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Theoretical and experimental limitations of shear-driven chromatography

Wednesday, 5 November, 2008 - 17:00
Campus: Brussels Humanities, Sciences & Engineering campus
Veronika Fekete
phd defence

The development of high-performance liquid chromatography (HPLC) in the years of 1965 to 1985 was undeniably a success story. However, an important limitation of the technique remained: in comparison to capillary gas chromatography the resolving power accessible in a reasonable analysis time is poor. The introduction of a range of miniaturized liquid phase techniques, exploiting either pressure or electrical propulsion, has recently opened new possibilities.

Since 1999, our group has been exploring shear forces to generate flows. The simplest way to generate these forces is by dragging a flat surface past a second flat surface held stationary. Using two surfaces to delimit a channel that is much wider than deep, the mean flow velocity in case of Newtonian flows is only dependent on the moving wall’s velocity, making it possible to work at unprecedentedly high velocities. We have shown that the liquid phase separation technique using such shear flows, baptised Shear-Driven Chromatography (SDC), is a very efficient reversed-phase liquid phase separation tool.

By elaborating a new coating protocol, we obtained reproducible results for the considered test separation of four coumarin dyes while using channels from different wafers. Furthermore, a series of mechanical and optical improvements enabled separations 8 mm far from the injection point in only 120 nm deep channels. These remarkable results offer unprecedented low plate heights.

Although the outstanding figures obtained with shear-driven chromatography are often very impressive, a fair way of comparing its kinetic possibilities to conventional analytical separation techniques is desirable in order to show the real opportunity that it may offer. With the extended kinetic plot method, it is now possible to compare pressure-, electrically-, and shear-driven devices. From the conventional HPLC, through UHPLC, to capillary zone electrophoresis, SDC proved to be the most powerful separation technique based on the resolution power per unit of time. Amongst all these techniques, it gives, by far, the shortest analysis time for a given plate number. However, it should be noted that kinetic plot method does not account for the mass loadability which is very poor for SDC. So is the range of samples that can be detected.

A possible way to increase the mass loadability of SDC is the use of porous layers. However, porous layers alone would increase the limit of detection. A promising solution to circumvent this problem is the combination of porous layers with locally deepened parts of the nanochannels forming detection grooves. The use of detection grooves without porous layer enhanced detection possibilities, but unfortunately, our results show that detection grooves combined with porous layers are difficult to work with. In particular, the difference in reaction to the etching procedure exhibited by the porous and the monolayer result in a groove shape that caused stagnant zones and convective flows disturbing the laminar flow.

Besides using detection grooves to enhance detection, a second approach was tested, namely the use of the more sensitive photomultiplier tube as detector. Although previous results seemed promising, our study revealed that stray light due to practical restrictions causes an important detector band broadening, and highly decrease the performance of the set-up.

Besides the reversed-phase liquid chromatography function of shear-driven channels, they can be used for direct measurements of effective diffusion coefficients. With a simple model assuming that diffusion events take place in parallel, it is possible to investigate the surface diffusion. Since it is a direct method, it has the advantage over the currently used peak parking method that it does not necessitate to estimate any factor that may influence the surface diffusion.

Finally, we conclude by noting that the shear-driven flow concept theoretically offers unprecedented separation speeds combined with the required strong resolving power. Unfortunately, at this point of the research, the lack of perfectly flat raw materials and a sufficiently sensible detection system limit the use of shear-flows as chromatographic flow drivers. It would therefore be interesting to explore alternative applications of the shear-driven concept, such as fundamental study of surface diffusion, optical restriction mapping of DNA and cell study in view of tissue engineering besides developing new detection schemes and injection procedures.