An optical apparatus and method comprising a light source, an array detector for area imaging and an optical cell assembly. The optical cell assembly comprises a chamber which is arranged to receive a sample of a material including an analyte, a fluid inlet and a fluid outlet coupled to the chamber. A fluid dissolution medium stream passes through the chamber such that the sample can dissolve into the dissolution medium. The chamber is in at least one light path created between the light source and the array detector. The array detector comprises a two dimensional array of detector locations arranged to provide an output signal indicative of the light absorbance of the analyte within the chamber such that the output of the array detector is indicative of the concentration profile of the analyte near the surface of the sample.