying our approach, in which four VCD spectra in different but selected sample orientations are recorded, and calculating their average, leads to the expected result, i.e., a zero line for achiral polymers. The advantage of this method for the elimination of artifacts from solid-state VCD spectra is that no further measurements are required (e.g., linear dichroism measurements or the determination of the orientation with the maximum anisotropy).
TL;DR: In this article, an apparatus and methodology for producing thermal detectors with spectral responsivities that mimic the absorptions of chemical analytes, and whose detector characteristics approach those of conventional broad-band thermal detectors is presented.
Abstract: Disclosed is apparatus and methodology for producing thermal detectors with spectral responsivities that mimic the absorptions of chemical analytes, and whose detector characteristics approach those of conventional broad-band thermal detectors. In an exemplary arrangement, the methodology provides for modification of a known Si-based thermal detector by adding a near-infrared dye absorbing film above a reflector deposited directly on the thermal detector element. The method is general to all types of thermal detectors that can be divided into separate absorber and thermal sensor components.
TL;DR: It is proven that the prototype of a portable instrument, based on the time-correlated single-photon counting method, provides lifetime measurements with good precision and it is shown that fluorescence lifetime determinations allow the distinguishing, on laboratory samples, of different red organic lakes having similar fluorescence spectra, achieving new boundaries in the nondestructive diagnosis of artwork materials.
Abstract: The prototype of a portable instrument, based on the time-correlated single-photon counting method, purposely assembled for in situ measurements of luminescence lifetimes on artwork surfaces, is here presented. Preliminary tests have been carried out using the portable instrument and the results have been compared with those obtained using a bench instrument. In this way we have proven that the prototype provides lifetime measurements with good precision. It is also shown that fluorescence lifetime determinations, coupled with steady-state fluorescence spectra, allow the distinguishing, on laboratory samples, of different red organic lakes having similar fluorescence spectra, achieving new boundaries in the nondestructive diagnosis of artwork materials. The first in situ application of the technique on an original work of art, The Book of Kells, held at Trinity College Library, Dublin, highlighted the diagnostic potential of coupled steady-state and time-resolved luminescence spectroscopy in the identification of organic colorants.
TL;DR: The results obtained from only two measurements with the electric field of the incident radiation parallel and perpendicular to the fiber axis has demonstrated that ATR spectroscopy can be used routinely in quantitative studies of the molecular orientation and conformation of macromolecules.
Abstract: Polarized attenuated total reflection (ATR) infrared spectroscopy is an efficient technique to determine the orientation and conformation of a large variety of samples, but it is more difficult to apply to very small specimens such as silk fibers. The Golden Gate single-reflection ATR accessory that uses diamond as an ATR element and a focalized beam turns out to be highly efficient to study quantitatively the orientation and conformation of a single silk fibroin filament of the silkworm Bombyx mori that is about 10 μm in diameter. For orientation measurements, rotating the sample instead of the electric field greatly simplifies the theoretical analysis and keeps the penetration depth of the infrared radiation constant. A sample holder that can be fitted on the ATR accessory has thus been developed to allow accurate rotation of the sample and to obtain spectra with a low, non-damaging, and reproducible pressure on the fiber. To validate the method, spectra have been recorded as a function of the angle θ between the fiber axis and the polarization of the incident radiation. The data have been fitted following the cosine square dependency of the absorbance with respect to the angle θ. The procedure has been applied to the spectral components of the amide I bands, as determined from spectral decomposition. Multiple angle measurements turn out to be quite useful to correct systematic angle errors and validate the accuracy of the curve-fitting parameters of the band decomposition. By using the calculated dichroic ratio, a parameter 〈P2〉 of −0.46 ± 0.01 has been calculated for the antiparallel β-sheets and −0.04 ± 0.02 for the remaining structures. From the orientation-insensitive spectrum A0, the amount of β-sheets has been estimated to 49 ± 3%. The results obtained from only two measurements with the electric field of the incident radiation parallel and perpendicular to the fiber axis has demonstrated that ATR spectroscopy can be used routinely in quantitative studies of the molecular orientation and conformation of macromolecules.
