Column-Only Band Broadening in a Porous Shell Radially Elongated Pillar Array Column.

In the quest for better performing separation media for liquid chromatography, micropillar array columns have received great interest over the past years. While previous research was mainly focused around micropillar array columns (µPACs) filled with cylindrical pillars, this contribution discusses µPACs with rectangular pillars, which, for the first time, have been anodized and hence carry a mesoporous shell. We report on a series of on-chip measurements of the band broadening and flow permeability in a µPAC with very wide radially elongated pillars (3·75 µm) and with an interpillar distance (2 µm) between that of the first (2.5 µm) and second generation (1.25 µm) of cylindrical µPACs. Because of the extreme flow path tortuosity, this type of µPAC can produce very large plate numbers over a short distance. Despite the relatively large interpillar distance, we obtain Hmin = 0.26 µm for a nearly unretained component (phase retention factor, k' ≈ 0.24) and Hmin = 0.79 µm for a retained component with k' ≈ 3. The kinetic performance in terms of separation impedance (Ei = 19) is considerably improved compared to cylindrical pillar µPACs (Ei in range 40-50) and is in excellent agreement with the theoretical value for an open tubular channel with a rectangular cross-section (Ei = 18). This shows that rectangular µPACs can be represented as a parallel bundle of interconnected open-tubular channels.

Microfluidic Validation of the Diffusional Bridging Effect Suppressing Dispersion in Multicapillary Flow Systems.

The efficiency of liquid chromatography separations could be strongly improved by changing the current packed bed columns by a bundle of parallel capillary tubes. In practice, however, the polydispersity effect, which emanates from the inevitable small differences in capillary diameter, completely ruins this potential. The concept of diffusional bridging, introducing a diffusive cross talk between adjacent capillaries, has recently been proposed to resolve this. The present contribution provides the first experimental proof for this concept and quantitatively validates its underlying theory. This has been accomplished by measuring the dispersion of a fluorescent tracer in 8 different microfluidic channels with different degrees of polydispersity and diffusional bridging. The observed degree of dispersion reduction agrees very well with the theoretical predictions, hence opening the road to the use of this theory to design a new family of chromatographic beds, potentially offering unprecedented performance.

On-Chip Comparison of the Performance of First- and Second-Generation Micropillar Array Columns.

Because of its dimensions, the recently introduced micropillar array columns are most suited for high-efficiency liquid chromatography separations in proteomics. Unlike the packed bed columns and capillary-based column formats, the micropillar array concept still has significant room to progress in terms of the reduction of its characteristic size (i.e., pillar diameter and interpillar distance) to open the road to even higher-efficiency separations and their applications. We report here on the on-chip comparison between first-generation (Gen 1) and second-generation (Gen 2) micropillar array columns wherein the pillar and interpillar size have been halved. Because of the on-chip measurements, the observed plate heights H represent the fundamental band broadening, devoid of any extra-column band-broadening effects. The observed reduction of H with a factor of 2 around the uopt-velocity and with a factor of 4 in the C-term dominated regime of the van Deemter-curve is in full agreement with the theoretically expected gain. This shows the pillar and interpillar size reduction could be effectuated without affecting the theoretical separation potential of the micropillar arrays. Compared to Gen 1, Gen 2 offers a 4-fold reduction of the required analysis time around the optimal velocity and about a 16-fold reduction in the C-term-dominated range.

Wafer-Scale Particle Assembly in Connected and Isolated Micromachined Pockets via PDMS Rubbing.

The present contribution reports on a study aiming to find the most suitable rubbing method for filling arrays of separated and interconnected micromachined pockets with individual microspheres on rigid, uncoated silicon substrates without breaking the particles or damaging the substrate. The explored dry rubbing methods generally yielded unsatisfactory results, marked by very large percentages of empty pockets and misplaced particles. On the other hand, the combination of wet rubbing with a patterned rubbing tool provided excellent results (typically <1% of empty pockets and <5% of misplaced particles). The wet method also did not leave any damage marks on the silicon substrate or the particles. When the pockets were aligned in linear grooves, markedly the best results were obtained when the ridge pattern of the rubbing tool was moved under a 45° angle with respect to the direction of the grooves. The method was tested for both silica and polystyrene particles. The proposed assembly method can be used in the production of medical devices, antireflective coatings, and microfluidic devices with applications in chemical analysis and/or catalysis.

Structured microgroove columns as a potential solution to obtain perfectly ordered particle beds.

We report on a novel concept to produce ordered beds of spherical particles in a suitable format for liquid chromatography. In this concept, spherical particles are either positioned individually (single-layer column) or stacked (multi-layer column) in micromachined pockets that form an interconnected array of micro-grooves acting as a perfectly ordered chromatographic column. As a first step towards realizing this concept, we report on the breakthrough we realized by obtaining a solution to uniformly fill the micro-groove arrays with spherical particles. We show this can be achieved in a few sweeps using a dedicated rubbing approach wherein a particle suspension is manually rubbed over a silicon chip. In addition, numerical calculations of the dispersion in the newly introduced column format have been carried out and demonstrate the combined advantage of order and reduced flow resistance the newly proposed concept has over the conventional packed bed. For fully-porous particles and a zone retention factor of k'' = 2, the hmin decreases from hmin = 1.9 for the best possible packed bed column to around hmin = 1.0 for the microgroove array, while the interstitial velocity-based separation impedance Ei (a direct measure for the required analysis time) decreases from 1450 to 200. The next steps will focus on the removal of occasional particles remaining on the sides of the micro-pockets, the addition of a cover substrate to seal the column and the subsequent conduction of actual chromatographic separations.