Methods of increasing the sensitivity of uv and visible spectrophotometric analysis have generally been confined to decreasing the volume of sample solution required. Some workers placed inserts in conventional cuvettes which limit the sample volume just to that occupied by the beam in passing through the cell. Others designed special absorption cells with apertures much less than the cross section area of the beam.
John H. Gould, "Construction and Use of Reflecting Multiple-Pass Absorption Cells for the Ultraviolet, Visible, and Near Infrared," Appl. Spectrosc. 25, 103-105 (1971)