*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 L**d** *

*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.*