Packing-material particle size chromatography

New templated polymer support materials have been developed for use as re versed-phase packing materials. Pore size and particle size have not usually been precisely controlled by conventional suspension polymerization. A templated polymerization is used to obtain controllable pore size and particle-size distribution. In this technique, hydrophilic monomers and divinylbenzene are formulated and filled into pores in templated silica material, at room temperature. After polymerization, the templated silica material is removed by base hydrolysis. The surface of the polymer may be modified in various ways to obtain the desired functionality. The particles are useful in chromatography, adsorption, and ion exchange and as polymeric supports of catalysts (39,40). 

This noninvasive method could allow the differentiation between the various packing materials used in chromatography, a correlation between the chromatographic properties of these materials that are controlled by the mass transfer kinetics e.g., the coliunn efficiency) and the internal tortuosity and pore coimectivity of their particles. It could also provide an original, accurate, and independent method of determination of the mass transfer resistances, especially at high mobile phase velocities, and of the dependence of these properties on the internal and external porosities, on the average pore size and on the parameters of the pore size distributions. It could be possible to determine local fluctuations of the coliunn external porosity, of its external tortuosity, of the mobile phase velocity, of the axial and transverse dispersion coefficients, and of the parameters of the mass transfer kinetics discussed in the present work. Further studies along these lines are certainly warranted. 

The purpose and role of the solid support is the accommodation of a uniform deposition of stationary phase on the surface of the support. The most commonly used support materials are primarily diatomite supports and graphitized carbon (which is also an adsorbent in gas-solid chromatography), and to a lesser extent, Teflon, inorganic salts, and glass beads. There is no perfect support material each has limitations. Pertinent physical properties of a solid support for packed column GC are particle size, porosity, surface area, and packing density. Particle size affects column efficiency by means of an eddy diffusion contribution in the van Deemter... 

FIGURE 7.2 Principles of size-exclusion chromatography (SEC). In this chromatographic method, the analyte does not interact with the surface of the stationary phase. Separation is achieved by the differential penetration and exclusion of the sample components in and out of the pores of the packing material. Particles of different sizes elute at different rates. Small molecules, which can penetrate into the pores of the stationary phase, elute later. On the other hand, a very large molecule, which cannot penetrate into the pore system, elutes earlier, in the dead volume of the column. The molecules of intermediate size, which can partially penetrate the pores of the stationary phase elute in the intermediary time, between very large and very small molecules. 

Larger diameter columns were also available for preparative chromatography. In later years, GPC analysis times were reduced and resolution was Improved by using shorter columns that were packed with smaller particle size material. A typical family of GPC columns that is available today contains 7pm particles... 

The factors that control separation and dispersion are quite different. The relative separation of two solutes is solely dependent on the nature and magnitude of the Interactions between each solute and the two phases. Thus, the relative movement of each solute band would appear to be Independent of column dimensions or particle geometry and be determined only by the choice of the stationary phase and the mobile phase. However, there is a caveat to this statement. It assumes that any exclusion properties of the stationary phase are not included in the term particle geometry. The pore size of the packing material can control retention directly and exclusively, as in exclusion chromatography or, indirectly, by controlling the access of the solute to the stationary phase in normal and reverse phase chromatography. As all stationary phases based on silica gel exhibit some exclusion properties, the ideal situation where the selective retention of two solutes is solely controlled by phase interactions is rarely met in practice. If the molecular size of the solutes differ, then the exclusion properties of the silica gel will always play some part in solute retention. 

A ternary liquid-liquid system for partition chromatography is prepared from a mixture of 2,2,4-trimethylpentane, ethanol and water (34 5 1). The less polar upper layer is used as the stationary phase. A diatomaceous material, Hyflow Super Cel (particle size, 7-11 jam), is used as the solid support. The columns (40 cm X 4 mm I.D.) are packed by the slurry technique, and the support material is coated in situ with the liquid stationary phase as described earlier [54]. A pre-column is inserted in order to maintain equilibrium between... 

GPC is a further special form of liquid chromatography. The separation column is packed with porous, polymer gels (e.g. polystyrene gel) as stationary phase. The particle size of the packing material and the size distribution of the pores are well defined and uniform. In GPC molecules are separated according to their effective size in solution, i.e., their hydrodynamic volume, and not according to their affinity for the support material. 

Particle chromatography using packed beds has attracted considerable attention of those interested in measuring particle size distribution of spherical particles in the submicron range (8,9, 1,14,16). There exist two complementary approaches to the use of this technique, according to the packing material employed ... 

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