Showing papers by "Peter R. Griffiths published in 1993"

Comparison of the Amide I/II Intensity Ratio of Solution and Solid-State Proteins Sampled by Transmission, Attenuated Total Reflectance, and Diffuse Reflectance Spectrometry

Abstract: The absolute and relative differences in amide I and amide II band intensities of albumin, β-lactoglobulin, and myoglobin as measured by attenuated total reflection infrared (ATR-IR) spectrometry, transmission of aqueous solutions, and KBr disks and diffuse reflectance (DR) spectrometry are compared. The amide I/II intensity ratios of the proteins sampled by ATR, DR, and transmission spectrometry of KBr disks were similar and were significantly different from the intensity ratios of the proteins in solution. The absolute amide II band intensity of dissolved proteins did not vary significantly with changes in pH. The difference in amide I/II intensity ratios between solution and adsorbed proteins was attributed to differences in secondary and possible tertiary structure. The thickness of each protein film was estimated with the use of the absorptivities calculated from the amide II band intensities of the KBr disk spectra. pH had a significant influence on the thickness of the adsorbed films. Differences in film thickness were attributed to a difference in the orientation of the protein molecules at the surface of the germanium internal reflection element.

Effects of Sample Dilution and Particle Size/Morphology on Diffuse Reflection Spectra of Carbohydrate Systems in the Near- and Mid-Infrared. Part II: Durum Wheat

Abstract: It is shown that reduction of particle size for ground durum wheat does not cause a concomitant decrease in the amount of specular reflection observed in the mid-infrared spectrum of the neat sample. Specular reflection can be reduced, but not completely eliminated, by dilution of the wheat in a nonabsorbing matrix such as KCl. The lack of dependence of mid-infrared diffuse reflection spectra of neat ground durum wheat on particle size is explained by the morphology of the surface of each particle and the low effective depth of penetration of mid-infrared radiation into particles of very high absorptivity. It is concluded that mid-infrared diffuse reflection spectra of wheats do not provide information characteristic of the overall composition of the wheat when the particle size exceeds a few micrometers, but that near-infrared diffuse reflection spectra will provide representative analytical data even when the particle size is large.

Step-Scan Fourier Transform Infrared Study on the Effect of Dynamic Strain on Isotactic Polypropylene

Abstract: Dynamic rheo-optical spectra of two isotactic polypropylene samples of different states of order were measured by step-scan FT-IR spectrometry. Experimental details and data collection considerations are discussed. The features in the dynamic spectra are found to be due to small wave-number shifts and absorption changes. The dynamic rheo-optical spectra depend strongly on the pretreatment of the samples, as well as on the polarization state of the infrared radiation used for analysis.

Step-scanning interferometer with digital signal processing

Abstract: A novel step-scan FT-IR spectrometer incorporating a digital signal processor for demodulation of the detector signal is described. The potential advantages of this method of signal processing are discussed and illustrated. The instrument is based on a commercial cube-corner interferometer which has been modified by replacement of the drive motor with a stepper motor-micrometer and piezoelectric transducer combination. The interferometer retardation is feedback controlled by a 48650 personal computer, which also controls the digital signal processor and collects spectral data. More than one phase modulation frequency can be imposed simultaneously, allowing for a multiplex advantage in photoacoustic depth profiling. Digital signal processing allows for simultaneous demodulation of multiple frequencies which would normally require several lock-in amplifiers. Data that illustrate the feasibility of these concepts are presented. The suitability of this instrument for double-modulation step-scan FT-IR measurements such as polymer stretching and electrochemically modulated step-scan FT-IR is also discussed.

