Efficiency gain limits of the parallel segmented inlet and outlet flow concept in analytical liquid chromatography columns suffering from radial transcolumn packing density gradients

The maximal gain in efficiency that can be expected from the use of the segmented column end fittings that were recently introduced to alleviate the effect of transcolumn packing density gradients has been quantified and generalized using numerical computations of the band broadening process. It was found that, for an unretained compound in a column with a parabolic packing density gradient, the use of a segmented inlet or a segmented outlet allows to eliminate about 60–100% of the plate height contribution (Htc) originating from a parabolic transcolumn velocity gradient in a dc = 4.6 mm column. In a dc = 2.1 mm column, these percentages change from 10 to 100%. Using a combined segmented in- and outlet, Htc can be reduced by about 90–100% (dc = 4.6 mm column) or 20–100% (dc = 2.1 mm column). The strong variation of these gain percentages is due to fact that they depend very strongly on the column length and the flow rate. Dimensionless graphs have been established that allow to directly quantify the effect for each specific case. It was also found that, in agreement with one's physical intuition, trans-column velocity profiles that are more flat in the central region benefit more from the concept than sharp, parabolic-like profiles. The gain margins furthermore tend to become smaller with increasing retention and increasing diffusion coefficient.

► The theoretical limits of performance gain that can be achieved using parallel segmented flow were investigated.
► The effect of two types of transcolumn gradients in packing density were investigated: parabolic and step-wise side-wall.
► The gain strongly depends on the residence time (column length and mobile phase velocity) and column diameter.
► Combining segmented in- and outlet increases the achievable performance gain.
► Columns with localized density gradients benefit more from this technique than gradual varying gradients.