Theoretical Comparison of Pillar Shape Influence on Band Broadening in Electrically and Pressure Driven flows through Ordered 2-D Porous Chromatographic Media

Despite great advantages in chromatography have been achieved, the search for better
separation media and methods resulting in lower theoretical plates in less time never
stopped. Since the introduction of capillary electro-chromatography (CEC) it is well
established that electrically driven (ED) flows produce much smaller theoretical plate
heights than pressure driven (PD) flows, yielding better separation efficiency over
high-performance liquid chromatography (HPLC) [1]. This is mainly due to the
characteristic plug-flow profile of the electroosmotic flow (EOF), which results in an
alternative way the ED velocity profile reshapes when passing through a tortuous pore
structure with undulating cross section [2]. Furthermore, the impact of packing
structure imperfections plays such a great role that it is considered the most important
contribution to the total band broadening in modern separation devices [3]. As it's not
possible to fix the particles of packed bed columns on well defined positions,
alternative systems based on micro machining structural arrays of pillars in a solid
substrate were developed [4].
We report on the impact of the pillar shape in these systems on the band broadening for
ED and PD flows. For this theoretical comparison we preferred our 'in-house' build
CFD package over commercial packages, which resulted in faster calculation speeds
and the possibility to investigate bigger problems more accurate. The plateheight
values were fitted to the well known Knox equation (h = Av1/3 + B/v + Cv) to provide a
comparison method for the band broadening results. We conclude that more elongated
shapes are to be preferred if they maintain a more uniform pore space. These data
perfectly a greed w ith findings f or PD flows in s imilar media [5]. F or ED flow s we
found similar results for the B-term while for the A-term in the Knox equation - an
indication for the degree of uniformity in the packing (eddy diffusion contribution)-
the value was smaller regardless of the used geometry. This is due to a more uniform
flow in the pores caused by the plug-flow profile of EOF. As could be expected proved
the C-term value lower for all the ED cases but with no significant preference for the
pillar shape.
[1] M.M. Dittmann, K. Wienand, F. Bek, G.P. Rozing, LC-GC, 13 (1995) 800
[2] A. Van Theemsche et al., Anal. Chem., 76 (2004) 4030-4037
[3] Knox, J. H., J. Chromatogr., A, 960 (2002) 7-18
[4] M. De Pra et al., Anal. Chem., 78 (2006) 6519-6525
[5] J. De Smet et al., Anal. Chem., 76 (2004) 3716-3726