QUANTITATIVE DIFFUSE REFLECTANCE AND TRANSMITTANCE INFRARED-SPECTROSCOPY OF NONDILUTED POWDERS

An adaptation of Kubelka's general model of diffuse reflectance and diffuse transmittance of light to nonideal scattering samples is proposed. It is applied to quantitative Fourier transform infrared spectrometry of nondiluted surface-derivatized silica powders. Corrections of the measurements are introduced in order to take into account common problems that often prevent a truly quantitative application of the DRIFTS analytical method. Effects on the measured data due to the background absorption of the sample and to the specular reflection on its surface are discussed and quantitatively corrected for. The possible existence of gradients of the optical properties of the powder medium is taken into account by a numerical adaptation of the model to inhomogeneities of the phenomenological absorption and scattering coefficients K and S. Model systems constituted of two types of silica powders of very different morphology are prepared. Known concentrations of molecules carrying a cyano functional group as chromophore were covalently anchored on the powder surface. Application of the adapted model gives a quite good description of the measured diffuse reflectance and diffuse transmittance in these cases where the simple Kubelka-Munk function fails.