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At least 1,333 records · Page 74Linked to original sources
PubMed2005

Determination of molecular orientation of uniaxially stretched polyamide fibers by polarized infrared spectroscopy: comparison of X-ray diffraction and birefringence methods.

Polarized infrared (IR) spectroscopy has been used to determine the crystalline and amorphous orientation of polyamide fibers. The transition moment angle of the band at 936 cm-1 of PA66 was determined to be 48 degrees using IR spectroscopy and birefringence measurement. The crystalline orientation of PA66 fibers was estimated from the band at 936 cm-1 while the amorphous orientation of PA66 fibers was obtained by an indirect method. The alpha crystalline orientation of PA6 has been obtained using the band at 930 cm-1 and the amorphous orientation of PA6 has been determined using the band at 1124 cm-1. Crystalline orientation increased rapidly at low draw ratios (DR<3) and increased slowly at higher draw ratios (DR>3) for both PA66 and PA6 fibers, while the amorphous orientation increased slowly throughout the whole extension range for PA66 fibers. A good correlation was found between the crystalline orientation values obtained by infrared spectroscopy and other methods such as X-ray diffraction for PA66 and PA6 fibers. On the basis of this observation, it has been concluded that polarized infrared spectroscopy can be used reliably to measure the orientation of polyamide fibers without combining with other techniques.

Birefringence↗
PubMed2005

Influences of monosaccharides and its glycosidic linkage on infrared spectral characteristics of disaccharides in aqueous solutions.

The infrared spectral characteristics of ten different types of disaccharides (trehalose, kojibiose, nigerose, maltose, isomaltose, trehalulose, sucrose, turanose, maltulose, and palatinose) and five different types of monosaccharides (glucose, mannose, galactose, talose, and fructose) in aqueous solutions (H2O and D2O) were determined. The infrared spectra were collected using the Fourier transform infrared attenuated total reflectance (FT-IR/ATR) method and comparisons between the degrees of absorption band-shift of the saccharide spectra in the H2O solution with those in the D2O solution with respect to the saccharide concentrations were done. The study revealed that the wavenumber shifts in the bands of mono- and disaccharides in the H2O and D2O solutions could be used as an indicator of the level of interaction between the saccharides and water. The study also focused on the glycosidic linkage position and the constituent monosaccharides and found that they have a significant influence on the infrared spectroscopic characterization of disaccharides in an aqueous solution.

Complex Mixtures↗
PubMed2005

Scatter correction of transmission near-infrared spectra by photon migration data: quantitative analysis of solids.

The scope of this work is a new methodology to correct conventional near-infrared (NIR) data for scattering effects. The technique aims at measuring the absorption coefficient of the samples rather than the total attenuation measured in conventional NIR spectroscopy. The main advantage of this is that the absorption coefficient is independent of the path length of the light inside the sample and therefore independent of the scattering effects. The method is based on time-resolved spectroscopy and modeling of light transport by diffusion theory. This provides an independent measure of the scattering properties of the samples and therefore of the path length of light. This yields a clear advantage over other preprocessing techniques, where scattering effects are estimated and corrected for by using the shape of the measured spectrum only. Partial least squares (PLS) calibration models show that, by using the proposed evaluation scheme, the predictive ability is improved by 50% as compared to a model based on conventional NIR data alone. The method also makes it possible to predict the concentration of active substance in samples with other physical properties than the samples included in the calibration model.

Algorithms↗
PubMed2006

Improved monochromatic imaging spectrometer.

Distortion inherent to a previously described system for acquisition of two-dimensional monochromatic spatial images is described. A solution is offered in the form of an improved instrument. The system uses a Czerny-Turner monochromator for spectral discrimination and a charge-coupled device (CCD) as the detector. A second Czerny-Turner monochromator, with identical dimensions, is added to correct the distortion, albeit with a slight loss in spatial resolution. With the earlier uncorrected arrangement, spatial resolution was on the order of 0.1 mm vertically and 0.3 mm horizontally, with a magnification of 0.52. With the same magnification, the new, corrected system offers spatial resolution of 0.1 mm vertically and 0.4 mm horizontally.

Color↗
PubMed2006

Subsurface Raman spectroscopy and mapping using a globally illuminated non-confocal fiber-optic array probe in the presence of Raman photon migration.

