Nuclear magnetic resonance and the design of chromatographic separations

Viscous fingering in packed chromatographic columns: Linear stability analysis

When a fluid is displaced by a less viscous one in a porous medium, a hydrodynamic instability appears leading to the formation of some kind of fingers of the upstream fluid invading the downstream one, hence the name "viscous fingering" (VF) given to this instability. In a LC column, such an instability is likely to appear at that of the two interfaces between the sample and the eluent which exhibits an unfavorable viscosity contrast. It leads to distorted peak shapes and contributes to peak broadening. This phenomenon has been observed for long in SEC and more recently in RPLC on elution peak shapes as well as with various methods of in-column visualization. A simplistic LC column model is described to explain the origin of the VF instability and its characteristics. The general principles for analyzing hydrodynamic instabilities are described and the results of the linear stability analysis performed by Tan and Homsy [C.T. Tan, G.M. Homsy, Phys. Fluids 29 (1986) 3549 [1]], at the onset of the VF phenomenon for a step interface between two fluids are here applied to typical operating conditions encountered in analytical LC. The most probable growth rate and wavelength (linked to the finger width) of the instability are given in terms of particle size and solute diffusion coefficient, with particular emphasis on the role of the carrier velocity. Previously published qualitative observations about VF in chromatography are examined and interpreted at the light of this theory. The role of the column geometry on the development of the instability, the possible sources of noise or fluctuations triggering the instability, and the various experimental situations in which a significant viscosity contrast is encountered in LC are discussed.

Monoliths for fast bioseparation and bioconversion and their applications in biotechnology

Monoliths have consolidated their position in bioseparation. More than 200 different applications have been reported in the past two decades and their advantages compared to conventional chromatography demonstrated. These include the high mass transfer efficiency due to the convective flow enabled by the macroporous character of the matrix. Recently plasmid DNA and viruses were separated with high efficiency and cryogels and monolithic superporous agarose were developed for capture of proteins from crude homogenates and separation of microorganisms or lymphocytes. Currently four companies manufacture monoliths mainly for analytical applications although monoliths with a volume of 0.8 liter are commercially available and 8 L are available as prototypes. A book entitled "Monolithic materials: preparation, properties and applications" was published in 2003 and became standard reference of the status of this area. This review focuses on the progress in monoliths that goes beyond the scope of this reference book. Less progress has been made in the field of bioconversions in spite of the fact that monolithic supports exhibit better performance than beads in enzymatic processing of macromolecules. It appears that the scientific community has not yet realized that supports for these applications are readily available. In addition, monoliths will further substantially advance bioseparations of both small and large molecules in the future.

Nuclear magnetic resonance studies of solvent flow through chromatographic columns: effect of packing density on flow patterns

NMR (nuclear magnetic resonance) techniques have been used to measure and characterise solvent flow through chromatographic columns. NMR imaging was used to track an injection of D2O. PGSE (pulsed gradient spin echo) NMR was used to measure the flow-rate dependence of axial and transverse apparent diffusion. A combination of these two techniques (dynamic NMR imaging) gave the spatial distribution of the local velocity and apparent diffusion through a cross-section of the column. Significant column wall effects were observed and these effects were found to be highly dependent upon the column packing density. The column performance was assessed in terms of the HETP (height equivalent to a theoretical plate) determined by the NMR techniques employed.

Refining the scale-up of chromatographic separations

The use of heavily loaded columns and complex processing conditions makes scale-up of chromatographic separations a non-trivial process. The wide ranges of process conditions that must be investigated demands that a large number of preliminary experiments must usually be made in small columns and laboratory-scale work stations. These preliminary data can be biased by improper column packing, poor distributors and dispersion in auxiliary apparatus, and it is important to understand these disturbing factors in detail. Moreover, it is precisely at this macroscopic level that our understanding of the chromatographic process is weakest, for large columns as well as small. This paper addresses three of these factors: Efficient elimination of peripheral effects and characterization of both header flow distribution and packing non-uniformity. This will be done using a variety of experimental and analytical approaches including nuclear magnetic resonance imaging, computational fluid dynamics and mass transfer, and careful experimentation.

In situ analysis of protein chromatography and column efficiency using magnetic resonance imaging

Magnetic resonance imaging has been used to visualize size-based protein separations inside operating chromatography columns. The effects of flow nonuniformity have been observed and analyzed quantitatively through concentration profiles of tracers measured inside the column. Analysis of these profiles provides local and averaged intracolumn plate height values for characterization of dispersion and flow nonuniformity. The magnetic resonance measurements compare favorably with conventional chromatographic measurements of column efficiency and provide more detailed insights into nonideal column performance.