Additionally, to take the different material and shape of the ATR into account, the focal length of the illumination system needed to be adjusted. This was implemented through a removable plano-concave ZnSe lens in the focusing beam path before the final focusing lens. A kinematic magnetic mount of the plano-concave lens allowed to change between prisms and flat carriers quickly. Sample Preparation. The materials used for the polymer blend studied in this work are a mixture of poly methyl methacrylate (Mw=350K, Sigma Aldrich) and polystyrene (Mw=250K, Sigma-Aldrich) spin cast from toluene (AnalaR Normapur, VWR) solution. For the measurements in water purified Milli-Q® water is used.