Method

Means for monitoring the permeation of a permeant of interest between a first substance 13 and a liquid 14 across an interface membrane 11, the permeant being formulated in either the first substance or liquid, comprising; providing a UV radiation source 2 to produce a UV beam 10, containing a sample of the first substance and liquid separated by the interface in a sample assembly 4 positioned downstream in the beam such that there is a path length throughout an area of the sample which is or includes an area of first substance or of liquid in which the permeant of interest is formulated, generating a UV beam onto the area of the sample, detecting UV radiation transmitted through or reflected from the sample using an area detector 7, and manipulating the detected UV radiation into a representation of a characteristic of the permeation.