Pellicular packing material

The dimensions of the packing material are important in achieving efficient separations and the flow pattern around particles causes mixing of solute and solvent and increases the dilution effects, contributing to the phenomenon known as band broadening (Figure 3.9). Pellicular packing materials are... 

Using a macroporous support with a pore size of 200 nm instead of a microporous one, the acrylate-based nanobeads can also be agglomerated electrostatically inside the pores after sulfonation. Based on this technology, a solvent-compatible, pellicular packing material with a relatively high ion-exchange capacity of about 190 pequiv/column (250 mm x 4 mm i.d.) is obtained. This development was commercialized under the trade name lonPac... 

Unlike the pellicular packings used for ion exchange, the packings used in ion exclusion are derived from totally sulphonated polymeric materials. Separation is dependent upon three different mechanisms Donnan exclusion, steric exclusion and adsorption/partitioning. 

Ion-exchange packing materials are traditionally formed from the emulsion copolymerisation of styrene and divinylbenzene, the latter polymer is used to provide cross linking and thus increase the rigidity of the beads. Ionic functional groups are chemically bonded to this backbone. Pellicular silica-based packing materials may also be used which are then coated with a synthetic ion-exchange resin but these tend to have comparatively less sample capacity. 

To protect the column from compounds that irreversibly adhere to or partition into the column packing, a precolumn may be used. A pellicular packing coated with C g material (Cjg Corasil II, Waters Associates) has proven beneficial, yet has a minimally detrimental effect on compound resolution. 

Guard columns may be filled either with pellicular material (see Fig. 3-13) of the same bonded phase as the analytical column or with the identical packing material as in the analytical column. Pellicular packings (35-40 /urn)... 

The only way to increase the rate of diffusion of solutes is to raise the temperature substantially, but this would lead to problems in thermally unstable analytes. The alternative is to reduce the distance through which the molecules diffuse. Efficient separation then requires the use of smaller particles for column packings. This apparently simple expedient has evolved into a new practice of LC that is competitive with GLC in speed and resolution of complex mixtures and applicable to many more materials than GLC. Packing materials comprised of particles as small as 5 pm are currently available. Smaller particles are extremely difficult to handle and give an almost impermeable column. To solve this problem, solid glass beads of 30-50 pm in diameter can be coated with a layer of porous material. These are called pellicular beads. The porous layer may serve as a solid stationary phase or be coated with a very thin layer of liquid stationary phase with an extremely large surface area. 

Conventionally, the phase ratio is the volume of the active moieties of the stationary phase divided by the volume of the mobile phase in the column. From the particular configuration of pellicular stationary phases described earlier, it follows that they have a relatively low phase ratio with respect to that of the conventional packing materials employed in HPLC. 

It is obvious that the cleaner the sample, the longer the column life. On the other hand extensive clean-up procedures are time consuming. A compromise can be the use of precolumns to protect the analytical column against contamination caused by impurities present in the sample. Precolumns can be prepared from less expensive material than the analytical columns, i.e. pellicular packings or large diameter particles, which can be easily dry-packed. However, microparticulate stationary phases have also been used. As HPLC analysis of alkaloids often deals with the determination of low concentrations of alkaloids in a complex matrix, some factors concerning the sample preparation which influence the detection limit are worth consideration. 

Most LC stationary phases are now either bare of surface derivatized micro-sporous silica particles of 10, 5, or 3 pm in diameter. These small-diameter packings provide very high efficiencies due to rapid mass transfer, in contrast to the much larger pellicular packings of the 1970s. In a survey of column usage taken in 1994, 24.6% of respondents reported using 1-3 pm materials 71.3% reported... 

Microporous particles (3-10 (im) give columns that are as much as 20 times as efficient as porous layer-bead or pellicular (40 pm) packings. Whilst modem LC is based almost exclusively on microporous packing materials it is informative to relate the advances in particle design with the attempts to eliminate the deleterious effects on column performance since the latter as expressed by H is related to experimental variables, such as, the particle size (dp), the nominal stationary phase thickness (ds) and the mobile phase velocity (u). 

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. 

---