A newly proposed concept in liquid chromatography (LC) positions spherical particles, either individually or stacked, in pockets forming microgrooves that act as perfectly aligned chromatographic columns.
Researchers from Vrije Universiteit Brussel in Brussels, Belgium and the University of Twente in Enschede, the Netherlands, have reported what they call a breakthrough in the production of beds of spherical particles to construct a perfectly ordered column for use in liquid chromatography (LC), laying out the proposed method’s theoretical separation efficiency and kinetic performance potential and therefore its improvement over conventionally packed beds in terms of order and reduced flow resistance (1).
The study in the Journal of Chromatography A conceptualizes the spherical particles to be positioned either individually, in a single-layer column, or stacked, in a multi-layer column, in micromachined pockets that create an array of microgrooves, achieved by a few sweeps of a dedicated rubbing approach of a particle solution over a silicon chip. This design, the researchers said, gives the microgrooves an inherent mechanism for correcting differences in velocity.
The separation performance of high performance liquid chromatography (HPLC) columns, made either of stainless steel or fused-silica based capillaries, has been shown to be significantly compromised by the randomness and heterogeneity of the packing structure in these slurry-packed cylinders. In recent years, alternative production approaches such as 3D printing have been suggested, but few of these ideas are currently commercially available. Micropillar array columns used for nano-LC in proteomics deliver enhanced performance through induced order and enlarged flow resistance, but are silicon-based and offer a limited choice of stationary phases.
The proposed microgroove array would have interconnected cylindrical pockets of a depth of between about 1 and 50 particle diameters, and a diameter of between about 1 and 5 particle diameters. For this experiment, cylindrical pockets with a diameter of 12 µm designed to receive individual 10 µm particles were fabricated, and the researchers showed how the microgroove structures would be able to be filled within a few tens of seconds.
Numerical calculations of the dispersion in this method show its potential efficacy. For fully porous particles and a zone retention factor of k” = 2, there is a decrease from hmin = 1.9 for the best possible packed bed column to hmin = 1.0 for the microgroove array, with interstitial velocity-based separation impedance (Ei), which directly measures required analysis time, decreasing from 1450 to 200.
The interstitial velocity-based separation impedance (Ei) is not a measure of time, but rather a measure of the resistance to flow that a sample experiences during HPLC separation. It is a dimensionless quantity that depends on various parameters of the HPLC system, such as the column dimensions, flow rate, and particle size. The value of Ei is used to estimate the time required for a given sample to be separated by the HPLC system, but it is not a direct measure of time.
While the pursuit of perfection in chromatographic columns may still be theoretical, this study does at least provide a possible framework. According to the researchers, next steps will include removal of particles which may occasionally remain on the sides of the micropockets, addition of a cover substrate to seal the column, and finally, conduction of actual separations.
(1) Vankeerberghen, B.; Verloy, S.; Jimidar, I.S.M.; Gardeniers, H.; Desmet, G. Structured microgroove columns as a potential solution to obtain perfectly ordered particle beds. J. Chromatogr. A 2023, 1700, 464031. DOI: 10.1016/j.chroma.2023.464031