TL;DR: The first two-dimensional resonance Raman spectra of TNT, RDX, HMX, and PETN are measured with an instrument that sequentially and rapidly switches between laser wavelengths, illuminating these explosives with forty wavelengths between 210 nm and 280 nm.
Abstract: The first two-dimensional (2D) resonance Raman spectra of TNT, RDX, HMX, and PETN are measured with an instrument that sequentially and rapidly switches between laser wavelengths, illuminating these explosives with forty wavelengths between 210 nm and 280 nm. Two-dimensional spectra reflect variations in resonance Raman scatter with illumination wavelength, adding information not available from single or few one-dimensional spectra, thereby increasing the number of variables available for use in identification, which is especially useful in environments with contaminants and interferents. We have recently shown that 2D resonance Raman spectra can identify bacteria. Thus, a single device that identifies the presence of explosives, bacteria, and other chemicals in complex backgrounds may be feasible.
TL;DR: This paper reports the development and field testing of a man-portable instrument based on laser-induced breakdown spectrometry (LIBS) for inspection and analysis of speleothems in Nerja Cave, aimed at evaluating the analytical performance of the instrument when operating in an unfriendly environment.
Abstract: This paper reports the development and field testing of a man-portable instrument based on laser-induced breakdown spectrometry (LIBS) for inspection and analysis of speleothems. The 50 mJ of a Q-switched Nd:YAG laser operating at 1064 nm was used to generate a plasma on the sample. Plasma emission was then guided using a fiber-optic cable to a 1/10 m spectrometer equipped with a charge-coupled device (CCD) array detector. Plasma light was automatically processed in order to obtain surface and in-depth information from the speleothems. A field campaign in the interior of Nerja Cave (a large karstic formation in the South of Spain) has been carried out, aimed at evaluating the analytical performance of the instrument when operating in an unfriendly environment. Identification analysis of the speleothems' alteration layers and depth profiles of Sr and Ca is carried out and reported.
TL;DR: Data obtained indicate both that it is a matrix-independent sampling process and that fs-LIBS can be used for the determination of Ca, Cu, Fe, K, Mg, Na, and P, but efforts must be made to obtain more appropriate detection limits for Al, Sr, and Zn.
Abstract: The aim of this work was to evaluate the performance of femtosecond laser-induced breakdown spectroscopy (fs-LIBS) for the determination of elements in animal tissues Sample pellets were prepared from certified reference materials, such as liver, kidney, muscle, hepatopancreas, and oyster, after cryogenic grinding assisted homogenization Individual samples were placed in a two-axis computer-controlled translation stage that moved in the plane orthogonal to a beam originating from a Ti:Sapphire chirped-pulse amplification (CPA) laser system operating at 800 nm and producing a train of 840 μJ and 40 fs pulses at 90 Hz The plasma emission was coupled into the optical fiber of a high-resolution intensified charge-coupled device (ICCD)–echelle spectrometer Time-resolved characteristics of the laser-produced plasmas showed that the best results were obtained with delay times between 80 and 120 ns Data obtained indicate both that it is a matrix-independent sampling process and that fs-LIBS can be used for the determination of Ca, Cu, Fe, K, Mg, Na, and P, but efforts must be made to obtain more appropriate detection limits for Al, Sr, and Zn
TL;DR: Pulsed terahertz attenuated total reflection (ATR) spectra of solid materials and liquids covering the 10cm−1 to 120 cm−1 region of the electromagnetic spectrum are recorded using a teraHertz pulsed spectrometer and silicon ATR modules.
Abstract: Pulsed terahertz attenuated total reflection (ATR) spectra of solid materials and liquids covering the 10 cm−1 to 120 cm−1 (0.3 THz to 3.6 THz) region of the electromagnetic spectrum are recorded using a terahertz pulsed spectrometer and silicon ATR modules. Pulsed terahertz ATR measurements are completed nondestructively using small amounts of sample (typically 1 mg for solids) and no sample preparation. Many terahertz analyses can be run in rapid sequence, minimizing the analysis time.