We report the use of a fiber-optic probe with global illumination and an array of 50 collection fibers (PhAT probe, Kaiser Optical Systems, Inc.) to obtain Raman spectra and 50 spatial element maps of polymers through overlayers of other polymers that are highly scattering. Band target entropy minimization (BTEM) is used to recover the spectra of the subsurface components and generate maps of their distributions. This approach to subsurface mapping is tested with model systems consisting of two or three layers of polyethylene, polytetrafluoroethylene (Teflon), and polyoxymethylene (Delrin) arranged in different geometries. Raman spectra and maps were obtained through overlayer thicknesses of up to 13 mm. Subsurface spatial resolution is achieved because each fiber views an asymmetric distribution of Raman scattered light from surface and subsurface components that depends on the position of the fiber relative to the depth and position of a component and the extent of photon diffusion through the system.

Equipment Design↗
PubMed2006

Novel, fourier filtering method that reuses interference-patterned spectra to extend the calibration set for thickness determination.

Determining the thickness of plastic sheets on the basis of near-infrared spectra by building a multivariate calibration model requires a relatively large sample set. In the thickness region, where just a few non-interference-patterned samples are available, it is a waste of information if interference-patterned spectra are excluded. After eliminating the interference pattern from the spectra (filtering), the calibration set can be extended with these filtered spectra. Fourier transformation of an interference-patterned spectrum versus wavenumber leads to a Fourier spectrum as a function of the optical path length containing an easily recognizable interference peak. Unfortunately, this peak coincides with components of the spectral information of absorbance, on which multivariate calibration is based. Hence, replacing the interference peak is a cardinal step of the filtering process. Since the Fourier spectrum versus optical path length function is not known, it has been shown that interpolated data over the remaining Fourier components can be substituted for the missing part of the spectrum. In this paper, a novel method is proposed that uses a linear approximation between the Fourier spectra and the thickness values so that the regression coefficients are calculated on components of all but the interference-patterned Fourier spectra and the corresponding thicknesses, and then the deleted components in the filtered spectrum are replaced. This latter method yields more detailed Fourier spectra. Reducing the disturbing effect of scattering is also discussed. The effectiveness of the filtering was tested on low-density polyethylene sheets. The performance of different calibration models with or without filtering was compared by significance tests on standard error of prediction values. Application of the new Fourier type filtering technique led to significant improvements in the calibration performance.

Algorithms↗
PubMed2006

Spectrometric determination of the refractive index of optical wave guiding materials used in lab-on-a-chip applications.

The design and optimization of light-based analytical devices often require optical characterization of materials involved in their construction. With the aim of benefiting lab-on-a-chip applications, a transmission spectrometric method for determining refractive indices, n, of transparent solids is presented here. Angular dependence of the reflection coefficient between material-air interfaces constitutes the basis of the procedure. Firstly, the method is studied via simulation, using a theoretical algorithm that describes the light propagation through the sample slide, to assess the potentially attainable accuracy. Simulations also serve to specify the angles at which measurements should be taken. Secondly, a visible light source and an optical fiber spectrometer are used to perform measurements on three commonly used materials in optical lab-on-a-chip devices. A nonlinear regression subroutine fits experimental data to the proposed theoretical model and is used to obtain n. Because the attainable precision using this method of refractive index determination is dictated by the uncertainty in the transmission measurements, the precision (with 95% confidence) for mechanically rigid samples, namely glass and poly(methyl methacrylate) (PMMA), is higher than those estimated for the elastomer sample (in-house-molded poly(dimethylsiloxane) (PDMS)). At wavelengths with the highest signal-to-noise ratio for the spectrometer setup, the estimated refractive indices were 1.43+/-0.05 (580 nm) for PDMS, 1.54+/-0.02 (546 nm) for glass, and 1.485+/-0.005 (656 nm) for PMMA. Accurate refractive index estimations with an average precision equal to 0.01 refractive index units (RIU) were obtained for PMMA and glass samples, and an average precision of 0.09 RIU for the PDMS molded slide between 550 and 750 nm was obtained.

Combinatorial Chemistry Techniques↗
PubMed2006

Shining new light on old principles: localization of evanescent field interactions at infrared-attenuated total reflection sensing interfaces.

A combined experimental and spectral ray tracing approach for identifying and evaluating evanescent field interactions with discrete surface deposits along a horizontal attenuated total reflection (HATR) element is presented. By experimentally depositing poly(styrene-co-butadiene) (PSCB) residues at fixed intervals along the measurement surface of a HATR crystal, distinct regions of evanescent field interaction with the surface deposits along the multi-reflection waveguide are visualized via infrared absorption features of PSCB. The infrared-attenuated total reflection (IR-ATR) measurements were confirmed by spectral ray tracing analysis simulating transmission-absorption spectra after modeling the polymeric surface deposits as thin-film IR absorbing cylinders. The presented analytical procedures and simulations provide a generic strategy for identifying and evaluating "active" sensing regions along ATR elements. Additionally, the simulated ATR setup along with the presented spectral ray tracing procedures provide a virtual platform aiding the development, optimization, and integration of deep-sea IR-ATR sensor probes with submersible mid-infrared spectrometers for in situ marine monitoring applications, which was the initial motivation for these studies.

