Showing papers in "Applied Spectroscopy in 2000"

Interval Partial Least-Squares Regression (iPLS): A Comparative Chemometric Study with an Example from Near-Infrared Spectroscopy

Abstract: A new graphically oriented local modeling procedure called interval partial least-squares (i PLS) is presented for use on spectral data. The i PLS method is compared to full-spectrum partial least-squares and the variable selection methods principal variables (PV), forward stepwise selection (FSS), and recursively weighted regression (RWR). The methods are tested on a near-infrared (NIR) spectral data set recorded on 60 beer samples correlated to original extract concentration. The error of the full-spectrum correlation model between NIR and original extract concentration was reduced by a factor of 4 with the use of i PLS (r=0.998, and root mean square error of prediction equal to 0.17% plato), and the graphic output contributed to the interpretation of the chemical system under observation. The other methods tested gave a comparable reduction in the prediction error but suffered from the interpretation advantage of the graphic interface. The intervals chosen by i PLS cover both the variables found by FSS and all possible combinations as well as the variables found by PV and RWR, and i PLS is still able to utilize the first-order advantage. Index Headings: Interval PLS; Variable selection; NIR, Principal variables; Forward stepwise selection; Recursively weighted regression; Beer; Extract.

Generalized Two-Dimensional Correlation Spectroscopy

Abstract: Publisher Summary Generalized two-dimensional (2D) correlation spectroscopy is described to show how this technique can be applied to a very broad range of spectral analysis problems. In this spectroscopy, the underlying similarity or dissimilarity among systematic variations in spectroscopic signal intensities is examined. It develops parallel to multiple pulse-based 2D spectroscopy techniques and employs a somewhat different approach to constructing 2D spectra. The signal variations are induced by an external perturbation or physical stimulus applied to the sample, and the pattern of the intensity variation is systematically analyzed by a simple cross-correlation technique. The correlation intensities thus obtained are displayed in the form of a 2D map defined by two independent spectral axes for further analysis. Such a 2D map is referred to as a 2D correlation spectrum, even though many of today's 2D correlation studies include applications in the field outside of spectroscopy, such as chromatography and microscopy. An illustrative example is given for the analysis of attenuated total reflection (ATR) IR spectra collected during the crystallization of biodegradable polymer poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) to demonstrate the merits of this technique, such as the enhancement of spectral resolution, detection of coordinated changes among spectral signals, and determination of relative directions and sequential order of intensity variations.

Determination of Two-Dimensional Correlation Spectra Using the Hilbert Transform

Abstract: A computationally efficient numerical procedure to generate twodimensional (2D) correlation spectra from a set of spectral data collected at certain discrete intervals of an external physical variable, such as time, temperature, pressure, etc., is proposed. The method is based on the use of a discrete Hilbert transform algorithm which carries out the time-domain orthogonal transformation of dynamic spectra. The direct computation of a discrete Hilbert transform provides a definite computational advantage over the more traditional fast Fourier transform route, as long as the total number of discrete spectral data traces does not significantly exceed 40. Furthermore, the mathematical equivalence between the Hilbert transform approach and the original formal definition based on the Fourier transform offers an additional useful insight into the true nature of the asynchronous 2D spectrum, which may be regarded as a time-domain cross-correlation function between orthogonally transformed dynamic spectral intensity variations.

Characterization of Laser-Induced Breakdown Spectroscopy (LIBS) for Application to Space Exploration:

Abstract: Early in the next century, several space missions are planned with the goal of landing craft on asteroids, comets, the Moon, and Mars. To increase the scientific return of these missions, new methods are needed to provide (1) significantly more analyses per mission lifetime, and (2) expanded analytical capabilities. One method that has the potential to meet both of these needs for the elemental analysis of geological samples is laser-induced breakdown spectroscopy (LIBS). These capabilities are possible because the laser plasma provides rapid analysis and the laser pulse can be focused on a remotely located sample to perform a stand-off measurement. Stand-off is defined as a distance up to 20 m between the target and laser. Here we present the results of a characterization of LIBS for the stand-off analysis of soils at reduced air pressures and in a simulated Martian atmosphere (5-7 torr pressure of CO2) showing the feasibility of LIBS for space exploration. For example, it is demonstrated that an analytically useful laser plasma can be generated at distances up to 19 m by using only 35 mJ/pulse from a compact laser. Some characteristics of the laser plasma at reduced pressure were also investigated. Temporally and spectrally resolved imaging showed significant changes in the plasma as the pressure was reduced and also showed that the analyte signals and mass ablated from a target were strongly dependent on pressure. As the pressure decreased from 590 torr to the 40-100 torr range, the signals increased by a factor of about 3-4, and as the pressure was further reduced the signals decreased. This behavior can be explained by pressure-dependent changes in the mass of material vaporized and the frequency of collisions between species in the plasma. Changes in the temperature and the electron density of the plasmas with pressure were also examined and detection limits for selected elements were determined. Index Headings: Laser-induced breakdown spectroscopy; LIBS; Laser spark; Soil analysis; Space exploration.

