Jeff Op De Beeck

Impact of the limitations of state-of-the-art micro-fabrication processes on the performance of pillar array columns for liquid chromatography

We report on the practical limitations of the current state-of-the-art in micro-fabrication technology to produce the small pillar sizes that are needed to obtain high efficiency pillar array columns. For this purpose, nine channels with a different pillar diameter, ranging from 5 to 0.5 μm were fabricated using state-of the-art deep-UV lithography and deep reactive ion etching (DRIE) etching technology. The obtained results strongly deviated from the theoretically expected trend, wherein the minimal plate height (H(min)) would reduce linearly with the pillar diameter. The minimal plate height decreases from 1.7 to 1.2 μm when going from 4.80 to 3.81 μm diameter pillars, but as the dimensions are further reduced, the minimal plate heights rise again to values around 2 μm. The smallest pillar diameter even produced the worst minimal plate height (4 μm). An in-depth scanning electron microscopy (SEM) inspection of the different channels clearly reveals that these findings can be attributed to the micro-fabrication limitations that are inevitably encountered when exploring the limits of deep-UV lithography and DRIE etching processes. When the target dimensions of the design approach the etching resolution limits, the band broadening increases in a strongly non-linear way with the decreased pillar dimensions. This highly non-linear relationship can be understood from first principles: when the machining error is of the order of 100-200 nm and when the target design size for the inter-pillar distance is of the order of 250 nm, this inevitably leads to pores that will range in size between 50 and 450 nm that we want to highlight with our paper highly non-linear relationship. This highly non-linear relationship can be understood from first principles: when the machining error is of the order of 100-200 nm and when the target design size for the inter-pillar distance is of the order of 250 nm, this inevitably leads to pores that will range in size between 50 and 450 nm.

On the Advantages of Radially Elongated Structures in Microchip-Based Liquid Chromatography

We report on the possibility to realize submicrometer plate heights using chromatographic pillar array columns filled with radially elongated diamond-shaped pillars, even when using a relatively large interpillar distance (2.5 μm) and axial pillar width (5 μm). It is demonstrated that the use of high aspect ratio radially elongated pillars which are 15 times wider in the radial than in the axial direction can lead to a fivefold reduction of the minimal plate height compared to beds filled with pillars with a similar interpillar distance but with an aspect ratio around unity (cylinders and diamonds).This increase in performance can be attributed to a decrease in longitudinal dispersion, reflected by a reduction of the B-term by a factor of about 25. Experiments were conducted at room temperature, as well as at elevated temperature (70 °C), where the B-term band broadening is known to be more critical. The main advantage of radially elongated pillar beds is that they enable a drastic reduction of the footprint of pillar array columns, allowing design of very long channels with a minimum of turns. Under retained conditions, a four-component laser dye mixture could be separated over a distance of only 1.5 mm.

Visualization and quantification of the onset and the extent of viscous fingering in micro-pillar array columns.

New experimental data of the viscous fingering (VF) process have been generated by studying the VF process in perfectly ordered pillar array columns instead of in the traditionally employed packed bed columns. A detailed quantitative analysis of the contribution of VF to the observed band broadening could be made by following the injected species bands using a fluorescence microscope equipped with a CCD-camera. For a viscosity contrast of 0.16 cP, a plate height increase of about 1 microm can be observed, while for a contrast of respectively 0.5 cP and 1 cP, additional plate height contributions of the order of 5-20 microm were observed. Citing these values is however futile without noting that they also depend extremely strongly on the injection volume of injected sample. It was found that, for a given viscosity contrast of 0.314 cP, the maximal plate height increase varied between 0.5 microm and 18 microm if the injection volume was varied between 3.0 nl and 32.7 nl. These values furthermore also strongly vary with the distance along the column axis.

Hydrodynamic chromatography of polystyrene microparticles in micropillar array columns

We report on the possibility to perform HDC in micropillar array columns and the potential advantages of such a system. The HDC performance of a pillar array column with pillar diameter = 5 microm and an interpillar distance of 2.5 microm has been characterized using both a low MW tracer (FITC) and differently sized polystyrene bead samples (100, 200 and 500 nm). The reduced plate height curves that were obtained for the different investigated markers all overlapped very well, and attained a minimum value of about h(min)=0.3 (reduction based on the pillar diameter), corresponding to 1.6 microm in absolute value and giving good prospects for high efficiency separations. The obtained reduced retention time values were in fair agreement with that predicted by the Di Marzio and Guttman model for a flow between flat plates, using the minimal interpillar distance as characteristic interplate distance.

Separations using a porous-shell pillar array column on a capillary LC instrument

We investigated the achievable separation performance of a 9-cm-long and 1-mm-wide pillar array channel (volume = 0.6 μL) containing 5 μm diameter Si pillars (spacing 2.5 μm) cladded with a mesoporous silica layer with a thickness of 300 nm, when this channel is directly interfaced to a capillary LC instrument. The chip has a small footprint of only 4 cm × 4 mm and the channel consists of three lanes that are each 3 cm long and that are interconnected using low dispersion turns consisting of a narrow U-turn (10 μm), proceded and preceded by a diverging flow distributor. Measuring the band broadening within a single lane and comparing it to the total channel band broadening, the additional band broadening of the turns can be estimated to be of the order of 0.5 μm around the minimum of the van Deemter curve, and around some 1 μm (nonretained species) and 2 μm (retained species) in the C-term dominated regime. The overall performance (chip + instrument) was evaluated by conducting gradient elution separations of digests of cytochrome c and bovine serum albumin. Peak capacities up to 150 could be demonstrated, nearly completely independent of the flow rate.

Micron-sized pillars for ion-pair reversed-phase DNA separations

In the present paper, the feasibility to construct micron-sized silicon pillar channels to be used in HPLC is studied. For this, a channel with flow-through pores of 1 μm and with critical sidewall dimensions below 1 μm was constructed using advanced deep-UV lithographic equipment. Integrating a 3-nL injection system on the chip directly in front of the separation channel and using elongated distribution structures, a very controlled and high aspect ratio sample definition across the relatively wide separation channel was accomplished. The system was evaluated in isocratic ion-pair RP mode, allowing the separation of a mixture of two components with, respectively, 300 and 400 base pairs in 5 s only.

The axial rearrangement mixer: working principles and in-depth investigation.

In the current paper, an axial rearrangement mixer is studied. The mixer aims to flatten out occasional composition fluctuations originating from the pump. In a first phase, dispersion in a single mixer channel is investigated using pulses introduced by a dedicated injection pulse generator. The mixing in the single channel is studied for different flow rates and a model is presented based on the experimental data. Next, the resulting signal for the whole mixer is studied.

Hydrodynamic chromatography separations in micro‐ and nanopillar arrays produced using deep‐UV lithography

We report on a series of hydrodynamic chromatography separations conducted in micropillar array columns with an interpillar distance spacing of, respectively, 1.00, 0.70, and 0.47 μm. The columns have been produced using state-of-the-art deep-UV lithography and deep reactive ion etching techniques. Despite the fact that the efficiency was smaller than theoretically possible (due to fabrication limitations and significant injection and detection band broadening), it was nevertheless possible to separate mixtures of fluorescein isothiocyanate (used as the t(0) -marker) and 20- and 40-nm polystyrene beads. With the smallest interpillar distance, a resolution of R(s) = 0.5 between the 20- and 40-nm particles could be obtained in 90s over a column length of 4 cm. The selectivity obtained in the pillar array columns was found to be very similar to that observed in packed-bed columns. By detecting the fluorescent signals in a 90-μm-deep detection groove at the end of the column, the signal-to-noise ratio could be enhanced up to 150 times.