Page 141 - Tandem Techniques
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were far superior to those of coiled tubes. Furthermore, the dispersion that takes place in serpentine
tubing is practically independent of the mobile phase linear velocity and consequently, such tubes can
be used over a wide range of flow rates. The authors first examined the effect of secondary flow on
band dispersion that took place in tubes of different radius coiled to different diameters. The theory of
Tijssen [9] was successfully employed to qualitatively describe the relationship between the variance
per unit length, (H), and the mobile phase velocity.
For the sake of simplicity, the equations that Tijssen derived for radial dispersion in coiled tubes are
given in terms of conventional chromatographic terminology. At relatively low linear velocities (but not
low relative to the optimum velocity for the tube) Tijssen derived the equation,
where (j) is a constant over a given velocity range, and the other symbols have the meaning previously
ascribed to them.
It is seen that the band variance is directly proportional to the square of the tube radius and the
relationship is very similar to that derived by Golay [1] for a straight tube.
At high linear velocities, Tijssen deduced that,
where (b), is a constant for a given mobile phase
and (y), is the ratio of the tube radius to the coil radius, and was given the term the coil aspect ratio.
Consequently,