Modeling and Measuring the Effect of Refraction on the Depth Resolution of Confocal Raman Microscopy

Abstract: A simple ray-tracing analysis has been used to predict the effect of refraction at the sample/air interface, on the depth resolution of confocal Raman microscopy. This analysis applies to the ''z-scanning'', or ''optical sectioning'', approach to obtaining depth profiles, where the laser beam is incident normal to the sample surface, and spectra are recorded sequentially as the focus is moved deeper into the material. It is shown that when a ''dry'' metallurgical objective (the most common configuration for commercial Raman microscopes) is used, both the position and the depth of focus increase dramatically as the beam is focused deeper into the sample. It quickly becomes impossible to obtain ''pure'' spectra of thin layers that are buried more than a few micrometers below the air interface. Equations are presented which model the intensity response expected when focusing through a coating into a substrate. The model requires knowledge of the sample refractive index, the numerical aperture, and the laser beam intensity distribution at the limiting aperture, of the objective. Given these values, one can predict the substrate Raman intensity as a function of the nominal focal point within the sample. For a 36μm coating on a thick substrate, we predict that even for a perfectly sharp interface (1μm), substrate bands rise slowly (over an apparent distance of 10μm or more), and are strong when the focus is apparently only ~18μm below the air/coating interface. This prediction was confirmed through experimental observation. The model was also used to analyze literature data that had been interpreted previously as showing interfacial diffusion in polymer laminates. The model correctly reproduced the main features of the observed data without invoking interfacial penetration - the optical aberrations alone accounted for almost all the observed broadening and the fact that the apparent thickness of the buried layer is also distorted. It was concluded that, with the use of this illumination geometry, it is very difficult to detect or quantify interfacial broadening unless it occurs on a very large scale indeed (tens of micrometers). It is concluded that ''optical sectioning'' cannot be recommended for quantitative depth profiling at significant depths using metallurgical objectives. The optimum practical solution is to cut a cross section and map laterally across the sample, thereby utilizing and maintaining the excellent (lateral) resolution of the Raman microprobe. An alternative solution is to use an immersion objective to minimize refraction at the sample surface.

Confocal Raman Microscopy: Why the Depth Resolution and Spatial Accuracy Can Be Much Worse Than You Think

Abstract: This paper describes continuing studies on the effect of refraction on the depth resolution and spatial accuracy of confocal Raman microscopy Previous work showed how the apparent position of interfaces, and their breadth, was grossly in error when "optical sectioning" was carried out with a high-power metallurgical objective (the usual configuration for commercial confocal Raman instruments) Simple equations were presented which model these effects given only the numerical aperture of the objective and sample refractive index This paper extends these studies to the measurement of the position and thickness of buried structures and shows how the refraction theory provides a good basis for interpreting Raman intensity-depth profiles that have been acquired by optical sectioning of complex structures To summarize the magnitude of the problem, it is typical for the apparent thickness and positional depth of a buried layer to be about half of the true values—hence, correcting for refraction is critical when interpreting intensity-depth profiles In order to minimize the effects it is possible to use a specialized objective (eg, an oil immersion objective) to reduce refraction at the sample surface Data are presented which show that the apparent position and thickness of structures obtained with such an objective are much closer to the actual values, even when the sample index is not perfectly matched by the coupling fluid The use of immersion objectives is highly recommended if depth profiling by optical sectioning is to be attempted

Luminescence Lifetime Imaging of Oxygen, pH, and Carbon Dioxide Distribution Using Optical Sensors

