Fluorocarbon fiber-optic Raman probe for non-invasive Raman spectroscopy

We report the development of a novel fiber-optic Raman probe using a graded index fluorocarbon optical fiber. The fluorocarbon fiber has a simple Raman spectrum, a low fluorescence background, and generates a Raman signal that in turbid media serves as an intense reference Raman signal that corrects for albedo. The intensity of the reference signal can easily be varied as needed by scaling the length of the excitation fiber. Additionally, the fluorocarbon probe eliminates the broad silica Raman bands generated in conventional silica-core fiber without the need for filters.     

Detection of albumin unfolding preceding proteolysis using Fourier transform infrared spectroscopy and chemometric data analysis

The hydrolysis of bovine serum albumin with protease K at 60 degrees C has been studied by means of infrared spectroscopy. Two-dimensional correlation spectroscopy (2DCoS) has been used to study spectral changes in the reaction. The use of the multivariate curve resolution-alternating least-squares method applied to infrared measurements allowed the recovery of pure infrared spectra and concentration profiles of the different species involved in the reaction. Special attention was paid to the careful inspection of residuals again using 2DCoS. In this way, a heat-induced unfolding step previous to protein hydrolysis was identified. The infrared spectra of the intermediate species showed a more disordered structure than native albumin, the decrease in alpha-helix conformation being especially noticeable. The formation of beta-sheet aggregates due to heating was detected too.     

Fourier transform infrared spectral analysis of degenerative cartilage: an infrared fiber optic probe and imaging study

A preliminary investigation into the diagnostic potential of an infrared fiber optic probe (IFOP) for evaluating degenerative human articular cartilage is described. Twelve arthritic human tibial plateaus obtained during arthroplasty were analyzed using the IFOP. Infrared spectra were obtained from IFOP contact with articular surface sites visually graded normal or degraded (Collins Scale grade 1 and grade 3, respectively). Comparisons of infrared spectral parameters (peak heights and areas) were made to elucidate spectral indicators of surface degeneration. IFOP spectral analysis revealed subtle but consistent changes between grades 1 and 3 sites. Infrared absorbance bands arising from type II collagen were observed to change with degradation. More degraded tissues exhibited increased amide II (1590-1480 cm(-1))/1338 cm(-1) area ratio (p=0.034) and decreased 1238/1227 cm(-1) peak ratio (p = 0.017); similar changes were seen with Fourier transform infrared imaging spectroscopy (FT-IRIS) analysis. Grades 1 and 3 cartilage showed consistent spectral differences in the amide II, III, and 1338 cm(-1) regions that are likely related to type II collagen degradation that accompanies cartilage degeneration. These results suggest that it may be possible to monitor subtle changes related to early cartilage degeneration, allowing for IFOP use during arthroscopy for in situ determination of cartilage integrity.     

Quantitative analysis of vitamin A degradation by Raman spectroscopy

Vitamin A is known to support cell growth promotion, maintenance, and differentiation of epithelial tissues. Retinol is currently used in cosmetic formulations and products to deliver these and other benefits to the skin. However, retinol is known to be unstable and, therefore, remains of great concern to the cosmetic industry. The decomposition pathways of the retinoids in general have been previously postulated and investigated mostly by high-performance liquid chromatography (HPLC) with UV/Vis spectroscopy. In our studies, we examined specific conditions at which retinol degrades and subsequently identified and quantified the products of retinol decomposition by Raman spectroscopy. We reveal which experimental settings and computational tools allow monitoring of in situ evolution of an all-trans retinol solution when submitted to UV light in the presence of oxygen.     

Nondestructive analysis and dating of historical paper based on IR spectroscopy and chemometric data evaluation

Sampling restrictions in analysis of cultural heritage materials narrow the choice of appropriate analytical methods considerably. In this work, near- and mid-FT-IR reflectance data were related to paper properties determined with classical analytical methods using partial least-squares. Nondestructive determination of properties, which are of importance for evaluation of the long-term stability of historical paper, i.e., ash content, lignin content, degree of polymerization of cellulose, pH, and aluminum content, is possible. With the use of a considerable sample set, satisfactory reliability was achieved for all properties but aluminum content. Considering that with age, chemical properties of paper change, dating of historical documents was attempted for the first time, also with success.     

Autofluorescence and diffuse reflectance spectroscopy of oral epithelial tissue using a depth-sensitive fiber-optic probe

Optical spectroscopy can provide useful diagnostic information about the morphological and biochemical changes related to the progression of precancer in epithelial tissue. As precancerous lesions develop, the optical properties of both the superficial epithelium and underlying stroma are altered; measuring spectral data as a function of depth has the potential to improve diagnostic performance. We describe a clinical spectroscopy system with a depth-sensitive, ball lens coupled fiber-optic probe for noninvasive in vivo measurement of oral autofluorescence and diffuse reflectance spectra. We report results of spectroscopic measurements from oral sites in normal volunteers and in patients with neoplastic lesions of the oral mucosa; results indicate that the addition of depth selectivity can enhance the detection of optical changes associated with precancer.     