Algorithms↗
PubMed2006

Transmission infrared spectroscopy as a probe of Nafion film structure: analysis of spectral regions fundamental to understanding hydration effects.

Transmission infrared spectroscopy was applied to investigate properties of the perfluorosulfonated polymer Nafion. Measurements were made on thin films formed by casting the polymer from solution onto ZnSe windows. Effects of water vapor permeation were studied. A complex band structure between 1350 and 1100 cm(-1) was analyzed qualitatively by fitting the region to Gaussian functions. Features associated with vibrational modes of -CF(2) and -SO(3)(-) groups were identified and observed to be sensitive to film hydration. The intensities of bands for the -SO(3)(-) modes increased with film hydration, while bands assignable to -CF(2) modes decreased. The results were applied to interpret infrared difference spectra of Nafion and shed light on the complicated features that appear. Vibrational bands for water were also examined. In partially hydrated films, the stretching mode of the free -OH group for interfacial water present in pores and channels of the polymer and bands for hydrated proton clusters were detected.

Fluorocarbon Polymers↗
PubMed2006

Coded aperture Raman spectroscopy for quantitative measurements of ethanol in a tissue phantom.

Coded aperture spectroscopy allows for sources of large étendue to be efficiently coupled into dispersive spectrometers by replacing the traditional input slit with a patterned mask. We describe a coded aperture spectrometer optimized for Raman spectroscopy of diffuse sources, (e.g., tissue). We provide design details of the Raman system, along with quantitative estimation results for ethanol at non-toxic levels in a lipid tissue phantom. With 60 mW of excitation power at 808 nm, leave-one-out and blind cross-validation of partial least squares (PLS) regression models achieve r(2) > 0.98. Leave-one-out cross-validation demonstrates prediction errors of <15% at the common legal limit for intoxication (17.4 mmol/L = 0.08% by vol) and the best blind cross-validation achieves <12% error at this concentration.

Algorithms↗
PubMed2006

Water evaporation analysis of L-phenylalanine from initial aqueous solutions to powder state by vibrational spectroscopy.

The water evaporation from L-phenylalanine (L-phe) aqueous solutions at different initial pH (0-13) was studied by vibrational spectroscopy. Next, the attenuated total reflection-Fourier transform infrared (ATR-FT-IR) spectra of aqueous solutions were compared to those recorded after drying for 72 h at 21 degrees C at appropriate initial pH values. Micro-Raman results collected after the water evaporation process are also presented and interpreted. Between pH = 2.5 and 8.76 a white non-transparent gel was observed, possibly due to the presence of the NaCl salt. The significant differences of the band intensities of L-phe functional groups noticed at pH near pK(a) values indicate the structural changes of L-phe molecules due to dimer formation (hydrogen bonds between the -COOH and -CO(2)(-) groups, and the -NH(3)(+) and -NH(2) groups). The presence of the hydrophobic interactions leads to the aggregation of L-phe molecules, most probably via phe-phe stacking as well as complexes of phe with Na(+) ions, HCl, or H(2)O molecules.

Hydrogen Bonding↗
PubMed2006

Classification of atherosclerotic rabbit aorta samples with an infrared attenuated total reflection catheter and multivariate data analysis.

The strongly overlapping infrared absorption features of atherosclerotic and normal rabbit aorta samples as governed by their water, lipid, and protein content render the direct evaluation of molecular characteristics obtained from infrared (IR) spectroscopic measurements challenging for classification. We have successfully applied multivariate data analysis and classification techniques based on partial least squares regression (PLS), linear discriminant analysis (LDA), and principal component regression (PCR) to IR spectroscopic data obtained by using a recently developed infrared attenuated total reflectance (IR-ATR) catheter prototype for future in vivo diagnostic applications. Training data were collected ex vivo from atherosclerotic and normal rabbit aorta samples. The successful classification results on atherosclerotic and normal aorta samples utilizing the developed data evaluation routines reveals the potential of spectroscopy combined with multivariate classification strategies for the identification of normal and atherosclerotic aorta tissue for in vitro and, in the future, in vivo applications.

Algorithms↗
PubMed1979

Falsely high refractometric readings for the specific gravity of pleural fluid.