Abstract: We present a modular system for time-resolved two-dimensional luminescence lifetime imaging of planar optical chemical sensors. It is based on a fast, gateable charge-coupled device (CCD) camera without image intensifier and a pulsable light-emitting diode (LED) array as a light source. Software was developed for data acquisition with a maximum of parameter variability and for background suppression. This approach allows the operation of the system even under daylight. Optical sensors showing analyte-specific changes of their luminescence decay time were tested and used for sensing pO2, pCO2, pH, and temperature. The luminophores employed are either platinum(II)-porphyrins or ruthenium(II)-polypyridyl complexes, contained in polymer films, and can be efficiently excited by blue LEDs. The decay times of the sensor films vary from 70mus for the Pt(II)-porphyrins to several 100 ns for the Ru(II) complexes. In a typical application, 7 mm-diameter spots of the respective optical sensor films were placed at the bottom of the wells of microtiter-plates. Thus, every well represents a separate calibration chamber with an integrated sensor element. Both luminescence intensity-based and time-resolved images of the sensor spots were evaluated and compared. The combination of optical sensor technology with time-resolved imaging allows a determination of the distribution of chemical or physical parameters in heterogeneous systems and is therefore a powerful tool for screening and mapping applications. Index Headings: Optical sensor films; Time-resolved imaging system; Microsecond decay time sensors.

Oil and fat classification by selected bands of near-infrared spectroscopy

Abstract: One hundred and four edible oil and fat samples from 18 different sources, either vegetable (Brazil nut, coconut, corn, sunflower, walnut, virgin olive, peanut, palm, canola, soybean, sunflower) or animal (tallow and hydrogenated fish), have been analyzed by highperformance gas chromatography (HPGC) and near-infrared spectroscopy (NIRS). Fatty acids were quantified by HPGC. The near-infrared spectral features of the most noteworthy bands were studied and discussed to design a filter-type NIR instrument. An arborescent structure, based on stepwise linear discriminant analysis (SLDA), was built to classify the samples according to their sources. Seven discriminant functions permitted a successive discrimination of saturated fats, corn, soybean, sunflower, canola, peanut, high oleic sunflower, and virgin olive oils. The discriminant functions were based on the absorbance values, between three and five, from the 1700-1800 and 2100-2400 nm regions. Chemical explanations are given in support of the selected wavelengths. The arborescent structure was then checked with a test set, and 90% of the samples were correctly classified.

Surface-enhanced infrared spectroscopy: A comparison of metal island films with discrete and nondiscrete surface plasmons

Abstract: A study of the surface-enhanced infrared absorption (SEIRA) spectroscopy of para-nitrobenzoic acid (PNBA) adsorbed on thermally evaporated silver films has been conducted to determine the effect of film architecture on the magnitude of the SEIRA enhancement. Ordered arrays of uniformly sized silver nanoparticles, termed periodic particle arrays (PPAs), were prepared on several different infrared transparent substrates (germanium, silicon, and mica) by nanosphere lithography (NSL). It was found that the ordered arrays deposited by NSL produced well-defined and intense surface plasmon resonance (SPR) bands in the infrared at frequencies between 1500 and 4000cm -1. The peak frequency of these infrared SPR bands depended on the array architecture and the substrate material. By appropriate design of the nanoparticle array, the infrared SPR band can be made to be coincident with the SEIRA sensitive infrared bands of the PNBA. The trends in the infrared SPR peak frequencies and band shapes were consistent with predictions from electrodynamic theory. The SEIRA responses per unit area of deposited metal obtained with the PPA-type films were at best comparable to results obtained with disordered silver and gold films deposited on the same substrate materials by thermal evaporation (i.e., in the absence of any NSL masking spheres). The results of this study are most consistent with theories and models that attribute SEIRA to the dielectric constant and optical extinction spectrum of the metal film. Index Headings: Infrared spectroscopy; Surface-enhanced infrared absorption; Plasmon; SEIRA.

Dual-Pulse LIBS Using a Pre-Ablation Spark for Enhanced Ablation and Emission

Abstract: In this paper we report the first observations of dual-pulse laser-induced breakdown spectroscopy (LIBS) signal enhancements by using a pre-ablation spark. In this technique a laser pulse is brought in parallel to the sample surface and focused a few millimeters above it to form an air plasma or air spark. A few microseconds later a second laser pulse, which is focused on the sample, ablates sample material and forms the LIBS plasma from which analyte emission occurs. In this way, large LIBS signal enhancements, 11- to 33-fold, are observed for copper and lead, respectively, relative to the signal in the absence of the air spark. In all cases where enhanced LIBS signals are seen, greatly enhanced sample ablation also occurs.

