Applied Spectroscopy

Spectroscopists' Calendar is a regular feature in Applied Spectroscopy. Your cooperation in submitting timely information for the column is appreciated. For publication in one issue of Applied Spectroscopy, send information no less than four months in advance of the actual event. Please send announcements of meetings and symposia of interest to spectroscopists to Mary Carrabba, Department of Chemistry, Southern Oregon University, 1250 Siskiyou Blvd., Ashland, Oregon 97520, e-mail: carrabbam@sou.edu, Ph: (541)261-9800. Announcements of short courses, schools, workshops or other educational activities should be sent to the SAS webmaster, Ron Williams (ron.williams@maconstate.edu).

WHAT'S NEW is provided as a service for our readers. It contains the latest news on the products, catalogs, tips, and supplies that manufacturers elect to highlight. Publication in WHAT'S NEW does not imply recommendation or endorsement by the Society for Applied Spectroscopy or the column editor. Contributions to WHAT'S NEW should be sent to Applied Spectroscopy, What's New Editor, 5320 Spectrum Drive, Suite C, Frederick, MD 21703 (Fax 301-694-6860; e-mail office@s-a-s.org). Although there is no charge to submit materials, preference is given to Applied Spectroscopy advertisers. Call 800-627-0932 for questions regarding advertising.

Applied Spectroscopy News is a monthly feature in the journal. It includes information from the Society for Applied Spectroscopy, news from other societies and institutions, announcements of meetings, schools, or other activities, and reports of symposia from recent conferences. If you have news items, a meeting announcement, or a report from a symposium that would be in interest to readers of Applied Spectroscopy, contact David J. Butcher, Applied Spectroscopy News Editor, 340C Stillwell Science Building, 246 Central Drive, Western Carolina University, Cullowhee, NC 28723, Ph: (828) 227-7646, email: david.butcher@analytchem.org.

Infrared (IR) spectroscopic imaging seemingly matured as a technology in the mid-2000s, with commercially successful instrumentation and reports in numerous applications. Recent developments, however, have transformed our understanding of the recorded data, provided capability for new instrumentation, and greatly enhanced the ability to extract more useful information in less time. These developments are summarized here in three broad areas—data recording, interpretation of recorded data, and information extraction—and their critical review is employed to project emerging trends. Overall, the convergence of selected components from hardware, theory, algorithms, and applications is one trend. Instead of similar, general-purpose instrumentation, another trend is likely to be diverse and application-targeted designs of instrumentation driven by emerging component technologies. The recent renaissance in both fundamental science and instrumentation will likely spur investigations at the confluence of conventional spectroscopic analyses and optical physics for improved data interpretation. While chemometrics has dominated data processing, a trend will likely lie in the development of signal processing algorithms to optimally extract spectral and spatial information prior to conventional chemometric analyses. Finally, the sum of these recent advances is likely to provide unprecedented capability in measurement and scientific in-sight, which will present new opportunities for the applied spectroscopist.

We studied pathological bioapatite from patients undergoing valvular replacement due to severe aortic and mitral stenosis. Three different types of mineralized human cardiac valves were analyzed. We used infrared and Raman spectroscopy to infer the presence of the carbonate group and evaluate the carbonate substitution in bioapatite structure. The Raman spectra showed that the pathological bioapatite is a B-type “carbonateapatite” (CO₃^2- for PO₄^3-) similar to the major mineralized products derived from normal biomineralization processes occurring in the human body. Fourier transform infrared spectra (FT-IR) confirmed the B-type carbonate substitution (CO₃^2- for PO₄^3-) and showed evidence for the partial replacement of [OH] by [CO₃] (A-type substitution). The carbonate content of the samples inferred by the spectroscopic measurements is in good agreement with the range of values estimated for biological apatite. On the contrary, the crystal size of the pathological apatite estimated using the percentage area of the component at 1059 cm⁻¹ of the infrared spectrum is in the nanometer range and it is significantly smaller than the crystal size of normal mineralized tissues.

