Chromatographic Columns’ Performances Improved with Elastic Flow Instability

Scientists from Waters Corporation in Milford, MA and Princeton University in Princeton, NJ investigated how using elastic flow instability can improve the kinetic performance of chromatographic columns. Their findings were published in the Journal of Chromatography A (1).

Column chromatography chemistry in the lab. | Image Credit: © sinhyu - stock.adobe.com

When it comes to analyte dispersion along random sphere packings, previous knowledge has shown that the minimum reduced plate height (RPH, the ratio of the minimum column plate height to the average particle diameter) of an infinite diameter column, which is characterized by a lack of border and wall effects, should be around 0.9 with fully porous particles, 0.7 for superficially porous particles, and 0.5 for non-porous particles. From an experimental viewpoint, minimum RPHs close to have been observed, but under abnormal conditions that cannot be implemented in production by manufacturers of modern high-performance liquid chromatography (HPLC) and ultrahigh-performance liquid chromatography (UHPLC) columns. While there have been decades of research and development, optimal efficiency for slurry-packed HPLC columns is still hindered by inherent long-range flow heterogeneity from the wall to the columns’ central bulk region.

In this study, the scientists show how this can be addressed using straightforward addition of a semidilute amount (500 ppm) of a large, flexible, and synthetic polymer (18 MDa partially hydrolyzed polyacrylamide, HPAM) to the mobile phase (1% NaCl aqueous solution, hereafter referred to as “brine”) during operation of a 4.6 mm 300 mm column packed with BEH 125 Å particles. Adding the polymer adds elasticity to the mobile phase, causing flow in interparticle pore space to become unstable above a threshold flow rate. Afterwards, they verified the development of this instability using pressure drop measurements of friction factor versus Reynolds number, which represents the ratio of inertial to viscous forces within a fluid to indicate the laminar or turbulent nature of a flow (2).

Elastic flow instability can stimulate a disordered flow state generated by large normal stresses, with the flow field having potential to evolve into a so0called elastic turbulence regime at surprisingly low Reynolds numbers (3). Previous works from these scientists showed that flow instability is characterized by large spatiotemporal fluctuations in pore-scale flow velocities that can potentially promote analyte dispersion across a column. When using axial dispersion measurements of a quasi-non-retained tracer thiourea, this possibility was confirmed, revealing that operative above instability onset can improve column efficiency by over 100%. These experiments suggest that elastic flow instabilities can be harnessed to mitigate the negative impact of trans-column flow heterogeneities on the efficiency of slurry-packed HPLC columns.

Overall, despite this approach having inherent limitations and constraints, the results of this study laid groundwork for future targeted polymer development that can impart elasticity when dissolved in commonly used LC mobile phases. These can help generate elastic flow instabilities to help improve the resolution of HPLC columns. However, more research is needed to create “polymer enhancers” that can be routinely added to the mobile phase to minimize flow heterogeneities in chromatography with different properties; these include sufficient elasticity to generate flow instability, compatibility with the use of common mobile phase compositions in different types of LC techniques, and minimal irreversible adsorption to packed particles’ surfaces.

References

(1) Gritti, F.; Chen, E. Y.; Datta, S. S. Harnessing an Elastic Flow Instability to Improve the Kinetic Performance of Chromatographic Columns. J. Chromatogr. A 2024, 1735, 465326. DOI: 10.1016/j.chroma.2024.465326

(2) Reynolds’ Number. Elsevier B.V. 2022. https://www.sciencedirect.com/topics/engineering/reynolds-number (accessed 2024-9-23)

(3) Sousa, P. C.; Pinho, F. T.; Alves, M. A. Purely-Elastic Flow Instabilities and Elastic Turbulence in Microfluidic Cross-Slot Devices. Soft Matter 2018, 14 (8), 1344–1354. DOI: 10.1039/c7sm01106g