Showing papers in "Applied Spectroscopy in 1988"

The Effect of Multiplicative Scatter Correction (MSC) and Linearity Improvement in NIR Spectroscopy

Abstract: Near-infrared (NIR) reflectance spectra of five different food products were measured. The spectra were transformed by multiplicative scatter correction (MSC). Principal component regression (PCR) was performed, on both scatter-corrected and uncorrected spectra. Calibration and prediction were performed for four food constituents: protein, fat, water, and carbohydrates. All regressions gave lower prediction errors (7-68% improvement) by the use of MSC spectra than by the use of uncorrected absorbance spectra. One of these data sets was studied in more detail to clarify the effects of the MSC, by using PCR score, residual, and leverage plots. The improvement by using nonlinear regression methods is indicated.

Comparison Among Several Numerical Integration Methods for Kramers-Kronig Transformation

Abstract: Several numerical integration methods are compared in order to search out the most effective method for the Kramers-Kronig transformation, using the analytical formula of the Kramers-Kronig transformation of a Lorentzian function as a reference. The methods to be compared involve the use of (1) Maclaurin's formula, (2) trapezium formula, (3) Simpson's formula, and (4) successive double Fourier transform methods. It is found that Maclaurin's formula, in which no special approximation is necessary for the pole part of the integration, gives the most accurate results, and also that its computation time is short. Successive Fourier transform is less accurate than the other methods, but it takes the least time when used without zero-filling. These results have important relevance for programs used to obtain optical constant spectra and to analyze spectral data.

Entrainment and Transport of Laser Ablated Plumes for Subsequent Elemental Analysis

Abstract: Several cell designs have been systematically evaluated for gas flow entrainment and transport of laser ablated material to a secondary excitation source for elemental analysis. The best cell is not limited to samples of particular size or shape and is insensitive to sample surface irregularity. An annular gas sheath around the cell results in a transient response sufficiently fast to permit depth and lateral sampling of single samples or rapid throughput of different samples but slow enough to give a steady signal with laser repetition rates ≥10 Hz. Entrainment and transport of ablated particulates have been investigated experimentally and by model calculation for a test material (Mo metal). The equations for predicting diffusive and gravitational loss of particles in a horizontal tube are presented and discussed. The major loss mechanism appears to be gravitational deposition of relatively large particles formed during ablation and possibly by coalescence within the transfer tube. Entrainment of ablated Mo by the cell and mass transport from the cell to the secondary source were determined to be ∼90% and ∼40% efficient, respectively. Shot-to-shot fluctuation in particle size may cause corresponding variation in transport efficiency when the upper end of the ablated particle size distribution exceeds the size limit for particle transport.

JCAMP-DX: A Standard Form for Exchange of Infrared Spectra in Computer Readable Form

Abstract: JCAMP-DX is a standard file form for exchange of infrared spectra and related chemical and physical information between spectrometer data systems of different manufacture, main-frame time-sharing systems, general purpose lab computers, and personal computers. It is compatible with all media: telephone, magnetic and optical disk, magnetic tape, and even the printed page (via optical reader). All data are stored as labeled fields of variable length using printable ASCII characters. A JCAMP-DX spectrum is a text file which can be viewed, corrected, and annotated with a text editor. The present focus is on infrared spectra, but JCAMP-DX can easily accommodate Raman, UV, NMR, mass, and other types of spectra, x-ray powder patterns, chromatograms, thermograms, and other plots which require the capability of representing contours as well as peak position and intensity. JCAMP-DX also provides for combining adequate information about the sample and method of observation with its spectrum.