Steel Analysis with Laser-Induced Breakdown Spectrometry in the Vacuum Ultraviolet:

Abstract: Laser-induced breakdown spectrometry (LIBS) with multiple pulse excitation has been applied for the multielemental analysis of steel samples in the vacuum ultraviolet. The emission of the induced plasma was coupled into a Paschen-Runge spectrometer equipped with photomultipliers linked to high-speed multichannel signal electronics. Time-resolved signal evaluation yields a significantly improved signal-to-noise ratio for the plasma emission periods after a multipulse excitation. Reference materials for low-alloy steel grades were used to calibrate the measurements. The investigations concentrated on the light elements phosphorus, sulfur, and carbon using emission wavelengths in the range from 178.28 nm to 193.09 nm. For the first time, limits of detection below 10 μg/g were achieved for the light elements phosphorus, sulfur, and carbon using LIBS. With these results the basis is established for future on-line applications of LIBS in the steel industry.

Development of a Hierarchical Classification System with Artificial Neural Networks and FT-IR Spectra for the Identification of Bacteria

Abstract: The practical value of elaborated vibrational spectroscopic techniques in medical and microbiological biodiagnostics depends strongly on the reliability, the speed, the ease of use, and the evaluation procedures of the acquired data. In the present study, artificial neural networks (ANNs) were used to establish a hierarchical classification system for microbial Fourier transform infrared (FT-IR) spectra suitable for identification purposes in a routine microbiological laboratory. A radial basis function network (RBF) proved to be superior for a top-level classification of the FT-IR spectra at the genus level. Species within these genera were sequentially further classified by using multilayer perceptrons (MLPs), which achieved a larger differentiation depth than RBF networks. The MLPs were trained with several learning algorithms. Best performance was achieved with the cascade correlation (CC) approach to determine the network topology combined with resilient propagation (Rprop) as the training algorithm. The final hierarchically organized model was able to discriminate between four genera of microorganisms comprising 42 different strains of Pseudomonacae, 33 strains of Bacillus, 46 strains of Staphylococcus, and 6 species and 24 strains of yeast genera Candida. Altogether, 145 strains from international microbial strain collections are comprised in 971 spectra. The species Candida albicans could be further classified with respect to susceptibility against the antibiotic drug fluconazole, which is of therapeutic relevance. Key factors for the classification results of the bacterial FT-IR spectra were the data pretreatment, the number of wavelengths selected by a feature extraction algorithm, the type of network, and the learning function used for the ANN training.

Dual polarization modulation: A real-time, spectral-multiplex separation of circular dichroism from linear birefringence spectral intensities

Abstract: A real-time, spectral-multiplex method for the complete separation of circular dichroism (CD) spectra from linear birefringence (LB) spectra is presented. The method, called dual polarization modulation (DPM), involves the introduction of a second source of polarization modulation after the CD sample. The first source of polarization modulation, as in conventional CD spectrometers, is located before the sample. Intensity signals at the detector in phase with each of the two polarization modulation frequencies are demodulated simultaneously in parallel and combined electronically in opposition to eliminate the LB spectrum by real-time cancellation. The accuracy of the cancellation can be adjusted electronically without the need to change the optical alignment of the instrument. The DPM method permits baseline-corrected CD spectra to be measured without the need for a subsequent CD background measurement.

Derivative Preprocessing and Optimal Corrections for Baseline Drift in Multivariate Calibration

Abstract: The characteristics of baseline drift are discussed from the perspective of error covariance. From this standpoint, the operation of derivative filters as preprocessing tools for multivariate calibration is explored. It is shown that convolution of derivative filter coefficients with the error covariance matrices for the data tend to reduce the contributions of correlated error, thereby reducing the presence of drift noise. This theory is corroborated by examination of experimental error covariance matrices before and after derivative preprocessing. It is proposed that maximum likelihood principal components analysis (MLPCA) is an optimal method for countering the deleterious effects of drift noise when the characteristics of that noise are known, since MLPCA uses error covariance information to perform a maximum likelihood projection of the data. In simulation and experimental studies, the performance of MLPCR and derivative-preprocessed PCR are compared to that of PCR with multivariate calibration data showing significant levels of drift. MLPCR is found to perform as well as or better than derivative PCR (with the best-suited derivative filter characteristics), provided that reasonable estimates of the drift noise characteristics are available. Recommendations are given for the use of MLPCR with poor estimates of the error covariance information.

