Extra Column Dispersion - Low Dispersion Connecting Tubes > Page 27


Low Dispersion Connecting Tubes

The ideal solution to conduit dispersion, where the sample valve and the detector sensor cell are coupled directly to the column, is, in practice, mostly impossible. Consequently, a conduit system that provides little or no dispersion would be extremely useful. In order to reduce dispersion due to Newtonian flow through an open tube, the parabolic velocity profile of the fluid must be disrupted to introduce rapid radial mixing. The parabolic velocity profile can be disturbed, and secondary flow introduced, into the tube, by deforming its regular geometry.

Dispersion that occurs in geometrically deformed tubes (squeezed, twisted and coiled) has been studied by Halasz (4, 5 and 6), 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 (8). In addition, the effect of secondary flow produced by employing serpentine shaped tubes has also been examined by Katz and Scott (9).

According to Tijssen, at very low linear velocities the variance per unit length (H) of a coiled tube is given by,


where (j) is a constant over a given velocity range and the other symbols have the meanings previously ascribed to them.