Nonuniform flow in chromatography columns: An examination of packing heterogeneity and viscous fingering during solute retention and separations

The main goal of the chromatographer is to develop a robust separation process that results in reproducible high product purity and concentration. However, desired chromatographic performance can be compromised by nonuniform flow distribution, heterogeneous packing, and flow instabilities such as viscous fingering. This presentation focuses on the application of high-speed magnetic resonance imaging (MRI) towards the noninvasive, direct visualization of flow nonuniformities within a chromatography column. Results will be presented for two studies.; In the first study, echo planar spectroscopic imaging and three-dimensional MRI was developed and used to demonstrate that solute retention and separation can modify viscous finger development within eluting sample bands decreasing solute resolution in some cases. The potential occurrence of viscous fingering during solute desorption has also been examined using MRI and numerical simulations revealing that fingering can be a problem for high capacity columns using step gradient elution but becomes less of a problem for low capacity columns and shallow gradient elution schemes.; In the second study, by varying the manufacturer's suggested protocol for two different chromatography media, we have examined the effects of slurry flow rate, bed compression, slurry concentration, and particle size on bed homogeneity in a 3.5 cm I.D. column. A new semi-quantitative technique was developed for studying flow nonuniformity within a packed chromatography column where high speed three-dimensional MRI was used to visualize solute band deformation within the columns and where solute velocities and localized contributions to band spreading were estimated using the 3D images. Our results show that bed over-compression and nonuniform frit permeability can lead to significant variations in the velocity field within the bed and that differences in packing flow rate can result in predictable variations in band asymmetry. Porous media theories incorporating wall friction and elastic bed deformation may account for some of the behavior observed. However, more complex theoretical approaches will likely be required to provide a description of packing behavior and bed stability that will be useful for process scaleup and development.