Realization of the Hadamard Multiplex Advantage Using a Programmable Optical Mask in a Dispersive Flat-Field Near-Infrared Spectrometer:

Abstract: Spectrometry and hyperspectral imaging are finding numerous applications for micro-optoelectromechanical systems (MOEMS). A type of MOEMS device in the form of a digital micromirror array (DMA) has been made commercially available by Texas Instruments USA for projector display applications. We use this device as a spatial light modulator (SLM) in a new type of flat-field, near-infrared dispersive spectrometer (NIRDMAS). Attributes of the DMA used in this manner are presented for discussion. Features that make a DMA attractive for spectrometry and imaging are described. A brief introduction to Hadamard transform (HT) techniques is presented to show that the DMA may be the best Hadamard encoding mask yet developed. A comparison of a conventional raster scanning (CRS) scan and a Hadamard transform spectrometry (HTS) scan with respect to the Hadamard multiplex advantage using a nonphoton noise-limited, single-element detector is presented. A signal-to-noise ratio comparison using four spectral lines from a mercury-argon calibration lamp demonstrates that the theoretical noise reduction is approached for the HTS scan compared to the CRS scan. Some future applications of MOEMS in spectrometry and hyperspectral imaging are suggested.

Characterization of Uranium Oxides Using in Situ Micro-Raman Spectroscopy

Abstract: A range of uranium compounds have been characterized using micro-Raman spectroscopy. It is possible to easily distinguish the oxides UO2, α-U3O8, and γ-UO3. Studies have also investigated the Raman spectra of uranium precursors used for the preparation of these oxides. In situ studies to probe the transformation of uranyl nitrate to uranium oxide show that the nitrate is decomposed below 300°C. Initially an oxide similar to UO3 is produced, and this oxide gradually transforms to U3O8 as the temperature is increased to 550°C. The use of in situ Raman studies provides a powerful technique to study the mechanism of transformation and the stability of uranium oxides under oxidizing conditions.

Stand-off Detection of Chemicals by UV Raman Spectroscopy

Abstract: Experimental results are reported on a mobile, stand-alone, solarblind ultraviolet (UV) Raman lidar system for the stand-off detection and identification of liquid and solid targets at ranges of hundreds of meters. The lidar is a coaxial system capable of performing rangeresolved measurements of gases and aerosols, as well as solids and liquids. The transmitter is a flash lamp pumped 30 Hz Nd:YAG laser with quadrupled output at 266 nm. The receiver subsystem is comprised of a 40 cm Cassegrain telescope, a holographic UV edge filter for suppressing the elastic channel, a 0.46 m Czerny-Turner spectrometer, and a time gated intensified charge-coupled device (CCD) detector. The rejection of elastic light scattering by the edge filter is better than one part in 105, while the transmittance 500 cm-1 to the red of the laser line is greater than 50%. Raman data are shown for selected solids, neat liquids, and mixtures down to the level of 1% volume ratio. On the basis of the strength of the Raman returns, a stand-off detection limit of ~500 g/m2 for liquid spills of common solvents at the range of one half of a kilometer is possible.

Enhancement of Aluminum, Titanium, and Iron in Glass Using Pre-ablation Spark Dual-Pulse LIBS

Abstract: In this paper, we report the first enhanced emission for elements in a nonmetal or nonconducting matrix, glass, with the use of a preablation spark The glass samples used in this work are prototypes of samples used to immobilize inorganic waste at the Savannah River Site Vitrification Facility We have found that using a pre-ablation spark results in larger signal enhancements, 11- to 20-fold for titanium, aluminum, and iron in glass compared to the metal under the same experimental conditions We also demonstrate that this method is more sensitive than single-pulse LIBS experiments for the direct solid sampling of vitrified waste glass

Pigment Identification in Painted Artworks: A Dual Analytical Approach Employing Laser-Induced Breakdown Spectroscopy and Raman Microscopy:

