The underlying operational principle of all flow based techniques is the impulse-response operation. In the case of FIA techniques, the initial square wave input, provided by sample injection is transformed into a response function by a modulator, within which two simultaneous processes occur:
physical dispersion of the sample zone within the carrier stream, and
chemical reaction of the sample constituents with the surrounding medium.
The physical processes are studied by means of dye tracer experiments, that yield
Residence Time Distribution curves (RTD). This experimental technique does not provide information on underlying mechanism of the dispersion processes, since it treats the flow system as a “black box”. It is, however a valuable tool since it allows study contribution of the principal components of the system (injector, reactor, detector) and parameters (flow rate, flow reversal amplitude) to the sample zone treatment – and ultimately allows optimization of system performance. Note that the impulse-response concept is applicable not only to constant flow rate methods (such as FI and chromatography) but also to methods based on programmable flow (such as Sequential Injection).