Pore size characterization of monolith for electrochromatography via atomic force microscopy studies in air and liquid phase

Modeling of flow in a polymeric chromatographic monolith

The flow behavior of a commercial polymeric monolith was investigated by direct numerical simulations employing the lattice-Boltzmann (LB) methodology. An explicit structural representation of the monolith was obtained by serial sectioning of a portion of the monolith and imaging by scanning electron microscopy. After image processing, the three-dimensional structure of a sample block with dimensions of 17.8 μm × 17.8 μm × 14.1 μm was obtained, with uniform 18.5 nm voxel size. Flow was simulated on this reconstructed block using the LB method to obtain the velocity distribution, and in turn macroscopic flow properties such as the permeability and the average velocity. The computed axial velocity distribution exhibits a sharp peak with an exponentially decaying tail. Analysis of the local components of the flow field suggests that flow is not evenly distributed throughout the sample geometry, as is also seen in geometries that exhibit preferential flow paths, such as sphere pack arrays with defects. A significant fraction of negative axial velocities are observed; the largest of these are due to flow along horizontal pores that are also slightly oriented in the negative axial direction. Possible implications for mass transfer are discussed.

Surface structure of size exclusion chromatography stationary phase

Chromatography is a widely used separation unit operation for separating nanomaterials such as proteins and enzymes, quantum dots and carbon nanotubes. An understanding of the chromatographic stationary phase on a nanoscale would be extremely helpful in improving the process and developing efficient and new materials. This study is an attempt to characterize the stationary phase in its swollen wet state using environmental scanning electron microscope (ESEM) and atomic force microscopy (AFM). Observation of the wet beads using ESEM is limited to a micron-range resolution. However, AFM can be used in wet mode to characterize the stationary phase in both wet and dry states with nanometric resolution. In the swollen state, microscale cracks were observed on the surface and this may explain the high mass transfer rate and lower back pressures of the stationary phase. The structures on the surface of the stationary phase depict that the micron-sized beads may be composed of nanometric beads.

Probing topography and tailing for commercial stationary phases using AFM, FT-IR, and HPLC

Atomic force microscopy was used to study surface characteristics of three chromatographic silica products: Agilent Zorbax SB300, Waters Symmetry 300, and Merck Chromolith. Each is modified with a monomeric C18 monolayer. Both topographic and adhesive force measurements were made for each product. Topographical images revealed that all three materials are as smooth as glass on the scale of 100 nm and below. Adhesive forces for all three materials were much lower and much more uniform than for chemically modified fused silica. FT-IR spectra for all three materials showed a low abundance of isolated silanols, thus explaining the low adhesion. Chromatograms of a cationic dye, 1,1'-didodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), ranging in concentration from 1 to 300 microM were obtained for each column. All three materials exhibited classic nonlinear tailing; the Zorbax exhibited fronting as well. Chromatographic simulations were performed for the Symmetry and Chromolith products to determine the number of strong adsorption sites. The AFM, FT-IR, and HPLC were all consistent in indicating that the Chromolith material had half as many strong adsorption sites as the Symmetry material. The Zorbax material exhibited a number of isolated silanols that was comparable to the other materials, yet its adhesive force suggested a less adsorptive material, and its chromatographic performance suggested a more adsorptive material. Its topography is discussed as a possible reason for its anomalous chromatographic behavior.

Assessing the macroporous structure of monolithic columns by transmission electron microscopy

A set of monolithic stationary phases representing a broad span of monomers and porogens have been characterized directly in their capillary chromatographic format by computational assessment of their pore structure from transmission electron micrographs obtained after in situ embedment of the monoliths in contrast resin, followed by dissolution of the fused-silica tubing, further encasement of the resin-embedded monolith, and microtomy. This technique has been compared to mercury intrusion, a more conventional technique for macroporosity estimation. Supplementing the embedding resin by lead methacrylate gave a negative staining, and the resulting micrographs showed a good contrast between the polymeric monoliths and the embedding resin that allowed studies on the pore formation and polymer development. The technique was also applied to a commercial monolithic silica column.

Experimental characterization and modelling of analytical monolithic column

Hydrodynamics, equilibrium and kinetics of adsorption in a silica-based monolithic column Chromolith Performance RP-18e (Merck KgaA, Germany) have been studied. The column permeability was calculated according to the Darcy law for laminar flow. The efficiency of the monolithic column was characterized through the height equivalent to a theoretical plate (HETP) for myoglobin, phenol and progesterone. The 2-D single channel mathematical model has been applied to describe the adsorption dynamics. Parabolic velocity profile, axial and radial diffusion in the monolith channel, linear driving force model for the mass transfer in the monolith channel skeleton wall and linear adsorption equilibrium were assumed. The mathematical model gives good prediction of the experimental elution peaks.

Recent advances in the control of morphology and surface chemistry of porous polymer-based monolithic stationary phases and their application in CEC

This review focuses on developments in the field of polymer-based monolithic columns for CEC published in the literature since the beginning of the year 2005. The possibility of in-situ preparation as well as easy control over their porous properties and surface chemistries clearly make monolithic separation media an attractive alternative to capillary columns packed with particles. Different variables such as polymerization conditions, morphology, and surface chemistry are shown to directly affect performance of monolithic capillary columns in terms of efficiency, analysis time, and retention.

Porous polyacrylamide monoliths in hydrophilic interaction capillary electrochromatography of oligosaccharides

Capillary electrochromatography (CEC) of oligosaccharides in porous polyacrylamide monoliths has been explored. While it is possible to alter separation capacity for various compounds by copolymerization of suitable separation ligands in the polymerization backbone, "blank" acrylamide matrix is also capable of sufficient resolution of oligosaccharides in the hydrophilic interaction mode. The "blank" acrylamide network, formed with a more rigid crosslinker, provides maximum efficiency for separations (routinely up to 350,000 theoretical plates/m for fluorescently-labeled oligosaccharides). These columns yield a high spatial resolution of the branched glycan isomers and large column permeabilities. From the structural point of view, some voids are observable in the monoliths at the mesoporous range (mean pore radius ca. 35 nm, surface area of 74 m2/g), as measured by intrusion porosimetry in the dry state.