Abstract: The combined application of two laser-based analytical techniques-laser-induced breakdown spectroscopy (LIBS) and Raman microscopy-for pigment identification on painted artworks is demonstrated. Detailed spectral data are presented from analyses performed on a 19th century Byzantine icon, which was examined in order to identify the pigments used in the original painted structure, as well as in interventions carried out subsequently for restorative purposes. LIBS measurements yielded elemental analytical data which suggest the presence of certain pigments and, in addition, provide information on the stratigraphy of the paint layers. Identification of most pigments and of the materials used in the preparation layer was performed by Raman microscopy. Index Headings: Laser-induced breakdown spectroscopy; LIBS; Raman microscopy; Artwork analysis; Pigment identification.

New prediction-augmented classical least squares (PACLS) methods: Application to unmodeled interferents

Abstract: A significant improvement to the classical least squares (CLS) multivariate analysis method has been developed. The new method, called prediction-augmented classical least squares (PACLS), removes the restriction for CLS that all interfering spectral species must be known and their concentrations included during the calibration. The authors demonstrate that PACLS can correct inadequate CLS models if spectral components left out of the calibration can be identified and if their spectral shapes can be derived and added during a PACLS prediction step. The new PACLS method is demonstrated for a system of dilute aqueous solutions containing urea, creatinine, and NaCl analytes with and without temperature variations. The authors demonstrate that if CLS calibrations are performed using only a single analyte's concentration, then there is little, if any, prediction ability. However, if pure-component spectra of analytes left out of the calibration are independently obtained and added during PACLS prediction, then the CLS prediction ability is corrected and predictions become comparable to that of a CLS calibration that contains all analyte concentrations. It is also demonstrated that constant-temperature CLS models can be used to predict variable-temperature data by employing the PACLS method augmented by the spectral shape of a temperature change of the watermore » solvent. In this case, PACLS can also be used to predict sample temperature with a standard error of prediction of 0.07 C even though the calibration data did not contain temperature variations. The PACLS method is also shown to be capable of modeling system drift to maintain a calibration in the presence of spectrometer drift.« less

Two-Dimensional Correlation Spectroscopy: Effect of Band Position, Width, and Intensity Changes on Correlation Intensities

Abstract: Simulation studies have demonstrated that the effect of a band shift may be completely removed from two-dimensional (2D) synchronous spectra if a shifting band simultaneously changes its intensity. In contrast, the corresponding asynchronous spectrum develops at least two peaks, even for a small shift coupled with an appreciable intensity variation. The separation between these peaks increases upon an increase in the bandwidth. If the spectral data are changing monotonically, the number and positions of the synchronous features can be readily determined from the difference between the first and the last spectrum in the series. The correlation spectrum calculated without the subtraction of reference spectrum, for a single band that shifts with constant intensity, is similar to that calculated without the subtraction of reference, for a band undergoing shift combined with significant intensity variations. The synchronous peaks resulting from the exponentially decaying intensity changes alone are at least 10 times more intense than the corresponding asynchronous peaks, whereas the analogous intensity ratio due to a moderate band shift is discernibly lower. This result proves that the asynchronous spectra are more sensitive to the band shift than the synchronous spectra. Also, the effect of noise is more apparent in the case of the asynchronous spectrum. The bandwidth variation alone generates noticeably weaker correlation intensity than that due to the band position or intensity changes. It has been shown that the asynchronous intensity strongly depends on the overall extent of the intensity changes at particular wavenumbers. As a result, the bands changing their intensities extensively but at similar rates may develop more intense asynchronicity than the bands with distinct difference in the rates of the intensity changes but smaller magnitude of these changes.

Novel SERS-Active Optical Fibers Prepared by the Immobilization of Silver Colloidal Particles

Abstract: A novel sensor based upon surface-enhanced Raman scattering (SERS) has been constructed by immobilizing colloidal silver particles onto the distal end of an optical fiber. This same single fiber was then used to both transport the exciting laser radiation and collect the Raman scattering from analytes sorbed onto the colloidal particles. The colloidal particles were immobilized by functionalization of the end of the optical fiber with (3-am inopropyl)trimethoxysilane prior to immersion of the fiber in silver colloid. Spectra were obtained from both 4-(5'-azobenzotriazol)3,5-dimethoxyphenylamine and crystal violet. The within-batch variation of a set of five fibers has been measured as approximately 10%. Raman imaging experiments demonstrated that the effects due to spatial variations in the intensity of the SERS recorded over the distal end of the fiber are removed by the use of a multimode fiber. Index Headings: Fiber optic; Raman; Surface-enhanced Raman scattering; SERS; Surface-enhanced resonance Raman scattering; SERRS; Sensors.

