Band dispersion in chromatography—a universal expression for the contribution from the mobile zone

It is generally assumed that the dispersion which is covered by the C term of Van Deemter type equations arises from processes occurring in the static zone, while the dispersion covered by the A term arises from processes occurring in the mobile zone. It is also now widely accepted that the contribution to h, the reduced plate height, from mobile zone processes increases with a modest power of ν, the reduced flow velocity. A reassessment of data acquired since the 1960s suggests that this power falls with increasing velocity, but may be relatively high at reduced velocities, ν, in the range 1–30. Data for a wide variety of materials over a wide range of ν have been re-examined and are well fitted by an equation of the form: h=B/ν+{1/A+1/(Dνn)}−1+Cν. With C≤0.02 in accordance with the theoretical value for slow equilibration in the static zone, n is found to be in the range 0.5–1.0 with the lower values applying to glass bead packings, and the higher values applying to porous spherical packing materials. The equation provides a decreasing power of velocity in the A term in agreement with experimental data. It is now clear that nearly all of the dispersion previously assigned to processes in the static zone actually occurs in the mobile zone. Accordingly, substantial improvements in column performance in LC may well be achieved by better packing of columns, or by designing structures such as monolithic beds and two dimensional designs on chips, which can provide more uniform structures than the beds of spherical particles widely used in current HPLC.