Step-scan FT-IR external reflection spectrometry of the electrode/electrolyte interface by potential modulation

Abstract: The application of conventional rapid-scanning Fourier transform infrared (FT-IR) spectrometry for infrared spectroelectrochemical measurements involving potential modulation is limited by the cross talk between the Fourier frequencies and the frequency at which the potential is modulated. In this report we show the first demonstration of electrochemically modulated infrared spectrometry using a step-scanning phase-modulated interferometer. The wavenumber of the absorption band of CO adsorbed on polycrystalline Pt electrode was modulated through the electrochemical Stark effect, and the detector signal was demodulated using two lock-in amplifiers, one for the phase modulation and the other for the potential modulation

Short-Wave near Infrared Spectra of Organic Liquids

Abstract: Multi-quanta vibrational transitions are discussed in terms of the local mode model in order to develop a better understanding of the features observed in the short-wave near infrared (700–1100 nm) spectra of organic liquids. It is demonstrated that the spectra of simple alkanes and alcohols exhibit local mode features, and that these features are characteristic of molecular structure and will, therefore, be useful for quantitative and qualitative analysis.

Use of Buffered Solvent Systems with Concentric Flow Nebulization Liquid Chromatography/Fourier Transform Infrared Spectrometry

Abstract: Identifiable Fourier transform infrared (FT-IR) spectra of species eluting from a high-performance liquid chromatograph (HPLC) in buffered mobile phases are shown. The spectra are measured with the use of an online solvent-elimination HPLC/FT-IR interface based on direct-deposition concentric flow nebulization. Both volatile and nonvolatile buffers were used, in mobile phases consisting of 63-100% water. Examples of spectra of components separated by microbore HPLC using these buffered systems are shown. For the systems using nonvolatile buffers, spectral subtraction is needed to obtain analyte spectra, and absorption bands due to the buffer are rarely completely eliminated. For systems using volatile buffers, very little or no spectral subtraction is needed at low buffer concentrations. Minimum identifiable quantities in the low-nanogram range were achieved.

Investigation of Polysaccharide Adsorption on Protein Conditioning Films by Attenuated Total Reflection Infrared Spectrometry.

Abstract: The study of microorganisms at the aqueous/metal interface is of great interest as their presence is often associated with surface deterioration or corrosion. Surfaces submerged in aquatic environments rapidly become fouled with organic macromolecules, such as proteins and/or glycoproteins. Bacteria adhering to this conditioning film typically produce extracellular polysaccharides that anchor the cells to the substrate. In a previous paper, the kinetics of polysaccharide adsorption and desorption on protein conditioning films formed on bare germanium internal reflection elements (IREs) were investigated in an effort to gain a better understanding of the fouling process. This work has been extended to the study of adsorption phenomena at an aqueous/copper interface. Alginic acid and dextran were exposed to protein-conditioned and untreated copper films. In the absence of a protein biofilm, greater erosion of the Cu film was observed with flowing saline than with a solution of alginic acid in saline, and more Cu was removed at pH 4.8 than at pH 7.0. The neutral polysaccharide was less corrosive than alginic acid. The presence of a film of bovine serum albumin (13.5 nm) conditioning film did not shield the Cu film from the corrosive effects of either polysaccharide. In all experiments, as Cu slowly eroded from the surface of the IRE, metal islands formed leading to the observation of surface-enhanced infrared absorption (SEIRA). The intensity of the C-O stretching band of adsorbed dextran on Cu islands was over an order of magnitude greater than would be observed with a uniform Cu film of the same thickness. Enhancement of the amide II band of BSA was also observed. In general, the effect of SEIRA complicated spectral processing and interpretation and thus interfered with the determination of surface coverage and polysaccharide adsorption/desorption kinetics on protein conditioning films. Copyright 1999 Academic Press.

Classification of Condensed-Phase Infrared Spectra by Substructures Using Principal Components Analysis

Abstract: Expert system rules have been developed through the use of principal components analysis (PCA) for discriminating compounds containing large molecular substructures from their condensed-phase infrared spectra. With the use of this approach, the presence of substructures such as those of barbiturates, cocaines, and amphetamines can be recognized automatically. The classification rule can be generated from a PCA of a small training set of infrared spectra of compounds containing the substructure of interest. One important use of this type of expert system is the analysis of direct-deposition capillary gas and supercritical fluid chromatographic separations In which many peaks are eluted and analyzed by FT-IR spectrometry but only one or two contain the substructure of interest. Classification rules for substructures are easy to generate with little or no knowledge of characteristic group frequencies.