Diffusion in FI and SI Channel.
In aqueous  solutions the diffusion coefficients of ions have values that at room temperature range from 6x10^(-10) to 2x10^(-9) m2/s. For biomolecules the diffusion coefficients range from 10^(-11) to 10^(-10) m2/s. Derived from the Fick's Law, the diffusion length Ld

allows an estimate of  distance traveled by an ion, or by a molecule of the diffusion coefficient D within a time interval of t. Thus  at a  room temperature biomolecules will cover a distance between 2 to 20 mm/sec. Since  a sample zone travels through a FIA channel at a  rate of 10cm/sec, diffusion has no influence on axial dispersion at these conditions. As far as radial dispersion is concerned in a typical 0.8mm I.D. channel, the element of fluid situated at the outer wall would reach the central streamline at best in 20 seconds ( if traveling in a straight line). Therefore a tail section of sample zone will not clear the flow channel sufficiently fast, to allow a practical sampling rate ( say 2 injections/minute) to be achieved. Since at strict laminar flow conditions the diffusion is the only
mean of mass transfer, mixing of sample with reagents will not take place fast enough and for this reason alone FI technique was not deemed to be feasible.

NOTE : Recently published work on  convection/diffusion dispersion in FI and SI system yielded diffusion coefficients two order of magnitude grater, than those obtained by a conventional method, confirming that the radial mass transfer is faster than it would be at  conditions of strict  laminar flow.
T. Sokalski, M. Kass, Ch. Mueller, A. Ivaska. J. Flow Injection Anal., Vol. 25, (2008) 156