Extra Column Dispersion - Low Dispersion Connecting Tubes > Page 28

Equation (14) is the same as the function derived from the Golay equation for a simple straight tube when operated at low mobile phase velocities. However, at higher velocities, the momentum of the fluid as it changes direction while flowing round a curved tube, introduces the radial flow that breaks up the parabolic velocity profile. As a consequence, the dispersion is reduced. At high linear velocities, Tijssen deduced that,


where (b) is a constant for a given mobile phase and (f) is the coil aspect ratio (the ratio of the tube radius to the coil radius),


Equation (14) shows that, at high velocities, (H) is now determined by (Dm) to the power of 0.14 and is inversely proportional to the coil aspect ratio and linear velocity. It follows, that, at low velocities, the band dispersion increases with (u), whereas at high velocities, the band dispersion decreases with (u). It follows that a curve relating (H) to (u) will show a maximum at a certain value of (u). The equation giving the value of (u) at which (H) is a maximum was deduced by Tijssen to be,