Disclosed is an optical flow cell (300′) comprising: a housing (910) forming; an enclosed and elongated fluid channel (920) arranged along a first axis (923); a first light guide (961) and a second light guide (962) generally concentrically arranged along a second axis (970) and on opposite side walls of the fluid channel, said first and second light guides having ends (961c,962c) removed in situ to provide a sensing gap (d).