Small-pore packings

The presence of the pores adds two parameters - the pore volume fraction and the pore radius. The predicted Rp Increases as the pore radius decreases suggesting a preference tor small pore packings. However, for a small pore radius of 1.0 pm a single value of the separation factor corresponds to two values of the particle diameter (13). Such double-valued behavior Is of course undesirable In an analytic technique. 

In packed beds of particles possessing small pores, dilute aqueous solutions of hydroly2ed polyacrylamide will sometimes exhibit dilatant behavior iastead of the usual shear thinning behavior seen ia simple shear or Couette flow. In elongational flow, such as flow through porous sandstone, flow resistance can iacrease with flow rate due to iacreases ia elongational viscosity and normal stress differences. The iacrease ia normal stress differences with shear rate is typical of isotropic polymer solutions. Normal stress differences of anisotropic polymers, such as xanthan ia water, are shear rate iadependent (25,26). 

From surface area to volume ratio considerations, the internal area is practically all in the small pores. One gram of the adsorbent occupies 2 cm as packed and has 0.4 cnP in small pores, which gives a surface area of 1150 m /g( or about 1 mi per 5 lb or 6.3 miVft of packing). Based on the area of the annular region filled with adsorbate, the solute occupies 22.5 percent of the internal pore volume and 13.5 percent of the total packed-bed volume. 

Although most PCHdC studies are conducted using columns packed with nonporous gels, the hydrodynamic separation also occur in SEC columns. This can be easily observed using small pore-size SEC columns (29) as shown in... 

In other experiments, PVC/plasticiser extracts (n-hexane) were separated by SEC using THF or chloroform as the mobile phase. Similarly, PE film was immersed in THF for several hours and the extract was concentrated by a factor of 20 prior to injection into a SEC system [51]. However, use of extraction techniques followed by injection into a SEC system for separation of low-MW additives is not the most obvious analytical approach in view of the relatively low resolution of conventional SEC in the low molecular mass range. For this purpose efficient column packing materials with small pore sizes are to be used. 

Despite their distinct advantages, on-line SPE and column-switching proce-dures do not always represent ideal separation techniques. In many cases, only a small number of samples can be analyzed before contamination of the precolumn by proteins occurs. Alternative techniques that prevent the adsorption of macromolecules onto column packings and allow direct injection of sample extracts are those based on use of specific LC columns. Shielded hydrophobic phase (27), small pore reversed-phase (28), and internal surface reversed-phase (29, 30) columns can be used to elute proteins in the excluded volumes, allowing small... 

The different methods of separation (RP-HPLC with small-pore or large-pore packing, IEC, and SEC) are complementary, and better separations are sometimes obtained using combinations of the various techniques. 

Porous microparticles are the most common stationary phase particles used in modern HPLC. The role of pore size is a critical one, as the pores provide the surface with which the sample interacts. Particles with small pores exhibit a high surface area and therefore have greater retention. Large molecules like proteins, however, may be excluded from the small pores, and for those molecules a packing with a larger pore size is preferable. The difference between porous particles, pellicular particles, and porous microparticles is illustrated in Figure 3.19. Porous particles are seldom used owing to low efficiencies and are not discussed further. 

Fig. 5.1. Scanning electron micrographs of continuous-bed columns prepared from ODS-silica particles packed in a capillary. Prepared from (a) 75 pm capillary packed with silica particles by sintering in the presence of NaHC03 [9], and (b) large pore (left) and small-pore (right) ODS particles by a sol-gel method [10]. Reproduced from refs. 9 and 10, with permission.

Since electroosmotic flow can exist in both the interparticle and intraparticle spaces, numerous studies have focused on the existence of intraparticle flow in CEC. Several groups have investigated the existence of electroosmotic flow in wide-pore materials [41-44], A model was developed to estimate the extent of perfusive flow in CEC packed with macroporous particles [41] by employing the Rice and Whitehead relationship. Results showed the presence of intraparticle EOF in large-pore packings (> 1000 A) at buffer concentrations as low as 1.0 mM. Additional parameters had been investigated [43,44] to control intraparticle flow by the application of pressure to electro-driven flow. Enhancement in mass transfer processes was obtained at low pore flow velocities under the application of pressure. The authors pointed out that macroporous particles could be used as an alternative to very small particles, as smaller particles were difficult to pack uniformly into capillary columns. 

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