A Spectrometer for Measuring Time-Resolved Infrared Linear Dichroism Induced by a Small-Amplitude Oscillatory Strain

Abstract: A spectrometer for detecting dynamic infrared linear dichroism (DIRLD) induced by a small-amplitude oscillatory strain is described. The strain-induced dynamic variations of absorbance and linear dichroism, as well as the normal static absorbance and dichroism, are measured simultaneously as functions of IR wavenumber, temperature, and strain frequency. The phase (temporal) relationships between the dynamic optical signals and the applied strain are also obtained. The instrument is sensitive enough to detect dynamic optical signals on the order of 10−4 absorbance units with a time resolution of about 14 μs. The dynamic dichroism signals arise from the strain-induced temporary reorientation of dipole-transition moments associated with the molecular vibrations of chemical functional groups. Both the rate and extent of reorientations are strongly influenced by the local molecular environment of the functional groups. Because of the specificity of IR absorbance bands to the individual submolecular structures, DIRLD spectroscopy is especially suited for the study of the intra- and intermolecular interactions and the detection of subtle changes in the local molecular environment. Example DIRLD spectra of atactic polystyrene are presented to demonstrate the potential utility of this spectroscopic technique.

Fourier Deconvolution of the Amide I Raman Band of Proteins as Related to Conformation

Abstract: Fourier deconvolution has been employed to enhance the resolution of the amide I Raman band of nine proteins found in milk and/or other foods. The broad band was resolved into several components. The overall shape of the amide I Raman band of proteins was found to be nearly Gaussian or to be composed of Gaussian components. A Gaussian function was therefore used for deconvolution. The results obtained were more detailed than those obtained with the Lorentzian approximation usually employed. The resolved band components were assigned to specific protein conformations. The frequencies and assignments are in good agreement with previous Raman work based on entirely different procedures. The band areas of the resolved components appear to reflect the fraction of any given conformation in a protein. Semiquantitative estimations of protein conformation are in reasonable agreement with data obtained by x-ray diffraction and by infrared methods.

Fiberoptic Oxygen Sensor Based on Fluorescence Quenching and Energy Transfer

Abstract: A new type of oxygen sensor is described that is based on electronic energy transfer from a donor (whose fluorescence is efficiently quenched by molecular oxygen) to an acceptor (which is less affected by oxygen). We use pyrene as the donor and perylene as the acceptor. The fluorescence emission band of the donor shows good overlap with the absorption band of the acceptor. When excited at 320 nm, the two-fluorophore system shows strong fluorescence at 476 nm, where pyrene itself is nonfluorescent. Although perylene is not efficiently quenched by oxygen, the system strongly responds to oxygen because fluorescence is quenched with an efficiency that by far exceeds the quenching efficiency for pyrene or pyrelene alone. The principle has been applied in order to devise a fiberoptic oxygen sensor by incorporating the two dyes in a polymer matrix that has been attached to the end of an optical fiber. Oxygen can be detected in the 0-150 kPa range with ±0.3 kPa precision. The detection limit is 60 Pa oxygen.

Partial Least-Squares Quantitative Analysis of Infrared Spectroscopic Data. Part I: Algorithm Implementation

Abstract: Various approaches to infrared multicomponent quantitative analysis including K-matrix, multivariate least-squares, principal component regression (PCR), and partial least-squares (PLS) are compared. The advantages and disadvantages of each are discussed. A particular implementation of the PLS method is detailed, with emphasis on the methods provided for calibration optimization and evaluation.

A Compact Surface Plasmon Resonance Sensor for Measurement of Water in Process

Abstract: An optical sensor based on surface plasmon resonance (SPR) for water measurement in process has been developed. The optical setup of this sensor is made in a simple and compact way, by the use of a laser diode as a point light source, providing an angularly spread (convergent) beam incident to the sensing part with imaging optics. Multichannel detection by a photodiode array and the Fourier transform optical setup eliminates the need for a mechanical angle scanner or a rotary stage in the system, resulting in a reduction in the size, the weight, and the cost of the sensor. Water concentrations in ethanol were measured by this sensor. Water concentrations of from 0.3% to 10% were obtained. Temperature dependence of the sensor and the method for compensation are discussed.

