Page 143 - Tandem Techniques

Page 124

It can be seen from equation (3.24) that, at the higher linear mobile phase velocities, the value of (H) depends on (Dm) taken to the power of
0.14 and inversely dependent on the coil aspect ratio and the linear velocity. According to equations (3.23) and (3.24), at low velocities the
band dispersion increases with (u), whereas at high velocities the band dispersion decreases with (u). It follows that a plot of (H) against (u)
should exhibit a maximum at a certain value of (H). By combining equations (3.23) and (3.24), an equation can be obtained that predicts the
value of (u) at which (H) is a maximum, and is given by

where (c) is a constant for a given solute and given mobile phase.

The above equations were employed to investigate the effect of tube radius and coil aspect ratio on the onset of radial mixing in coiled tubes.
The properties of the four different coils are shown in Table 3.3.

Table 3 Physical Dimensions of Coiled Tubes Examined
Tube r (cm.) L(cm) r(coil cm) (y) L(coil)(cm.)
1 0.019 365.8 0.5 0.038 18.5
2 0.020 365.0 0.085 0.235 65.8
3 0.0127 998.0 0.0765 0.166 128.0
4 0.0127 337.5 0.0498 0.0255 73.7

The curves relating (H) and (u) are shown in Figure 3.5. It can be seen that at low linear velocities, where radial mixing is still poor, the
values of (H) increases as (u) increases. Furthermore, the dispersion in coiled tubes (1) and (2) of larger radii is greater than that in tubes (3)
and (4) which had smaller radii. At high linear velocities, where radial mixing commences, the values of (H) decrease as (u) increases. As the
range of linear velocities is approached where radial mixing dominates, the solute dispersion becomes independent of the linear velocity (u).
It is also seen