Page 140 - Tandem Techniques

Page 122

It is also interesting to note that changing the length of the tube has the same effect on both dispersion
and pressure drop. Reducing (1) will linearly reduce variance of the dispersion and at the same time
proportionally reduce the pressure drop across the tube. It follows that adjusting the tube length is by far
the best method of controlling dispersion. Furthermore, by making (1) as small as possible, both the
dispersion and the pressure drop can be minimized. However, depending on the overall geometry of the
tandem system, the extent to which any tube component of the interface can be reduced in length may
also be limited. In practice, the internal diameter of any tubular conduit contained in an interface,
should not be made less than 0.012 cm (0.005 in. I.D.). This is not merely to limit the pressure drop that
will occur across it, but for a more mundane, but very important reason. If tubes of less diameter are
employed, they will easily become blocked.

An alternative approach to restricting the dispersion in tubular interface conduits is to utilize low
dispersion tubing. Low dispersion tubing has only recently been used in chromatographic systems and
is not, at this time, readily available commercially. Nevertheless, such conduits could be very useful in
chromatography/spectrometer interfaces and consequently they will be briefly discussed.

Low Dispersion Connecting Tubes

As already stated, the dispersion in simple open tubes results from the parabolic velocity profile of the
fluid flow. In order to reduce this dispersion, the velocity profile of the fluid must be disrupted to allow
rapid radial mixing. The parabolic velocity profile can be disturbed, and secondary flow introduced
into the tube, by deforming its regular geometry. The dispersion in geometrically deformed tubes
(squeezed, twisted and coiled) has been extensively studied by Halasz [6-8], and the effect of radial
convection (secondary flow) on the dispersion introduced in tightly coiled tubes has been examined
both theoretically and experimentally by Tijssen [9] The effect of secondary flow produced by
employing serpentine shaped tubes has also been studied by Katz and Scott [10]. It was found that the
dispersion characteristics of serpentine tubing