Reflectance spectroscopy for diagnosis of epithelial precancer: model-based analysis of fiber-optic probe designs to resolve spectral information from epithelium and stroma

Reflectance spectroscopy is a promising technology for detection of epithelial precancer. Fiber-optic probes that selectively collect scattered light from both the epithelium and the underlying stroma are likely to improve diagnostic performance of in vivo reflectance spectroscopy by revealing diagnostic features unique to each layer. We present Monte Carlo models with which to evaluate fiber-optic probe geometries with respect to sampling depth and depth resolution. We propose a probe design that utilizes half-ball lens coupled source and detector fibers to isolate epithelial scattering from stromal scattering and hence to resolve spectral information from the two layers. The probe is extremely compact and can provide easy access to different organ sites.     

Nontoxic and chemically stable hollow optical fiber probe for fourier transform infrared spectroscopy

Remote spectroscopy systems based on hollow optical fiber probes are proposed and experimental results using a Fourier transform spectroscope are presented. A hollow optical-fiber probe with a silver and polymer inner coating is used to deliver incoherent light to a target and another separate hollow fiber is used to collect the reflected light. The reflectance spectra of teeth, skin, and oral mucosa were successfully measured with the probe even from surfaces with reflectances lower than 0.5%. The preliminary results obtained using attenuated total reflection spectroscopy are also presented. This remote infrared spectroscope is useful for endoscopic measurements inside the body because it is flexible, durable, nontoxic, and has the low transmission losses associated with hollow-fiber-based probes.     

Quantitative analysis of polyethylene blends by Fourier transform infrared spectroscopy

The quantitative analysis of binary polyethylene (PE) blends by Fourier transform infrared (FT-IR) spectroscopy has been achieved based on the ratio of two absorbance peaks in an FT-IR spectrum. The frequencies for the absorbance ratio are selected based on structural entities of the PE components in the blend. A linear relationship between the absorbance ratio and the blend composition was found to exist if one of the absorbance peaks is distinct to one of the components and the other peak is common to both components. It was also found that any peak resulting from short-chain branching in copolymers (such as linear low-density polyethylene (LLDPE) or metallocene-catalyzed LLDPE (mLLDPE)), is suitable for use as the peak that is designated as being distinct to that component. In order to optimize the linearity of the equation, however, the selection of the second common peak is the most important and depends on the blend system studied. Indeed, under certain circumstances peaks that are not spectrally distinct can be used successfully to apply the method. The method exhibits potential for the routine analysis of PE blends that have been calibrated prior to its application.     

Identification of clay minerals by infrared spectroscopy and discriminant analysis

Identification of clay minerals based on chemometric analysis of measured infrared (IR) spectra was suggested. IR spectra were collected using the diffuse reflection technique. Discriminant analysis and principal component analysis were used as chemometric methods. Four statistical models were created for separation and identification of clay minerals. More than 50 samples of various clay mineral standards from different localities were used for the creation of statistical models. The results of this study confirm that the discriminant analysis of IR spectra of clay minerals could provide a powerful tool for identification of clay minerals. Differentiation of muscovite from illite and identification of mixed structures of illite-smectite were achieved.     

Correlation of limestone beds using laser-induced breakdown spectroscopy and chemometric analysis

Correlation of limestone beds is commonly based on a variety of features, including the age of the bed, the fossil assemblage, internal sedimentary structures, and the relationship to other units in the stratigraphy. This study uses laser-induced breakdown spectroscopy (LIBS) to correlate 16 limestone beds from Kansas, USA, using three multivariate techniques: (1) soft independent modeling of class analogy (SIMCA) classification, (2) a partial least squares regression, 1 variable (PLS-1) model in which the spectra are regressed against a matrix of the indicator variables 1 through 16, and (3) a matching algorithm that consists of a sequence of binary PLS-1 models. Each gravel-sized limestone particle was analyzed by one LIBS shot; ten spectra were averaged into a single spectrum for chemometric analysis. The entire spectrum (198-969 nm wavelength) is used for multivariate analysis; the only preprocessing is averaging. The SIMCA and PLS-1 models fail to discriminate among the beds, which are chemically similar. In contrast, the matching algorithm has a success rate of 95% to 96%, using half of the spectra to train the model and the other half of the spectra to validate it. However, 100% success can be accomplished by accepting the classification of the majority of spectra for a given bed as the correct classification. This study indicates that LIBS can be applied to complex geologic correlation problems and provide rapid, accurate results.     