Calibration Procedure to Derive IRRAS Spectra from PM-IRRAS Spectra

Abstract: A calibration procedure is presented to relate Fourier transform infrared reflection-absorption spectroscopy (IRRAS) spectra obtained by polarization modulation (PM) and conventional techniques. This procedure concerns only PM-IRRAS measurements obtained with a lock-in amplifier. At the first stage, it consists in measuring two calibrated spectra, by adding a linear polarizer after the sample. The corresponding corrected PM-IRRAS spectra display band intensities close to those observed on IRRAS spectra (at about 6%), but the value of the baseline is lower. The calibration procedure can be performed at the second stage, by normalizing the corrected PM-IRRAS spectra of the sample and the bare substrate. In this case, the band intensities and the baseline of the normalized corrected PM-IRRAS and IRRAS spectra are similar.

Infrared Spectroscopy of Human Cells and Tissue. Part VI: A Comparative Study of Histopathology and Infrared Microspectroscopy of Normal, Cirrhotic, and Cancerous Liver Tissue

Abstract: Infrared spectra of selected regions of normal, cirrhotic, and neoplastic liver tissue samples are reported. Spectra of these tissues may vary for a number of reasons that may or may not be related to the state of health or disease. With the use of histochemical and immunohistochemical methods, the spectral variations are interpreted in terms of the biochemical and pathological processes involved. This paper lays the groundwork for correlation of infrared spectroscopic and pathological/histochemical results for other tissues, to be reported in subsequent communications.

Comparison of Multivariate Calibration Techniques Applied to Experimental NIR Data Sets

Abstract: The present study compares the performance of different multivariate calibration techniques applied to four near-infrared data sets when test samples are well within the calibration domain. Three types of problems are discussed: the nonlinear calibration, the calibration using heterogeneous data sets, and the calibration in the presence of irrelevant information in the set of predictors. Recommendations are derived from the comparison, which should help to guide a nonchemometrician through the selection of an appropriate calibration method for a particular type of calibration data. A flexible methodology is proposed to allow selection of an appropriate calibration technique for a given calibration problem. Index Headings: Calibration; Multivariate; Method comparison; NIR; Nonlinearity; Clustering.

Enhanced Chemical Classification of Raman Images in the Presence of Strong Fluorescence Interference

Abstract: Raman spectra and spectral images containing severe fluorescence interference are analyzed by using a variety of correlation and classification algorithms, both before and after preprocessing with the use of the Savitzky–Golay second-derivative (SGSD) method (and other related methods). Spectral correlation coefficient, principal component, and minimum Euclidean distance analyses demonstrate superior suppression of background and noise interference in Raman spectra when using SGSD preprocessing. The tested spectra include fluorescence interference that is more intense than the Raman features of interest and also contains broad background peaks that vary in shape and intensity from sample to sample. The high chemical information content of the SGSD-processed Raman spectra is demonstrated by using quantitative comparisons of correlation coefficients in a series of synthetic Raman spectra with either different or identical large backgrounds. The practical utility of SGSD in chemical image classification is illustrated by using an experimental Raman image of sugar microcrystals on substrates with large interfering background signals. The functional equivalence of SGSD and other windowed preprocessing algorithms is discussed.

