Page 144 - Tandem Techniques

Page 125

the maximum value of (H), for any particular coil, occurs at different values of (u) depending on the
combined values of (r) and (y). In general, it would seem that a high coil aspect ratio reduces both the
maximum value of (H) and the value of (u) at which it occurs.

Figure 3.6
Curves Relating Plate Height against Mobile
Phase Velocity for Different Coiled Tubes

It is interesting to note that although the straight tube theory of Golay is not applied to coiled tubes, his
equation can be employed to qualitatively explain the shape of the curves given in Figure 3.5. At low
values of the mobile phase velocity the effect of longitudinal diffusion dominates, but as the velocity
tends to approach the optimum, the resistance to mass transfer term begins to increase and the value of
(H) also rapidly increases. However, at higher velocities, the magnitude of the diffusivity of the solute
in the solvent dramatically increases as a result of induced radial flow, eventually reducing the
resistance to mass transfer factor to virtually zero. This results in a corresponding dramatic reduction in
the value of (H). Finally, at very high velocities, the greatly reduced longitudinal diffusion effect again
dominates. At this point, the value of (H) is very small and, in fact, decreases even further as the mobile
phase velocity is increased.