Continuous constant flow rate is an inefficient   platform  for performing mixing, separations, incubation, and monitoring, because these operations require different times frames for optimum performance. Therefore transporting  samples trough the flow path at a constant flow rate, compromises  efficiency of an assay, as much  as it as it would  impair driving of a car, equipped with only one forward gear. Curiously, almost all flow injection protocols are still   based on continuous flow basis, the legacy of Auto Analyzer .

Programed Flow Injection (pFI) introduced for the  first time in this Tutorial (1.2.21.), employs  different flow rates within an individual assay cycle in order  to enhance the analytical readout, reduce reagent consumption and waste generation.

For assays carried out  in  a traditional  cFI system,  operated at constant flow rate,  the only way to increase the incubation time and sensitivity of measurement, is to increase the length of reaction coil.  This increases dispersion of sample, decreases sampling  frequency and generates an excessive volume of chemical waste. By slowing flow rate, when the sample is in a short reaction coil , the incubation time  is increased, without increase of dispersion and the sensitivity of assay is enhanced.   A conventional cFI instrument can be upgraded in this way, if the flow rate of  peristaltic pump can be programmed, to mix sample with reagent at fast 100% flowrate , incubate reaction mixture at 2% flow rate, and monitor signal at 50% flow rate (Section 1.2.21.)

Flow Programming for Flow Injection

Once you have exhausted all possibilities and failed, there will
be one solution, simple and obvious, highly visible to everyone else.   

The miniaturized pFI on lab-on-valve platform uses an advanced flow programming, similar to Sequential Injection. It applies reversed flow to inject sample into holding coil,  upstream from the multiposition valve, and combination of forward and reversed flow to merge sample with reagent at a confluence point situated at the central port of the multiposition valve (2.2.42.B.). The result is a substantial reduction of regent consumption and waste generation. Since microfluidic manipulations can be combined in many different ways, thy same instrumental setup will accommodate a variety of reagent based assays without need for reconfiguration of the flow path (1.2.25.)