Determination of Particle Size in Powders by Scatter Correction in Diffuse Near-Infrared Reflectance

Abstract: Diffuse near-infrared reflectance spectroscopy has traditionally been an analytical technique for determining chemical compositions in a sample. We will, in this paper, focus on light scattering effects and their ability to determine the mean particle sizes of powders. The reflectance data of NaCl, broken glass, and sorbitol powders are linearized and submitted to the Multiplicative Scatter Correction (MSC), and the ensuing parameters are used in subsequent multivariate calibrations. The results indicate that particle size can, to a large degree, be determined from NIR reflectance data for a given type of powder. Up to 99% of the partical size variance was explained by the regression.

Evaluation of an Isolated Droplet Sample Introduction System for Laser-Induced Breakdown Spectroscopy

Abstract: An isolated droplet generator (IDG) has been used as a novel solution sample introduction system for laser-induced breakdown spectroscopy (LIBS). The IDG converts liquid samples to equally-spaced, uniformsized droplets. The droplets are introduced into the region where a high-powered Nd:YAG laser beam is focused. The resulting analyte emission is spectrally and temporally resolved. The dependence of analyte emission intensity on several parameters (droplet radius, droplet production frequency, etc.) is examined. A flow injection analysis (FIA) manifold has been used to introduce samples to the IDG for quantitative analysis. Performance characteristics of the FIA/IDG/LIBS method are given. Calibration curves linear over 3 orders of magnitude have been obtained for several elements. Detection limits for solutions with the FIA/IDG/LIBS system are in the low ppm range.

A detector for liquid chromatography and flow injection analysis using surface-enhanced Raman spectroscopy

Abstract: A successful interface between a high-performance liquid chromatograph and a Raman spectrometer is described. Surface-enhanced techniques are utilized to overcome the sensitivity problem inherent to conventional Raman spectrometry by adding a Ag sol to the chromatographic effluent in a post-column mixing coil. The system is designed so that Raman spectra may be obtained from chromatographic effluent or from flow injection analysis effluent. A common organic dye (pararosaniline hydrochloride) is used to evaluate the reproducibility, dynamic range, and analytical capabilities of the system. The SERS instrument is found to be a viable detector for HPLC and FIA, capable of providing structural information with a sensitivity comparable to that of other commonly used HPLC detectors.

Surface-Enhanced Raman Spectroscopy in the Near-Infrared

Abstract: Near-infrared Raman spectroscopy has been used to obtain Surface-Enhanced Raman Spectra (SERS) of pyridine on both silver and gold electrodes. The enhancement factor is higher than that found for visible excitation, and the band intensities are somewhat different. There is no indication of photochemical damage or change of the sample when near-infrared radiation is used, even at relatively high power levels.

In Situ FT-IR Analysis of a Composite Curing Reaction Using a Mid-Infrared Transmitting Optical Fiber:

Abstract: The use of a mid-infrared transmitting fiber to carry the infrared beam of an FT-IR spectrometer outside of the optical bench is reported. In addition it is demonstrated that it is possible to analyze samples using the fiber as an internal reflection element. The fiber is covered with a protective coating which can be removed for a short region, to allow contact with the sample over a controlled length. Two examples of the use of an optical fiber for remote sampling are discussed. The first shows that a spectrum of a liquid sample (2-butanone) can be easily measured, and the second shows how a fiber can be used to monitor the progress of curing reactions in thermoset composite materials. The spectrum of a resin was recorded before the cure by embedding the optical fiber in the graphite fiber/polyimide matrix resin prepreg, and then the progress of the cure was monitored during the curing process. This type of remote sampling shows tremendous potential for opening totally new areas of usage for FT-IR spectrometry, including the studies of hazardous materials, enclosed reactions, and processes that do not allow samples to be taken inside the spectrometer.

Spherical Raman cell

Abstract: There have been continual efforts over the years to improve the quality of experimental Raman data, with particular attention being given to improved sources, detectors, and better resolution and rejection of stray light by the utilized monochromators. However, little attention has been given to the design of the sampling cells utilized for the recording of the Raman spectra of liquids.

