Long Optical Path Flow Cells
In pursuit for higher sensitivity of spectrophotometric assays, much attention has been given to Liquid  Core Waveguides (LCW) , while simple, less expensive and fouling free, home made alternative, the Gath's cell is available.

Dallas T. and Dasgupta P.K. TrAC, 23, 385 (2004)

Garth Klein, Private Communication 2008