Flow Injection (FI), the first generation of FIA techniques, is a flow based technique. It can be performed  at a constant flow rate (cFI) or, as for the first time described in this chapter in a programmed flow mode (pFI). In FI  simplest form, the sample zone is injected into a flowing carrier stream of reagent. As the injected zone moves downstream, the sample solution disperses into the reagent, and a product begins to form at the interface between the sample zone and the reagent. A detector placed downstream records the color or another parameter as it changes due to the passage of the derivatized sample material through the flow cell. The detector readout is therefore the result of two processes that take place simultaneously: sample dispersion and ensuing chemical reactions. (Ruzicka & Hansen 1975, Theory 1978)

Advances in hardware and computer control, allow further refinement of FI technique, by applying flow programming, so that  each step of an assay protocol, is performed at an optimized flow rate. The advantage of flow programming is the increased sensitivity and detection limit of most reagent based assays, achieved by adjusting the flow rate and incubation time from the computer keyboard, without need for system reconfiguration. Also pFI compared to cFI uses less reagents and generates smaller volumes of toxic waste .

J. Ruzicka & E. H. Hansen, Anal. Chim. Acta , 78, 145 (1975)
J. Ruzicka & E.H. Hansen, “Flow Injection Analysis” 2nd ed. J. Wiley, N.Y. 1988