NIR Radiative Transfer Investigations to Characterise Pharmaceutical Powders and Their Mixtures

Abstract

A measurement and data evaluation technique to determine quantitatively and separately the scattering and absorption coefficients of disperse media has been applied to pharmaceutical powders and their mixtures. This technique uses a 3-flux approximation of the equation of radiative transfer to evaluate analytically directional-hemispherical reflectance and transmittance data with the boundary conditions adjusted to the experimental set-up in order to derive the scattering and absorption coefficient. The performance of this technique is demonstrated by applying it to a variety of pharmaceutical powders and comparing it to Kubelka–Munk's 2-flux approximation. By applying this technique, more information about the investigated powders (e.g. particle size, degree of agglomeration or chemical composition) compared to diffuse reflectance measurements on optically thick samples can be obtained. From the latter only the ratio of the scattering to the absorption coefficient is determined. Using lactose and ascorbic acid, the effects of varying particle size and particles size distribution on the scattering and absorption coefficients are shown. Additionally this technique has been applied to powder mixtures with different contents of acetylsalicylacid and microcrystalline cellulose to demonstrate the usefullness of this technique for quantitative analysis.