Integration of uniform porous shell layers in very long pillar array columns using electrochemical anodization for liquid chromatography

Abstract

Electrochemical anodization has been applied to grow porous shell layers of 300 nm (30 nm pores) in 5 μm diameter pillar array columns (PACs) with a spacing of 2.5 μm. Using turn structures preceded and followed by the flow distributor structures recently introduced by our group and filled with radially elongated pillars, columns with quasi unlimited channel lengths could be conceived. The uniformity of the porous PAC was assessed by determining local plate heights along the channel, which appeared to be constant. Minimal (absolute) plate heights (H) between 4 and 6 μm were obtained at optimal flow rates when imposing increasing retention factors. Upon measuring the surface area involved in chromatographic retention as an indicator of the available surface area, an increase in the surface area by a factor of about 30 compared to that of non-anodized pillars was found. On reconfiguring a commercial HPLC instrument to enable on-chip injections, 90% of the performance (expressed in theoretical plates) could be maintained for a 1 m column, while for a 25 cm column severe losses were still observed. As the corresponding pressure drop for optimal operation of retained components is on the order of 10 bar per m only, portable and cheaper HPLC devices with high efficiencies become realistically conceivable.

Advances in and prospects of microchip liquid chromatography

“…A variety of methods to increase surface area for stationary phase support have been reported. Electrochemical anodization was used to create 300 nm porous layers on 5 μm pillar arrays to increase the surface area by a factor of 30 compared to non-porous pillars (Figure 3) [56]. This electrochemical method (a progression from techniques reported earlier [57,58]) was found to be beneficial in both its uniformity across the entire column and the tunability in pore size achieved by varying the applied potential.…”

“…This electrochemical method (a progression from techniques reported earlier [57,58]) was found to be beneficial in both its uniformity across the entire column and the tunability in pore size achieved by varying the applied potential. The increased surface area allowed for significantly higher retention than the non-anodized pillars while also demonstrating plate heights on the order of 6 μm [56]. Other methods to help increase sample loadability (and/or analyte retention) have also been either modeled or demonstrated experimentally.…”

Assessment and numerical search for minimal Taylor–Aris dispersion in micro-machined channels of nearly rectangular cross-section

“…In a recent paper [23], where smaller dimensions could be achieved with advanced deep UV lithography, the employed 1.25 m spacing (32 m depth) resulted in a plate height of 2.0 m, which was roughly two times higher than what can be expected based on work with a spacing twice as large [10,11]. This was attributed to the occurrence of vertical differences in throughpore width, in turn caused by the vertical taper that is inevitably present on the diameter of the etched pillars.…”

“…During the last decade, so-called pillar array columns have been evaluated as an alternative for the particle and monolithic columns conventionally used to perform liquid chromatography [1][2][3][4][5][6][7][8][9][10][11]. Because of the maximal degree of uniformity with which micro-pillar arrays can be fabricated, they seem ideally suited to conduct chromatographic separations, as this is a separation method where any degree of band broadening caused by disorder and heterogeneity directly translates into a considerable loss in performance.…”

“…Because of the maximal degree of uniformity with which micro-pillar arrays can be fabricated, they seem ideally suited to conduct chromatographic separations, as this is a separation method where any degree of band broadening caused by disorder and heterogeneity directly translates into a considerable loss in performance. Stimulated by promising 2D computational fluid dynamics (CFD) simulations [12][13][14], a series of extensive axial dispersion (band broadening) measurement studies has been conducted and reported upon in literature [1][2][3]10,11]. When the measurements were performed in non-porous silicon pillar array columns with a small depth, i.e., with a small depth over pillar spacing (distance between pillars, e.g.…”