Pellicular particles

Packings for HPLC can be further described as either pellicular or porous. Pellicular particles are made from spherical glass beads, which are then coated with a thin layer of stationary phase. For example, a porous layer can be deposited onto the glass bead to produce a porous layer or a superficially porous particle. The porous layer can in turn be coated with liquid stationary phase or reacted to give a bonded stationary phase. Pellicular particles are generally less efficient than the porous layer of superficially porous particles. 

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. 

Pellicular particles (Fig. 3-13, center) are commonly available in the 37 to 50-p.m size range. The name pellicular implies that these particles are composed of a solid core which is coated with a porous layer. These particles have an advantage in that mass transfer is much faster than for porous... 

Pellicular Particles (Thin layer silica fused to solid glass bead)... 

The pellicular particles consist of a solid core with a 2 to 3 pm crust etched onto the surface. They can also be about 50 pm in diameter, but their surface area is much less than that of the fully porous particles. Because of the smaller diffusion distances (2 pm in and 2 pm out), they are highly efficient and excellent for analytical separations. 

Pellicular particles technical term for a synthetic HPLC column packing material consisting of microglass beads, 10-50 pm in diameter with a 2-3 pm surface film of active stationary phase material. These packings have only about 10% of the capacity of microporous materials but produce columns with greater separation efficiencies due to the very rapid equilibrium processes that occur with such regular particles and thin stationary phase films. 

Since efficiencies for microparticulate columns are very high (optimum H values of 0.01-0.03 mm), only short columns (15-25 cm) are required for analytical HPLC, as can be seen in Figure 21.14C. The use of 5- m particles implies greatly increased column back-pressures compared to those produced by the larger porous or pellicular particles, as suggested by Equation 21.13. However, these short columns exhibit moderate back-pressures (less than 200 atm) when used at flow rates of... 

The preceding analysis applies spedfically to porous particles. However, several workers have suggest that pellicular, small-particle columns might be especially suited for these separations [p. 243 or Ref. (79)]. Some of the inherent di vantagesof very small partides could be avoid, while taking advantage of the smaller C term [Eq. (33)] of pellicular particles. Finally, columns packed with very snoall nonporous pa des hold similar promise, and l- rm-particle columns of this type have now been reported (7/7). 

Figure 4.60 Surface-agglomerated pellicular particles used in low-capacity anion-exchange columns for 1C.

Repack top of column with pellicular particles of same bonded phase functionality. Continue using the column in reverse flow direction. 

Nonporous particles, with a solid core and a thin porous surface layer (pellicular particles), have occasionally been used for macromolecules. The reason is that the slow diffusion velocity of macromolecules results in less band broadening in combination with the thin layers. However, the nonporous particles have a much smaller loading capacity than the totally porous particles. 

Two basic types of packings have been used in LC, pellicular and porous particle. The original pellicular particles were spherical, nonporous. glass or polymer beads with typical diameters of 30 to 40 pm. A thin, porous layer of silica, alumina, a polystyrene-divinyl-benzene synthetic resin, or an ion-exchange resin was deposited on the surface of these beads. Small porous microparticles have completely replaced these large pellicular particles. In recent years, small (- 5 pm) pellicular packings have been reintroduced for separation of proteins and large biomolecules. 

In the recent development of particle technology targeted for liquid chromatography, the use of core-shell (or superficially porous, fused core, shell, pellicular) particles has received considerable attention. Shell particles manifest the advantages of porous particles as well as some benefit of nonporous particles. 

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