IR Microscopic Imaging of Pathological States and Fracture Healing of Bone

Abstract: The application of IR microscopic imaging to the study of bone disease and fracture healing is demonstrated. Samples of normal and osteoporotic human iliac crest biopsies were prepared and examined at ~6-10 μm spatial resolution and 8 cm-1 spectral resolution with a 64 × 64 MCT focal plane array detector coupled to a Fourier transform infrared (FT-IR) microscope and a step-scanning interferometer. Two spectral parameters, one that monitors the extent of mineral (hydroxyapatite) formation in the tissue and another that monitors the size/perfection of the crystals, were compared in the samples generated from normal and pathological tissues. The average mineral levels in the osteoporotic sample were reduced by ~40% from the normal. In addition, the crystal size/perfection was substantially enhanced in the disease state. The applicability of IR imaging techniques to the study of therapeutic intervention was also investigated in a study of the effects of estrogen therapy on fracture healing in rat femurs. Femurs were examined by IR microscopic imaging 4 weeks after fracture. IR imaging showed that the mineral level was enhanced in estrogen-treated samples. In addition, the crystals were larger/more perfect in the treated specimens. These data demonstrate the utility of IR spectroscopic imaging for the study of pathological states of hard tissue.

Semi-quantitative Laser-Induced Breakdown Spectroscopy for Analysis of Mineral Drill Core

Abstract: An investigation is reported in the use of time-resolved laser-induced breakdown spectroscopy (LIBS) for mineral assaying applications. LIBS has potential for the rapid on-line determination of the major and minor constituents of mineral drill core samples. In this work a Q-switched Nd:YAG laser is used to test as-received lengths of drill core, with remote LIBS signal acquisition via a bare optical fiber bundle coupled to a spectrometer. A novel normalization scheme, based on integrating the total plasma emission, is demonstrated as a method for correction of signal variations due to the uneven surface geometry of rock. Averaged intensities of atomic emission for the elements Cr, Cu, Fe, Mn, and Ni show good linear correlations, with coefficients of R2 = 0.92-0.99, against laboratory assay values. Limitations in the comparison of the results of surface analysis to bulk compositions are discussed, with emphasis on mining applications of LIBS.

Two-Dimensional Correlation Spectroscopy Study of Visible and Near-Infrared Spectral Variations of Chicken Meats in Cold Storage

Abstract: This paper reports the generalized two-dimensional (2D) visible/near-infrared (Vis/NIR) correlation spectroscopy study of chicken meats in cold storage. The 2D visible correlation analysis revealed that, besides the previously reported three absorption bands around 445, 485, and 560 nm ascribed to DeoxyMb, MetMb, and OxyMb, there is an additional band around 635 nm that could be assigned to SulfMb, a product from the reaction of myoglobin with H2S generated by bacteria. Unlike the spectral intensity reduction of the 440 and 560 nm bands, the intensities of the 490 and 635 nm bands increase with storage time. The asynchronous 2D visible correlation spectra indicated that OxyMb and MetMb produce SulfMb first; then complicated reactions such as the oxygenation and oxidization of DeoxyMb and the oxidization of OxyMb follow as storage is prolonged. In addition, several close and separated bands appearing around the 440, 490, and 560 coordinates could be a result of the changes in the molecular environment of the heme pigment portion. Hence, the decreasing intensity and the splitting of the 440 and 560 nm bands are responsible for the discoloration of meats. The 2D correlation spectra in the NIR region showed that the O-H/N-H bands change their spectral intensity before the C-H groups during the storage, suggesting a coordination process for hydrophilic O-H and N-H groups. It revealed two main possibilities: (1) water species interact with other meat components, and (2) the meat proteins undergo proteolysis and denaturation processes, which is associated with the development of tenderization during storage (aging). In addition, the asynchronous spectra correlating the spectral bands in both the visible and NIR regions suggested that the discoloration occurs earlier than other developments, such as tenderization process.

Raman Spectroscopy for the in Situ Identification of Cocaine and Selected Adulterants

Abstract: We demonstrate the in situ identification of crack cocaine and cocaine·HCl by using a fiber-optic Raman probe and a portable Raman spectrograph. The Raman spectrum of freebase cocaine (crack) is obtained in just seconds without any sample preparation, and differs significantly from that of cocaine·HCl. We also show that the Raman spectra of these drugs are easily distinguishable from common cutting agents and impurities such as benzocaine and lidocaine. Another advantage of using Raman spectroscopy is that the drugs can be identified while contained in transparent containers, such as clear plastic evidence containers that are used to store drug evidence and to maintain chain of custody. We also demonstrate the in situ Raman identification of drugs separated by thin-layer chromatography. We discuss the utility of surface-enhanced Raman spectroscopy (SERS) in toxicological drug screening and present preliminary SERS data for cocaine in solution using colloidal silver. We believe this to be the first published SERS spectrum of freebase cocaine.