Pellicular beads

Applied Science Labs. Vydac Pellicular silica Pellicular ion exchangers Bonded phases on pellicular beads... 

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. 

Column Partisil SCX-10 (250x4.6 nm ID), protected with a precolumn (30x2.8 mm ID) packed with 30 pm pellicular beads, mobile phase methanol - water (1 4) containing 0.2% ammonium phosphate and 0.1% potassium chloride (pH 4.5), flow rate 1 ml/min, temperature 50° C. Peaks 1, psilocybin 2, psilocin 3, dimethyltryptamine (internal standard). Chromatogram a standard mixture, detection UV 267 nm chromatogram b mushroom extract, detection UV 267 nm chromatogram c standard mixture, fluorescence detection ( excitation 267 nm, emission 335 nm) chromatogram d mushroom extract, fluorescence detection. 

The first reversed-phase separation of ergot alkaloids was reported by Jane and Wheals in connection with the analysis of LSD. Pellicular beads with chemically bonded octadecyl groups were used in combination with methanol - 0.1% aqueous ammonium carbonate (3 2) as mobile phase However, the mobile phase proposed by Vivilecchia et al.6 for the separation of the dihydro-ergotoxine alkaloids on octadecyl columns (acetonitrile - aqueous ammonium carbonate) has been... 

Jusko and Poliszczuk reported the analysis of theophylline in biological fluids by using a chemically bonded strong cation-exchanger on pellicular beads (Table 11.2). 0.66 Aqueous... 

Figure 7.7 Types of HPLC packings (a) pellicular beads (25-50 g), (b) microporous particles (20-40 g) with longer pores (c) microporous particles (3-10 g) with short pores.

Porous layer or pellicular beads are composed of thin porous layers of adsorbent surrounding solid impregnable cores. The thin layer in PLB facilitates rapid mass transfer. Therefore, high v can be achieved while conditions close to equilibrium can be maintained so that efficiency of separation is not harmed. Particle diameter ranges from 25 to 50g and the layer thickness is about lg. PLB can be used in liquid-liquid chromatography as well as in liquid-solid chromatography. 

Both microporous and pellicular bead silica and alumina can be used for HPLC in the adsorption mode (liquid-solid chromatography). Pellicular beads, however, are rarely used now. While the pellicular beads (Table 10.2) mostly are spherical in shape, microporous particle column materials (Table 10.3) can be either irregular in shape or spherical. Theoretically irregular particles should give higher efficiencies but spherical materials pack together better. 

The non-polar pellicular beads with their long hydrocarbon chains are especially useful for the reversed-phase mode. Samples that are insoluble in water but soluble in alcohol or other water-miscible organic solvents e.g. dioxan, acetonitrile and tetrahydrofuran, are candidates for reversed-phase chromatography. 

Because none of these morphologies is perfect for the preparation of chromatographic separation media, numerous hybrid materials have also been developed in an attempt to preserve advantages of both gels and macroporous materials while trying to avoid their drawbacks. In addition, nonporous micro-pellicular beads are available for the rapid separation of biopolymers. 

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. 

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. 

Pellicular materials consist of a solid spherical bead of relatively large... 

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

The first separation on a chemically modihed surface with an aqueous eluent, which later got the name reversed-phase, was also invented by Horvath Figure 1-4, he demonstrated the hrst reversed-phase separation of fatty acids on pellicular glass beads covered with graphitized carbon black. 

Pellicular or controlled surface porosity particles were introduced in the late 1960s these have a solid inert impervious spherical core with a thin outer layer of interactive stationary phase, 1-2 pm thick [13]. Originally, the inner sphere was a glass bead, 35-50 pm i.d., with a thin active polymer film or a layer of sintered modified silica particles on its surface. Such particles were not very stable, had very low sample load capacities because of low surface areas and are not used any more. Nowadays, this type of material is available as micropellicular silica or polymer-based particles of size 1.5 to 2.5 pm [14]. Micropellicular particles are usually packed in short columns and because of fast mass-transfer kinetics have outstanding efficiency for the separation of macromolecules. Because the solutes are eluted as very sharp narrow peaks, such columns require a chromatograph designed to minimise the extra-column contributions to band broadening. 

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