A surface plasmon apparatus includes a light source, a sensor unit for Surface Plasmon Resonance (SPR) which includes a transparent sensor structure forming at least one wall of a cavity, the wall being defined by a concave inner surface and a convex outer surface, wherein the inner surface is provided with a layer of a conductive material capable of supporting a surface plasmon, a flow structure in the cavity so as to form at least one compartment for sample between the flow structure and the inner wall of the cavity, a detector for detecting reflected light from the sensor unit, and a processing unit.