Among the variety of available hyperspectral imaging systems, the line-scan technique stands out for its short acquisition time and good signal-to-noise ratio. However, due to imperfections in the camera lens and, in particular, optical components of the imaging spectrograph, the acquired images are spatially and spectrally distorted, which can significantly degrade the accuracy of the subsequent hyperspectral image analysis. In this work, we propose and evaluate an automated method for correction of spatial and spectral distortions introduced by a line-scan hyperspectral imaging system operating in the short wavelength infrared (SWIR) spectral range from 1000 nm to 2500 nm. The proposed method is based on non-rigid registration of the distorted and reference images corresponding to two passive calibration objects. The results of the validation show that the proposed method is accurate, efficient, and applicable for calibration of line-scan hyperspectral imaging systems. Moreover, the design of the method and of the calibration objects allows integration with systems operating in diffuse reflectance or transmittance modes.

Zinc oxide is a prevalent industrial-age pigment that readily reacts with fatty acids in oil-based paints to form zinc carboxylates. Zinc stearate aggregates are associated with deterioration in late nineteenth and twentieth century paintings. The current study uses both conventional and synchrotron Fourier transform infrared spectroscopy (FT-IR) to investigate metal carboxylate composition in a range of naturally aged artists' oil paints and reference paint film draw-downs. The paints contain zinc oxide alone or in combination with lead white, titanium white, and aluminum stearate and are prepared with linseed and safflower oils. Attenuated total reflectance (ATR)-FT-IR using the conventional source identifies marked differences in carboxylate profiles between exposed and protected surfaces in a large number of samples. Synchrotron FT-IR microspectroscopy of thin paint cross-sections maps metal carboxylate distributions at high spatial resolution and resolves broad concentration gradients and micrometer-scale phase separation of carboxylate species. Aluminum stearate, a common paint additive, is found to influence the distribution of zinc carboxylates more strongly than pigment composition or oil type. The presence of aluminum stearate results in higher concentrations and more pronounced separation of saturated C16 and C18 chain zinc carboxylates in the margin of paint nearest the polyester substrate. The presence of aluminum stearate in association with zinc oxide has a clear influence on zinc carboxylate formation and distribution, with potential implications for long term stability of vulnerable paintings.

A library of poly(2-oxazoline)s with varying length of the alkyl side-chain has been investigated by variable-temperature Fourier transform infrared (FT-IR) spectroscopy. These polymers are suitable for studies of structure-property relationships as their cationic ring-opening polymerization and the relatively facile monomer synthesis enable a control of the molecular structure. In this contribution, the number of carbon atoms in the linear side-chain is systematically varied from a short methyl to a long nonyl group. Previous studies showed that the sample library can be split in two groups: poly(2-oxazoline)s with a short side-chain (methyl-, ethyl-, and isopropyl-) exhibit hygroscopic behavior, while those with longer side-groups (butyl- and longer) were found to be semi-crystalline. To gain further insight into the mechanisms of hydrogen bonding and crystallization, temperature-dependent infrared (IR) spectroscopy has been applied in the current study. The processes involved have been monitored by generalized two-dimensional correlation spectroscopy (2DCOS) and perturbation-correlation moving-window two-dimensional correlation spectroscopy (PCMW2D) in the C=O stretching region around 1645 cm⁻¹. These advanced analysis techniques provided valuable additional information on the material behavior during heating. As water is removed from the samples in the course of the heating process, it was possible to clearly distinguish between “loosely associated” and hydrogen-bonded water. Furthermore, the melting process of the semi-crystalline samples could be depicted. For the poly(2-isopropyl-2-oxazoline) even a crystallization process could be monitored in the temperature range between glass transition and melting.

