Abstract: A flow cell for transporting fluid in a radiant energy field includes a cell structure having an open channel, and one or more end caps having a protrusion extending therefrom, wherein the protrusion may be inserted into the open channel to create a fluid seal. In a particular embodiment, the end caps include open channels for transporting fluid and radiant energy therethrough, and the open channel in the cell structure includes an efficient radiant energy transmission lining that is spaced from the end cap protrusions, thereby forming a gap volume in the flow cell open channel, which gap volume may be calibrated such that radiant energy losses may be standardized in respective flow cells transporting fluids having various indexes of refraction.