For 128 pleural fluids, the relationship between the protein content and the reading for the specific gravity obtained from a refractometer calibrated for urinary specific gravity was analyzed. The refractometer gave falsely high levels for the specific gravity of the pleural fluid. A reading for 1.019 (rather than 1.016) corresponded to a concentration of protein of 3.0 gm/100 ml, and each deviation of 0.005 (rather than 0.003) coresponded to a concentration of 1 gm/100 ml; however, determinations of the concentration of protein in the pleural fluid directly from the refractometer's scale for protein (calibrated for serum) was rapid and accurate. Calculation of the protein content of the pleural fluid from the reading for specific gravity on the refractometer is erroneous and sometimes misleading.

False Positive Reactions↗
PubMed2004

The density and refractive index of adsorbing protein layers.

The structure of the adsorbing layers of native and denatured proteins (fibrinogen, gamma-immunoglobulin, albumin, and lysozyme) was studied on hydrophilic TiO(2) and hydrophobic Teflon-AF surfaces using the quartz crystal microbalance with dissipation and optical waveguide lightmode spectroscopy techniques. The density and the refractive index of the adsorbing protein layers could be determined from the complementary information provided by the two in situ instruments. The observed density and refractive index changes during the protein-adsorption process indicated the presence of conformational changes (e.g., partial unfolding) in general, especially upon contact with the hydrophobic surface. The structure of the formed layers was found to depend on the size of the proteins and on the experimental conditions. On the TiO(2) surface smaller proteins formed a denser layer than larger ones and the layer of unfolded proteins was less dense than that adsorbed from the native conformation. The hydrophobic surface induced denaturation and resulted in the formation of thin compact protein films of albumin and lysozyme. A linear correlation was found between the quartz crystal microbalance measured dissipation factor and the total water content of the layer, suggesting the existence of a dissipative process that is related to the solvent molecules present inside the adsorbed protein layer. Our measurements indicated that water and solvent molecules not only influence the 3D structure of proteins in solution but also play a crucial role in their adsorption onto surfaces.

Adsorption↗
PubMed2004

Imaging cells and tissues with refractive index radiology.

Can individual cells, including live cells, be imaged using hard x rays? Common wisdom until now required sophisticated staining techniques for this task. We show instead that individual cells and cell details can be detected in culture solution and tissues with no staining and no other contrast-enhancing preparation. The sample examined can be much thicker than for many other microscopy techniques without sacrificing the capability to resolve cells. The key factor in our approach is the use of a coherent synchrotron source and of contrast mechanisms based on the refractive index. The first successful tests were conducted on a variety of cell systems including skin and internal leaf cells, mouse neurons, rabbit fibroblast cells, and human tumor cells.

Animals↗
PubMed2004

Intrinsic curvature in the VP1 gene of SV40: comparison of solution and gel results.

DNA restriction fragments that are stably curved are usually identified by polyacrylamide gel electrophoresis because curved fragments migrate more slowly than normal fragments containing the same number of basepairs. In free solution, curved DNA molecules can be identified by transient electric birefringence (TEB) because they exhibit rotational relaxation times that are faster than those of normal fragments of the same size. In this article, the results observed in free solution and in polyacrylamide gels are compared for a highly curved 199-basepair (bp) restriction fragment taken from the VP1 gene in Simian Virus 40 (SV40) and various sequence mutants and insertion derivatives. The TEB method of overlapping fragments was used to show that the 199-bp fragment has an apparent bend angle of 46 +/- 2 degrees centered at sequence position 1922 +/- 2 bp. Four unphased A- and T-tracts and a mixed A3T4-tract occur within a span of approximately 60 bp surrounding the apparent bend center; for brevity, this 60-bp sequence element is called a curvature module. Modifying any of the A- or T-tracts in the curvature module by site-directed mutagenesis decreases the curvature of the fragment; replacing all five A- and T-tracts by random-sequence DNA causes the 199-bp mutant to adopt a normal conformation, with normal electrophoretic mobilities and birefringence relaxation times. Hence, stable curvature in this region of the VP1 gene is due to the five unphased A- and T- tracts surrounding the apparent bend center. Discordant solution and gel results are observed when long inverted repeats are inserted within the curvature module. These insertion derivatives migrate anomalously slowly in polyacrylamide gels but have normal, highly flexible conformations in free solution. Discordant solution and gel results are not observed if the insert does not contain a long inverted repeat or if the long inverted repeat is added to the 199-bp fragment outside the curvature module. The results suggest that long inverted repeats can form hairpins or cruciforms when they are located within a region of the helix backbone that is intrinsically curved, leading to large mobility anomalies in polyacrylamide gels. Hairpin/cruciform formation is not observed in free solution, presumably because of rapid conformational exchange. Hence, DNA restriction fragments that migrate anomalously slowly in polyacrylamide gels are not necessarily stably curved in free solution.

Birefringence↗