The present invention discloses a flow cell optical detection system comprising a light source, a flow cell and a light detector, wherein the light detector is arranged in a separate detector unit that is arranged to be releasably attached to a detector interface, the detector interface being in optical communication with the light source and comprises optical connectors for optically connecting the flow cell and the detector unit in the light path from the light source, and wherein the flow cell is an interchangeable unit arranged to be held in position by the detector unit when attached to the detector interface.