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.