Packing-material particle size

If one considers the effects of packing material particle size on linear velocity, it has been determined that if one decreases particle size by a factor of two, the pressure increases as its square. Thus, the productivity increase is potentially jeopardized for an esthetic increase in resolution. One needs to test the actual resolution achieved in chromatograms of "unresolved peaks" by taking fractions through the region of interest and analyzing their composition. 

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). 

The HETP itself has to be estimated because no rigorous methods exist for its evaluation. Its value typically depends on the type and size of the packing material particles, and is usually supplied by the packing vendor. 

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 for GSC), to a lesser extent. Teflon, inorganic salts and glass beads. There is no perfect support material because each has limitations. Pertinent physical properties of a support for packed-column GC are particle size, porosity, surface area, and packing density. Particle size impacts column efficiency via the A term or eddy diffusion contribution in the van Deemter expression (Equation 2.44). The surface area of a support is governed by its porosity, the more porous supports requiring greater amounts of stationary phase... 

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. 

In 1972-1973 Knox et al. [3, 4, 5] examined, in considerable detail, a number of different packing materials with particular reference to the effect of particle size on the reduced plate height of a column. The reduced plate height (h) and reduced velocity (v) were introduced by Giddings [6,7] in 1965 in an attempt to form a rational basis... 

FIGURE 1.3 15- m macroporous packing material, (a) Narrow particle-size distribution, (b) Close-... 

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