This paper demonstrates the high potential of a web camera to be utilized as a low-cost multichannel fiber-optic spectrometer suitable for either educational or quality-control purposes in small and medium enterprises. The key idea is to arrange N input optical fibers in a line and position an external dispersive element to separate incoming optical beams into their associated spectral components in a two-dimensional (2D) space. With a commercial web camera, each set of the spectral components is imaged through a plastic lens onto the 2D image sensor of the web camera. For the demonstration, a five-channel webcam-based fiber-optic spectrometer is implemented where each channel is calibrated by selected reference light sources. The constructed spectrometer can perform wavelength analysis of the spectral irradiance in the range of 400 to 655 nm. Experimental results also show that peak operating wavelengths of five light-emitting diodes and a laser pointer can be determined with a wavelength measurement error of less than 10.5 nm. The total cost of the webcam-based five-channel fiber-optic spectrometer is only approximately US$92.50 and effectively performs to the desired results.

A laser spectroscopic technique is described that combines transmission and resonance-enhanced Raman inelastic scattering together with low laser power (< 30 mW) and good spatial resolution (< 200 μm) as operational features. The monitoring of the transmitted inelastic scattering provides an increased signal-to-noise ratio because the low fluorescence background and, on the other hand, the resonant character of the laser excitation, leads to enhanced analytical sensitivity. The spectroscopic technique was applied to investigate the carotenoid content (specifically the β-carotene concentration) of distinct samples that included fruits, reaching a detection limit of the order of hundreds of picograms in solid samples, which is below the level needed for typical food control analysis. Additional features of the present development are direct sampling, noninvasive character, and fast analysis that is not time consuming. From a theoretical point of view, a model for the Raman signal dependence on the sample thickness is also presented. Essentially, the model considers the sample to be homogeneous and describes the underlying physics using only three parameters: the Raman cross-section, the laser-radiation attenuation cross-section, and the Raman signal attenuation cross-section. The model was applied successfully to describe the sample-size dependence of the Raman signal in both β-carotene standards and carrot roots. The present technique could be useful for direct, fast, and nondestructive investigations in food quality control and analytical or physiological studies of animal and human tissues.

A high-throughput screening system has been developed to rapidly produce, screen, and assess the usefulness of organically modified silane (ORMOSIL)-based xerogel films formed on the surface of porous silicon (pSi) surfaces. The ORMOSILs tested include methyltriethoxysilane, n-octyltriethoxysilane, n-hexyltriethoxysilane, n-propyltriethoxysilane, 2-cyanoethyltriethoxysilane, phenyltriethoxysilane, benzyltriethoxysilane, vinyltriethoxysilane, tetraethoxysilane, and hexafluoroethyltriethoxysilane. Xerogel microarrays were pin-printed on the surface of O₃ oxidized pSi using a computer-controlled robotic pin-printer. The fragile pSi required careful pin-printing parameter optimization to simultaneously ensure sufficient sol application and limit pin-induced damage. These multi-functional xerogel-pSi microarrays were exposed to harsh conditions (0.1 mM NaOH, 15 min) to determine the extent to which the xerogel protected the pSi. Microarray assessment included multispectral photoluminescence and infrared imaging. Results demonstrate that the more hydrophobic/nonpolar xerogel films (n-octyltriethoxysilane, n-hexyltriethoxysilane) protect the pSi surface the most and maintained the pSi photoluminescence. Also, unlike xerogel material doped with a reporter molecule, the uniformity of the printed feature plays a role in the protection of the pSi material underneath. Areas with thinner xerogel distributions allowed the permeation of NaOH whereas the thicker areas prohibit pSi exposure to NaOH.

Raman spectroscopy is one of the major characterization methods employed over the last few decades as a nondestructive technique for the study of heterogeneous catalysts and related catalytic reactions. However, the promise of practical applicability on millimeter-sized catalyst bodies, such as extrudates, has not been fulfilled completely. Large fluorescence signals and the highly scattering nature of the extrudates often hamper its practical usage. Different approaches to overcome this problem were examined, including the use of time-resolved Raman spectroscopy (TRRS), spatially offset Raman spectroscopy (SORS), surface-enhanced Raman spectroscopy (SERS), and combinations of these techniques. This paper demonstrates that especially TRRS can provide chemical information at depth within catalyst bodies, overcoming fluorescence background signals and allowing for visualization of analytes at different depths. It also examines the application of time-resolved SERS within catalyst bodies to gain insight into localized activity. With these options a wider applicability of Raman spectroscopy for industrial catalysis research becomes within reach.

