Showing papers by "Peter R. Griffiths published in 1990"

Performance characteristics of a real-time direct deposition gas chromatography/fourier transform infrared spectrometry system

Abstract: In this paper, the real-time measurement of the infrared spectra of subnanogram quantities of analytes that have been separated by gas chromatography (GC) is demonstrated for the first time. Eluites trapped as small spots on a moving window held at 77 K are passed through the beam of a Fourier transform infrared (FT-IR) spectrometer that is 100 {mu}m square. Real-time GC/FT-IR spectra obtained from injected quantities as low as 50 pg are shown and the effect of postrun signal-averaging in improving the signal-to-noise ratio is demonstrated. With this technique, subnanogram quantities of n-alkanes at least up to C{sub 22}H{sub 46} can be discriminated. Identification of polar, hydrogen-bonded analytes is accomplished by spectral searching against standard libraries of compounds prepared as KBr disks. For less polar analytes, spectral searching against a database of vapor-phase spectra is also successful.

Combined deconvolution and curve fitting for quantitative analysis of unresolved spectral bands

Abstract: Curve-fitting programs usually only yield accurate parameters for unresolved spectral bands when the separation of neighboring bands exceeds their average half-width. By reduction of the width of each spectral feature in a heavily overlapped band multiplet by Fourier self-deconvolution (FSD), the conditioning of curve-fitting algorithms is improved to the point that the area of each band in the multiplet can be estimated with much improved accuracy. The practical limitations of this approach are discussed in this paper. Most FSD programs are strictly applicable to Lorentzian bands; the effect of increasing the Gaussian fraction of isolated bands on the accuracy of band area estimation has also been investigated. Finally, the application of combined deconvolution and curve fitting to synthetic band multiplets in which the relative peak absorbances and bandwidths are varied is shown to yield accurate estimates of band areas until the band separation approaches the width of the instrument line-shape function, which is determined by the truncation, exponentiation, and apodization of the Fourier domain array.

Effect of Scattering Coefficient on Diffuse Reflectance Infrared Spectra

Abstract: The application of Kubelka-Munk theory for thin samples for which the Kubelka-Munk function would not be expected to yield a quantitative relationship between reflectance and analyte concentration is examined. For thin samples, the effective penetration depth of radiation is dependent on both the absorption and the scattering coefficients. Different penetration depths result in inaccurate relative reflectance values being measured, with the magnitude of the error being dependent on the absorption coefficient of the sample and the scattering coefficient and thickness of the reference. For diffusely reflecting samples, previous experimental results are explained on the basis of the magnitude of the scattering coefficient for the surrounding matrix. The scattering coefficient of ground potassium chloride was determined experimentally to be approximately 50 cm−1. It is shown that "infinite depth" for nonabsorbing diffusely reflecting materials would not be achieved with thicknesses less than 1 mm, necessitating the use of the full Kubelka-Munk equations to explain band intensities of thin samples.

Comparison of diffuse reflectance and diffuse transmittance spectrometry for infrared microsampling

Abstract: Etude comparative des deux techniques d'analyse. Influence de l'epaisseur de l'echantillon, donc de la volatilite de la phase mobile, dans le cas de couplage de la spectrometrie IR avec la chromatographie

Coblentz Specifications for Infrared Reference Spectra of Materials in the Vapor Phase above Ambient Temperature: Prepared by the Spectral Evaluations: Committee of the Coblentz Society

Abstract: A. Introduction. These specifcations are designed to guide evaluators in selection of spectra for inclusion in Coblentz reference data bases; they replace the specifications published earlier. 1 For comparison and clarity, the format of these specifications will follow that of those specifications previously stated. ~ Class II spectra are defined 2 as reference spectra obtained on high-quality commercial instrumentation, operated at maximum efficiency under conditions consistent with acceptable laboratory practice. Class I spectra are defined 3 as \"standard spectra\" because they are of sufficient quality to be acceptable as physical constants of the substances under precisely defined conditions of measurement, and further refinements in spectrophotometric technique will not be expected to change them significantly. It has been the opinion of the Board of Managers of the Coblentz Society that the specifications for such standard spectra should be defined by international agreement and will not be specifically approached in this document. Class III spectra are defined ~ as those which have been obtained with sufficient accuracy to be useful in the identification of unknown ma-