Chemiluminescence (CL) is a process of producing electromagnetic radiation as a result of a chemical reaction. The radiation may be UV, visible, or near infrared, and the energy is often provided by an oxidant. Since the method does not rely on an external light source, there is no background illumination and therefore a  well shielded flow cell yields a well defined response with a low background . Because the light output is transient, the challenge in instrument design is in efficient rapid mixing of sample with reagents, short  travel time between the mixing point and the detector, and in interfacing of the detector with the flow cell.
Almost  all  flow based chemiluminescence systems have used coiled flow cell interfaced directly with a photomultiplier (A). The alternative is a linear flow cell connected to PMT by  optical fiber, designed by Li and Dasgupta. Their pioneering work used 15 cm long liquid core waveguide (LCW),  for flow injection assay of ammonia.
In this section performance of the coil cell is evaluated by means of  GLOCELL (C) (Global FIA) which uses a precisely machined coil (D) (volume 160 microL) interfaced with PMT by a sapphire window (J. M. Terry The principal component  of the linear flow cell is a straight piece of 0.8mm I.D. PEEK tubing precisely aligned with 0,5mm I.D. quartz optical fiber cemented into PEEK tubing of the same diameter as the PEEK tube serving as a flow cell (E). This cell is not  LCW, it is a collimator and therefore it has to be held straight by external tubing, which also serves as a light  shield. (2.3.6). The volume  of 7cm long cell shown here is 35 microL.

Flow Cell Design for Chemiluminescence
As seen on the enclosed photographs, the linear flow cell (F) is more compatible with LOV platform then the GLOCELL (C) and it is also easier to fabricate and to modify.  When testing performance of these flow cells the same PMT (P25532, Sens-Tech, UK ) was used. All experiments were carried at 35 C by thermostating the holding coil.

J. M. Terry, J. L. Adcock, D. C. Olson, D. K. Wolcott, C. Schwanger, L. A. Hill, N. W. Barnett,
P. S. Francis. Anal. Chem., 2008, 80, 9817
J. Li, P.K.Dasgupta,  Anal. Chim. Acta. 398(1999)33.