Page 124 - Tandem Techniques

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phase at different positions across the tube. This velocity profile causes the solute contained in the
mobile phase close to the wall to move very slowly and that at the center to move more rapidly.

Figure 3.1
The Parabolic Velocity Profile of a
Fluid Passing Through an Open Tube

As a consequence, any solute molecules in the center of the tube will be carried rapidly down the center
of the conduit, and away from those situated close to the walls. This process will obviously result in the
dispersion of the solute band. Furthermore, the dispersion will occur irrespective of there being
stationary phase on the walls, as in a chromatographic column, or if the walls are bare, as in a simple
tubular chromatography conduit. It will be seen that there is another dispersion process that takes place
in an open tube, but the contribution is usually not significant at the flow rates normally employed in
chromatographic development.

The band spreading that takes place in conduits containing local concentrations of solute vapor, carried
by a flow of inert gas, was critically examined by Golay in the late 1950s. Golay employed electrical
analogies to develop an equation that quantitatively described the dispersion of a band of solute vapor
in an open tube, the internal wall of which was coated with stationary phase. The Golay equation [1], as
it is called, expressed the variance contribution per unit length of tube, in terms of the velocity of the
mobile phase through the tube and certain