Since both chromatography and flow injection are based on forward flow, it was natural to focus on this apparent similarity and apply the theory of flow, that served chromatographers so well, to flow injection. Indeed, a well respected theoretician (Snyder 1980), advanced an argument that flow injection would work only in narrow, extremely long conduits that will have to be served by high pressure pumps. Also the assumption that FI techniques operate within the limits of laminar flow (Vanderslice 1981, 1986) was based on calculation of Reynolds number while ignoring the purpose and experimental setup that O. Reynolds himself had purposely imposed.
Yet another point to consider, is the difference in goals between chromatography and flow injection. Thus chromatography is designed to separate analytes, as they travel downstream to be detected. In contrast, flow injection techniques are designed to mix the target analyte with reagents reproducibly and rapidly, in order to form a detectable product, on the way to the detector.
In flow injection, this is achieved by means of confluence points that add reagent(s) evenly along the entire length of dispersing sample zone. The theoretical treatment or computer modeling of this process yet has to be done.
In sequential injection, the process of mixing by dispersion is, in absence of confluence points, far more complex. As the zones are sequentially stacked into the holding coil, they penetrate each other within the parabolic flow profile. It is essential to promote this process (see Chapter 2 section 2.2.8) through selection of seqenced volumes and subsequent flow programming.
For practicing analyst the theory of FIA techniques does not offer much for everyday use. Yet it is challenging and intriguing topic, that offers insight into processes which still need to be far better understood.