Raman Cross Sections of Chemical Agents and Simulants

Abstract: The differential Raman scattering cross sections of the chemical agents GA, GB, HD, and VX, and the chemical agent simulants DMMP and DIMP have been measured with visible and UV wavelength excitation. These compounds proved to be weak Raman scatterers with cross sections for the strongest lines on the order of 6 to 10 × 10−30 cm2/sr/molecule with 514.5-nm excitation. Excitation at 363.8 nm yielded only a slight preresonance enhancement of the Raman cross sections. Because of the small cross sections and the lack of enhancement, remote Raman detection of chemical agents appears doubtful.

Near-Infrared Surface-Enhanced Raman Spectroscopy. Part I: Copper and Gold Electrodes

Abstract: Surface-enhanced Raman spectra of pyridine on copper and gold electrodes in the near-infrared were obtained with a Fourier transform Raman spectrometer. Surface-enhanced Raman spectra were observed for pyridine adsorbed on copper and gold electrodes while a Nd:YAG laser (1.064 μm) was used for excitation. Good-quality spectra were recorded for 0.08 mM pyridine on a copper electrode with a single oxidation-reduction cycle, whereas for a gold electrode, several ORCs were necessary. A very intense low-energy Raman band was observed on both metals at positive potentials, which may be due to a metal-oxide vibrational mode.

Application of Multidimensional Analyses to the Extraction of Discriminant Spectral Patterns from NIR Spectra

Abstract: A method for extracting NIR discriminant spectral patterns without any reference to chemical values is suggested. First, groups of samples have to be defined a priori. Second, this method involves two procedures: the application of Principal Component Analysis (PCA) on spectral data and Factorial Discriminant Analysis (FDA) on the PC scores. Discriminant spectral patterns are assessed as linear combinations of PCA eigenvectors with weights determined by FDA. This method was applied on an illustrative collection of wheat semolina conditioned at 3 levels of water concentration. Three groups were made on the basis of the 3 water levels. Two discriminant spectral patterns representative of water concentration were extracted. They showed the commonly known bands of water at 1940 and 1450 nm. The band at 1450 nm was resolved into two bands at 1410 and 1460 nm, which could be assigned to free and bound OH, respectively. The discriminant spectral patterns therefore modeled the relative proportion of free and bound water. Suitability of the method for other applications is discussed.

Imaging with Functional Group Maps Using Infrared Microspectroscopy

Abstract: The interface of a two-dimensional computer-controlled stepper-motor-driven microscope stage with an infrared microspectroscopy system is explained in terms of its usefulness in performing microscopic imaging. Images are obtained by reducing spatially specific infrared spectra to functional group maps. The maps are constructed at a given infrared absorption frequency and are a function of the intensity versus × and y spatial coordinates. The use of the technique is demonstrated through two examples of polymer characterization. Transmittance infrared spectra reduced to five images based on absorptions at 1735, 1241, 1016 (ester functionalities), 1892 (crystalline band), and 908 (vinyl unsaturation) cm−1 show that a polyethylene film contains an inclusion of an acetate-containing material. Reflectance infrared spectra reduced to an image based on a ratio of the absorptions at 1738 cm−1 (ester carbonyl) and 1705 cm−1 (acid carbonyl) show that the streak in a film of an ethylene-acrylic acid/ethylene-methacrylate copolymer blend on aluminum foil contains more of the ester component of the blend.

Angular Dependence of Diffuse Reflectance Infrared Spectra. Part III: Linearity of Kubelka-Munk Plots

Abstract: The linearity of plots of the Kubelka-Munk function against concentration is investigated for weak and strong absorption bands in the spectrum of a typical organic analyte, caffeine. The linear region is extended when measurements are made with an off-axis optical geometry, in comparison to an in-line configuration. For the latter configuration, the linearity is improved when crossed polarizers are placed before and after the sample.

