Pellicular ion-exchange resin

PELLICULAR ION EXCHANGE RESINS IN CHROMATOGRAPHY Csaba Horvath... 

As particle sizes become small, the pressures required for maintenance of reasonable flow rate become high and usually a balance must be struck between flow rate and diffusion rate limitations. Pellicular ion-exchange resins are ones in which the exchange sites are limited to a thin surface layer. With beads of small diameter they are useful in high-pressure, high-speed separations of small samples. 

Proteins are isolated and characterized with different chromatographic techniques. Depending on the protein, ion-exchange (lEX), size-exclusion (SEC), affinity (IMAC), hydrophobic interaction (HIC), and reversed-phase chromatography (RPLC) may be applied. Traditionally, separation materials with low mechanical stability and limited resolution were used. In recent years, HPLC materials were developed that offered the separation power required by the protein chemist. Now, pellicular ion-exchange resins as well as organic polymer monoliths exhibit high resolution at relatively short retention times, while... 

New types of ion exchange resins have also been developed to meet the specific needs of high-performance liquid chromatography (HPLC) (Chapter 8). These include pellicular resins and microparticle packings (e.g. the Aminex-type resins produced by Bio-Rad). A review of the care, use and application of the various ion exchange packings available for HPLC is given in Ref. 19. 

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. 

The different stationary phases can also be classified on the basis of their physical structure. Pellicular materials, the particles of which consist of a hard (glass) core covered by a thin layer of an ion-exchange resin, may be used if a moderate efficiency and a small ion-exchange capacity are acceptable, but not if the column is required to have a high... 

Ordinary ion exchange resins are used with the vertical gravity flow columns. HPLC columns are pellicular, 5 -10 pm diameter beads usually, with either strong acid or strong base functional groups. 

On ion-exchange resins the rate of diffusion through the stationary phase, to and from the ion-exchange site, is relatively slow. This slow mass transfer results in a large contribution towards band broadening. Pellicular materials with only a thin layer of resin were evolved to... 

The choice of the amine can have a substantial influence on the selectivity of the separations, even for simple inorganic ions. The elution behavior is a function of the hydrophobicity of the functional group (5,6). As an example, the retention of inorganic anions on pellicular latex-based ion-exchange resins prepared from different amines is shown in Table 12.1 (5). 

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. 

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