A chemometric approach for brain tumor classification using magnetic resonance imaging and spectroscopy

A new classification approach was developed to improve the noninvasive diagnosis of brain tumors. Within this approach, information is extracted from magnetic resonance imaging and spectroscopy data, from which the relative location and distribution of selected tumor classes in feature space can be calculated. This relative location and distribution is used to select the best information extraction procedure, to identify overlapping tumor classes, and to calculate probabilities of class membership. These probabilities are very important, since they provide information about the reliability of classification and might provide information about the heterogeneity of the tissue. Classification boundaries were calculated by setting thresholds for each investigated tumor class, which enabled the classification of new objects. Results on histopathologically determined tumors are excellent, demonstrated by spatial maps showing a high probability for the correctly identified tumor class and, moreover, low probabilities for other tumor classes.     

Influence of high temperatures on a fiber-optic probe for temperature measurement

The use of optical fiber in a temperature probe or sensor for optical pyrometry in the 100-1000 °C range is affected by the low thermal stability of classical fibers. We have studied the different sources of perturbations induced by exposure to high temperature. Two specific fibers especially suited for a high-temperature environment were tested and compared. Low (100 °C/min) and very fast (100 °C/s) fiber heating was performed to evaluate its influence on the guided flux and the induced error on temperature measurement. The metallic-coated fiber shows a reproducible temperature error that can be predicted. This important result permits the development of an uncooled fiber probe for temperature monitoring in high-temperature environments such as aerospace engines.     

Forensic analysis of poly(ethylene terephthalate) fibers by infrared spectroscopy

Poly(ethylene terephthalate) (PET) is one of the most commonly employed polymers in the textile industry. Its relevance as a source of evidence in the reconstruction of criminal cases is nevertheless very limited because the properties and morphologies of fibers from different producers tend to be very similar. By integrating bands, obtained on single fibers by infrared (IR) microscopy, associated with trans and gauche conformation and to the O-H end-groups of the molecules, a method is proposed that can discriminate otherwise similar PET fibers. The absorbancies at 1370 and at 846 cm(-1) relative, respectively, to the gauche and trans conformation, were measured and ratioed. The end-group content was evaluated by ratioing the absorbancies of the signals at 3440 and at 874 cm(-1). Relative standard deviation (R. S. D.) was 1% for repetitive analyses on the same location of the same single fiber. Precision was reduced if the ratios were measured along the length of a single fiber (R. S. D. = 3%) and even further when different fibers of the same sample were examined (R. S. D. varied from 2 to 10%). This simple method can greatly enhance the evidential value of PET fibers by subclassifying them, thus helping the Court to better assess their significance.     

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.     

Grazing-angle fiber-optic fourier transform infrared reflection-absorption spectroscopy for the in situ detection and quantification of two active pharmaceutical ingredients on glass

Fourier transform infrared reflection-absorption spectroscopy has been used with a fiber-optic grazing-angle reflectance probe as a rapid, in situ method for trace surface analysis of acetaminophen and aspirin at loadings of approximately 0-2 microg cm(-2) on glass. Partial least-squares multivariate regression permits the loadings to be quantified, simultaneously, with root-mean-squared errors of prediction of RMSEP approximately 0.1 microg cm(-2) for both compounds. The detection limits are estimated to be LD approximately 0.2 microg cm(-2).     

Cartesian-structure analysis in cast films by advanced infrared multiple-angle incidence resolution spectroscopy

Multiple-angle incidence resolution spectroscopy (MAIRS) has been improved to be an advanced algorithm so that the Cartesian structure in organic thin films can be analyzed. The infrared MAIRS technique was originally proposed as a totally new spectroscopic technique to reveal structural anisotropy in thin films on an infrared-transparent substrate, which yields both in-plane- (IP; X and Y) and out-of-plane (OP; Z)-mode spectra from an identical sample. Since this technique employs an analytical concept based on a signal decomposition of light intensity (not absorbance spectra), the algorithm intrinsically has high potential for further development. In the present study, the theoretically deduced matrix that correlates the light intensity to the angle of incidence has been modified to further decompose the IP-mode spectrum into X and Y components. As a result, anisotropic measurements of infrared spectra of thin film have become possible for the X, Y, and Z directions (Cartesian coordinate) simultaneously. With this advanced algorithm, the Cartesian structural changes in a cast film prepared on a germanium substrate have readily been analyzed, and a change from the biaxial to the uniaxial film structure with aging has spectroscopically been revealed.