Multi-wavelength transmission (MWT) ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy, a technique underappreciated for particle characterization, is systematically explored using a set of NIST traceable standards over the nominal size range of 20 to 20 000 nm. Experimental results demonstrate that the particle size distributions obtained from MWT spectral data are in excellent agreement with the values reported by the manufacturer. In addition, it is shown that quantitative information on the particle concentration can be obtained—which is not currently accessible from commercially available light scattering instrumentation. The results validate that MWT UV-Vis-NIR spectroscopy has a considerable dynamic range for particle size measurements and offers significant advantages over other particle characterization techniques. Among these are the simplicity of the instrumentation and the measurements and the wealth of quantitative information contained in the MWT spectra. Most importantly, with standardized measurement protocols and standardized spectrometer configurations, MWT measurements can be used to provide the user and the manufacturer of particles with traceable data (i.e., the spectra and the quantitative analysis) for quality assurance.

A method for selective sampling and analysis of explosive residues on solid surfaces based on laser-induced breakdown spectroscopy (LIBS) is presented. Organic explosives are difficult to analyze when present as residues on organic materials. Under these circumstances LIBS suffers from the limitations imposed by the limited spectroscopic information available for the analysis. Since ablation and subsequent plasma formation are sensitive to the beam focal conditions and the pulse energy deposited on the surface, the choice of an appropriate set of experimental conditions increases the surface sensitivity of the analysis and hence a selective inspection of the residue in the absence of spectral contribution from the organic support analyzed. 2-Mononitrotoluene (MNT), 2,6- dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT) are used as model residues, whereas nylon and Teflon are used as illustrative surfaces of daily life objects. The results demonstrate that selective sampling is successfully achieved in all cases when the plasma formation threshold of the residues and the object is substantially different. Plasma imaging demonstrates that the species distribution along the plume changes with beam focal conditions, which is exploited here to further increase the selectivity of the approach.

Thermal barrier coatings (TBC) are used widely on a range of components that operate at high temperatures. We report measurement of the factor that is required to convert the Raman shift to stress for air plasma sprayed yttria (7 wt %) stabilized tetragonal zirconia (ZrO₂) (YSZ) thermal barrier coatings. The factor is evaluated for the as-coated condition and also following a heat treatment at 1000 °C for 1050 h. Two Raman bands at 608 cm⁻¹ and 640 cm⁻¹ have been investigated in a diamond anvil cell under hydrostatic pressure up to ~24 GPa. In the range of zero to ~1.6 GPa, a linear behavior was observed in terms of the shifts of these two Raman bands with a gradient similar to dense bulk tetragonal ZrO₂. From these measurements the factors to convert wavenumber shift to stress have been derived. The application of these conversion factors to stress measurement in TBC coated test specimens and components is discussed.

A multiplexed diode-laser sensor system based on second harmonic detection of wavelength modulation spectroscopy (WMS) is developed for application at elevated temperatures with two near-infrared diode lasers multiplexed using a frequency-division multiplexing scheme. One laser is tuned over a H₂O line pair near 7079.176 and 7079.855 cm⁻¹, and another laser is tuned over a pair of CO₂ and CO lines near 6361.250 and 6361.344 cm⁻¹. Temperature and concentrations of H₂O, CO₂, and CO could be measured simultaneously by this system. In order to remove the need for calibration and correct for transmission variation due to beam steering, mechanical misalignments, soot, and windows fouling, the WMS-1f normalized 2f method is used. Demonstration experiments are conducted in a heated static cell. The precision of temperature and the concentrations for H₂O, CO₂, and CO are found to be 1.57%, 3.87%, 3.01%, and 3.58%, respectively. These results illustrate the potential of this sensor for applications at high temperatures.