Analytical Jet Spectroscopy of Tyrosine and its Analogs Using a Pulsed Laser Desorption Volatilization Method

Abstract: Pulsed laser desorption is used as a means of volatilizing nonvolatile and thermally labile molecules for entrainment into a supersonic jet expansion. The jet expansion provides ultracold molecules whose sharp spectral features are probed by resonant two-photon ionization spectroscopy in a time-of-flight mass spectrometer. Such jet-cooled spectra are demonstrated for tyrosine and related structural analogs. Despite the similarity between these tyrosine-based compounds, electronic spectroscopy is shown to be a sensitive probe of small structural changes in these related biological compounds.

Analysis of Intact Tablets by Near-Infrared Reflectance Spectrometry

Abstract: A typical pharmaceutical tablet is too small for analysis in an ordinary NIRA instrument. An intact tablet will not begin to fill a solid-sample holder designed for use with a powdered sample. Grinding the sample is likewise unproductive, as a single tablet also does not provide enough powder for the sample cup. Potential NIRA applications such as the detection of product tampering and even routine quality control are unnecessarily complicated by this grinding requirement. A method of analyzing single, intact tablets using a double-reflecting aluminum sample holder is described in this report. The integrity of the sample is preserved during the analysis, allowing the tablet to be sold or consumed after the procedure.

A solution to the artifact problem in Fourier transform vibrational circular dichroism

Abstract: Using a newly constructed FT-IR vibrational circular dichroism (VCD) instrument, we have found that elimination of the ellipsoidal collection mirror before the detector and its replacement by a lens leads to a significant improvement in the absorption artifact problem seen previously in FT-IR/VCD. In the mid-IR region, we have been able to measure VCD of a single enantiomer for molecules such as α-pinene, 3-methyl-cyclohexanone, and dimethyltartrate. More importantly, this reduction in artifact level brings the FT-IR/VCD band shape of some particularly-difficult-to-measure bands, such as carbonyl stretches, into better agreement with those found in dispersive measurements. These results imply that the dispersive results are reliable, though of lower resolution than those obtained with the use of FT-IR/VCD.

Ultraviolet Resonance Raman Spectra of Escherichia coli with 222.5-251.0 nm Pulsed Laser Excitation

Abstract: Resonance Raman spectra of the gram-negative organism, Escherichia coli, have been obtained with 222.5-, 230.6-, and 251.0-nm excitation, and the results have been compared with those reported earlier for 242.4-nm excitation. Major changes in bacterial spectra have been observed with changes in exciting wavelength. The origins of the major peaks in each spectrum have been explained primarily in terms of contributions of nucleic acid bases and aromatic amino acids. As an aid in making assignments, spectra of aromatic amino acids, nucleosides, and mixtures of the two have been obtained at each wavelength used to excite bacterial spectra. Background fluorescence has been observed to be negligible below 251 nm. Selective excitation of bacterial nucleic acid and protein components has been done with ease. Results suggest that an extension of the exciting wavelength range to 190-220 nm will allow the selective excitation of additional cell components.

Intriguing Absorption Band Behavior of IR Reflectance Spectra of Silicon Dioxide on Silicon

Abstract: Infrared reflectance spectra of a thermally grown 30-nm SiO2 film on a Si wafer were measured as a function of incident angle and polarization Spectra measured with s-polarized light resemble the published extinction coefficient for SiO2 The p-polarized spectra show significant distortions at all incident angles Bands change in frequency and intensity and can even invert as the incident angle increases beyond the Brewster angle of the Si substrate Spectral simulations using the classical electromagnetic equations reproduce these distortions

Estimation of the Effective Dielectric Constant of Cyclodextrin Cavities Based on the Fluorescence Properties of Pyrene-3-Carboxaldehyde

