FTIR microscopy and confocal Raman microscopy for studying lateral drug diffusion from a semisolid formulation

Fourier transform infrared (FTIR) microscopy was applied to obtain information on lateral drug diffusion of dithranol in artificial acceptor membranes. Lateral (2D) drug distribution into an artificial membrane was investigated on an area of 300 μm × 1000 μm with a lateral resolution of 25 μm × 25 μm by integrating a specific IR band located at 1430 cm−1. The concentration profiles show a heterogeneous distribution of dithranol particles resulting in non-uniform drug diffusion. Use of the FTIR microscope either in the transmission or in the reflection mode was restricted to a thickness of the DDC membrane <15 μm. The third dimension (depth profile) was analysed by means of confocal Raman microscopy (CRM). In an artificial membrane, the depth range from a minimum of 1.5 μm up to a maximum of 49 μm was analysed for dithranol distribution.