Particle Shape and Size

Originally, reversed phases were produced from irregularly shaped particles of silica which, after air jet grinding was introduced, were well rounded but not perfectly spherical in shape. The silica was cleaned by various washing procedures to remove heavy metals that were thought to cause peak tailing. This material, when converted to a reversed phase, packed well and provided columns of the expected efficiency (c. L/1.6dp, where L is the column length and dp the particle diameter). The introduction of spherical silica, which was claimed to pack more easily and was manufactured from silica of greater purity, has now largely replaced the irregular silica. In fact, the retentive properties and efficiencies derived from the spherical silica do not seem to be strikingly different from those produced from irregular silica, if packed properly.

Reversed phases are mostly produced in four particle sizes: 3, 5, 10 and 20 |im. The 3 |im material is usually provided in columns 3 or 5 cm long, giving about 5000 and 8000 theoretical plates respectively. These columns can be operated at 1-2 mL min~1 and thus provide fairly fast separations. The 5 | m packings are usually supplied in columns 5 and 10 cm long and the 10 | m particles in columns 10 and 20 cm long. A column 20 cm long packed with 10 | m particles can be expected to provide efficiencies of about 12 000 theoretical plates at a flow rate of 1 mL min "1

Figure 2 The adsorption isotherms for some aliphatic alcohols on a reversed phase. 1, Butanol; 2, propanol; 3, ethanol; 4, methanol. (Reproduced with permission from Scott, 1993.)

0 1.0 2.0 Acetophenone in the mobile phase <%w/W

Figure 3 The adsorption isotherm of acetophenone between a reversed phase and an aqueous solvent mixture containing 40.4%w/v acetonitrile. (Reproduced with permission from Scott and Kucera, 1977.)

0 1.0 2.0 Acetophenone in the mobile phase <%w/W

Figure 3 The adsorption isotherm of acetophenone between a reversed phase and an aqueous solvent mixture containing 40.4%w/v acetonitrile. (Reproduced with permission from Scott and Kucera, 1977.)

and is thus very useful in the separation of complex mixtures. The 20 |im packing is almost exclusively used for preparative columns.

Reversed-phase columns are slurry packed, usually employing a methanol-water mixture or ethanol as the packing solvent. There are many proprietary solvent mixtures used in packing reversed-phase columns and the technique has assumed an artificial cloak of mystery. The important aspect of packing is to use a wetting agent (there are many that can be used successfully) to ensure the particles do not clump together in the packing solvent but are completely dispersed. This can be checked under a low powered microscope. The packing, dispersed in the solvent and wetting agent (often by sonic vibration), is packed by suddenly applying about 8000 psi pressure to the particle dispersion contained in a high pressure reservoir, forcing the slurry through the column; the packing is retained by an appropriate terminal frit.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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