In vivo fluorescence lifetimes of chlorophyll-a (chl-a) and nicotinamide adenine dinucleotide phosphate (NADPH) were obtained from the green microalgae Haematococcus pluvialis under normal and nutrient-stressed conditions (green stage and red stage, respectively), using two-photon excitation provided by a laser generating pulses in the femtosecond range, and a Leica microscope setup. Analysis of the fluorescence lifetime decay curve revealed two separate lifetime components in all our measurements. A short-lifetime component for chl-a of ~250 ps was completely dominant, contributing more than 90% of overall intensity in both green-stage and red-stage cells. Green-stage cells inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea (DCMU) displayed a significant chl-a lifetime increase for the short component. However, this was not the case for red-stage cells, in which DCMU inhibition did not significantly affect the lifetime. For green-stage cells, we found a short NADPH (free) lifetime component at ~150 ps to be completely dominating, but for red-stage cells, a longer component (protein bound) at ~3 ns contributed as much as 35% of the total intensity. We hypothesize that the long lifetime component of NADPH is connected to photoprotection in the cells and coupled to production of astaxanthin. DCMU does not seem to affect the fluorescence lifetimes of NADPH.

With the increasing use of microfluidics, there is a need for a rather general experimental approach in order to monitor and characterize transport effects. Indeed, micro-fabrication methods have allowed the inclusion of numerous new structures and devices within microfluidics channels, and such alterations in flow patterns should impact solute transport characteristics. In the present contribution, Raman microscopy is combined with band-target entropy minimization analysis (BTEM) in order to rapidly assess and map concentration profiles in various regions of a microfluidics device. Two isotopomers, CHCl₃ and CDCl₃, are contacted under laminar conditions. Special consideration is given to the point of contact between the two liquids, transport in straight sections, transport in curved sections, and wall effects. Break-through curves confirmed that stagnation of fluid at the wall is not occurring, despite substantial wall roughness. Since the methods used in the present study are quite general, they should be useful in rapidly accessing transport effects when fluids (also in conjunction with colloids, suspensions, and solids) are contacted in the presence of both simple as well as complex geometries.

Zinc oxide has been widely used as a white artist pigment since the end of the eighteenth century. The luminescence properties of this compound have received great interest during the last decades for promising applications in different fields of material science, but their diagnostic implications in the cultural-heritage context have been poorly exploited. This paper is intended to provide a clear picture of the luminescence behavior of zinc white in oil paintings. With this aim, three white pigments and three highly pure (analytical grade) zinc oxides were studied as powder substrates and as painting models by ultraviolet–visible (UV–VIS) fluorescence and Fourier transform infrared (FT-IR) spectroscopy. The quenching of the luminescence intensity of the UV excitonic emission due to self-absorption and multiple scattering phenomena has been investigated, pointing out the possible difficulty of detecting this signal with negative consequences in the diagnostics of works of art. By contrast, the UV emission is notably enhanced by interaction with the binder, whereas the visible emission decreases. This phenomenon is probably due to the formation of covalent bonds between zinc atoms and carboxylates from the lipidic medium that are chemisorbed on zinc oxide surfaces.

We present calculations of absorption spectra arising from molecular vibrations at THz frequencies for molecular clusters of the explosive HMX using density functional theory (DFT). The features of these spectra can be shown to follow from the coupling of vibrational modes. In particular, the coupling among ground-state vibrational modes provides a reasonable molecular-level interpretation of spectral features associated with the vibrational modes of molecular clusters. THz excitation from the ground state is associated with frequencies that characteristically perturb molecular electronic states, in contrast to frequencies, which are usually substantially above the mid-infrared (mid-IR) range, that can induce appreciable electronic-state transition. Owing to this characteristic of THz excitation, one is able to make a direct association between local oscillations about ground-state minima of molecules, either isolated or comprising a cluster, and THz absorption spectra. The DFT software program GAUSSIAN was used for the calculations of the absorption spectra presented here.