Abstract: The microscopic dielectric constant of the cyclodextrin (CD) interior is a fundamental property of these microheterogeneous environments. Polarity and dielectric constant play a crucial role in the understanding of spectroscopy and chromatography. Bulk measurements of these properties are not suitable for the evaluation of inclusion cavities, and frequently luminescent probes are employed to estimate these parameters. Recently, polycyclic aromatic hydrocarbon probes have been used to evaluate the effective polarity of the CD cavity, and the approximate formation constants were estimated for inclusion complexes of the variety: PAH + CD ⇌ [PAH − CD] (1) [PAH − CD] + CD ⇌ [CD − PAH − CD] (2) [PAH − CD] + PAH ⇌ [PAH − PAH − CD] (3) [PAH − CD] + [PAH − CD] ⇌ [CD − PAH − PAH − CD] (4)

In situ measurement of the FT-IR spectra of phospholipid monolayers at the air/water interface

Abstract: External reflection Fourier transform infrared spectroscopy has been applied to the in situ measurement of the infrared spectra of insoluble phospholipid monolayer films that were spread at the air/water interface. The spreading conditions for the phospholipids [1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)] were chosen to mimic those of liquid-condensed and liquid-expanded surface films, respectively. It is shown that the infrared reflection spectra can identify vibrations due to the hydrocarbon acyl chains, carbonyl ester, and phosphate groups for these monolayer films on water. Detailed examination of the frequencies and polarization properties of the C-H stretching region shows that the liquid-condensed DSPC acyl chains are in a rigid, mostly all-trans configuration, while the liquid-expanded DMPC acyl chains are highly fluid. The spectra demonstrate that the external reflection FT-IR experiment can differentiate between the physical conformation of monomolecular films when water is used as a reflective substrate.

Aqueous Infrared Pharmaceutical Analysis of Two Choline Compounds by Flow Injection Analysis Using the CIRCLE Cell

Abstract: The advent of the cylindrical internal reflectance (CIRCLE) cell has solved the difficult problem of quantitation of solutes in aqueous solutions by infrared spectroscopy. By a combination of flow injection analysis (FIA) and Fourier transform infrared (FT-IR) spectroscopy with the CIRCLE cell, a fast, reproducible technique suited for the determination of two choline compounds of pharmaceutical interest—succinylcholine chloride and bethanechol chloride—was developed. Quantitation was carried out with the use of either the ester band at 1075 cm−1 for bethanechol or the trimethyl quaternary ammonium band at 953 cm−1 for succinylcholine. Linearity of standard solutions was achieved from about 0.5 to at least 50 ppt for both compounds, and detection limits of 0.02% w/v were possible. A sample throughput of 60 samples/h was estimated. Analysis of pharmaceutical preparations was performed after minor sample clean-up, which was necessary only in the case of the bethanechol chloride tablets.

Micro-Raman spectrum of triuranium octoxide, U3O8

Abstract: Uranium oxides are important nuclear reactor fuels and there are at least nine oxides known, many of which are mixed valence and polymorphic. Transmission-IR spectroscopy has proven to be a useful method for characterizing the oxides. For instance, IR spectra and vibrational assignments have been reported for UO2, UO2.05, UO2.08, UO2.18, U4O9, and U3O8. Some of the results have recently been verified for UO2 and U3O8 with the use of a metal light pipe technique. Previous attempts to obtain complementary Raman data were frustrated because of the extremely dark colors of the materials. We have now shown that it is possible to surmount this problem using micro-Raman spectroscopy, and we have reported characteristic Raman spectra of UO2, U4O9, U3O7, and U3O8. However, no assignments were proposed for the observed spectra.

Step Scan Interferometry in the Mid-Infrared with Photothermal Detection

Abstract: A medium resolution mid-infrared FT-IR instrument (IBM Instruments IR/44) has been modified to do step scanning; this has been done with the use of concepts previously applied to both near- and far-infrared instruments. In this paper we illustrate the method used for driving the mirror in the step scan mode and present some preliminary results from using the instrument with photothermal detection. At the current state of development, results obtained with the use of phase modulation indicate that this method produces significantly higher signal-to-noise ratios than does the use of amplitude (chopper) modulation to generate the photothermal signal.