Experimental investigation of the band broadening originating from the top and bottom walls in micromachined nonporous pillar array columns.

We report on the experimental investigation of the effect of the top and bottom wall plates in micromachined nonporous pillar array columns. It has been found that their presence yields an additional c-term type of band broadening that can make up a significant fraction of the total band broadening (at least if considering nonporous pillars and a nonretained tracer). Their presence also induces a clear (downward) shift of the optimal velocity. These observations are, however in excellent quantitative agreement with the theoretical expectations obtained from a computational fluid dynamics study. The presently obtained experimental results, hence, demonstrate that the employed high aspect ratio Bosch etching process can be used to fabricate micromachined pillar arrays that are sufficiently refined to achieve the theoretical performance limit.

A discussion of the possible ways to improve the performance of silica monoliths using a kinetic plot analysis of experimental and computational plate height data.

Using kinetic plots to analyse the performance of some of the best silica monoliths found in the literature shows that the current generation of silica monoliths outperform particulate beds only in the high plate-number region (roughly N > 40000). The plots also reveal the existence of a so-called 'forbidden region' wherein no existing chromatographic support seems to be able to operate. To investigate several possible approaches to intrude this forbidden region, computational fluid dynamics simulations of the flow field and band-broadening characteristics of a simplified structural mimic of real silica monoliths were made for five different porosities (epsilon = 0.38, 0.49, 0.60, 0.72, 0.86). It was found that entering the forbidden region will necessitate new synthesis methods, yielding either a strong improvement of the structural homogeneity (if assuming constant domain size conditions) or a decrease of the domain size (if assuming constant homogeneity conditions).

Theoretical comparison of the band broadening in nonretained electrically and pressure-driven flows through an ordered chromatographic pillar packing.

Using a well-validated computational fluid dynamics simulation method, based on a multi-ion transport model, a detailed analysis of the differences in band broadening between pressure-driven (PD) and electrically driven (ED) flows through perfectly ordered, identical chromatographic pillar packings has been made. It was found that, although the eddy-diffusion band-broadening contributions were nearly completely absent in the considered structure, the ED flow still yields much smaller plate heights than the PD flow. This difference could be fully attributed to the different ways in which the ED and PD velocity profiles reshape when passing through a tortuous pore structure with undulating cross section. Whereas in the PD case the parabolic tip of the band front is continually squeezed and extended each time it passes a pore constriction, the ED flow displays some kind of band front restoring mechanism, with which the fluid elements of the band front are (at least partly) laterally re-aligned after each pore constriction passage. This could be clearly visualized from a series of step-by-step images of the progression of a sharply "injected" species band moving through the packing under ED and PD conditions.