Irregular column packings

Analytical hplc these days is nearly always done with microparticulate column packings, which are small porous particles, usually spherical or irregular silica, with nominal diameters of 3,5 or 10 fxm. They combine the best features of the other two types, having high efficiency as well as a large surface area. In bulk, the appearance of a microparticulate silica resembles that of a fine talcum powder. With microparticulates, dry packing methods result in column beds that are unstable under pressure, so they are packed into columns using a slurry of the material in a suitable solvent and under considerable pressure. 

The particle size of the stationary phase material plays a very vital and crucial role in HPLC. In fact, high-elficiency-stationary-phase materials have been researched and developed exclusively for HPLC having progressively smaller particle size termed as microparticulate column packings. These silica particles are mostly uniform, porous, with spherical or irregular shape, and having diameter ranging from 3.5 to 10 pm. 

Column packed with Cyclobond I, /3-cyclodextrin bonded to irregular silica gel. 

Recent technological advancements made spherical particles widely available and relatively inexpensive. Columns packed with spherical particles exhibit signihcantly higher efficiency, and columns packed with irregular particles are seldom used and are becoming nonexistent for analytical scale separations. 

The portion of the peak broadening which arises due to the irregularity of the column packing is called Eddy diffusion. It is approximated by Eq. (19) ... 

The compaction of the bed can be carried out both mechanically or hydraulically. In a mechanical scheme, the bed is compressed to a controlled volume, while the hydraulic compression uses a controlled pressure. Most of the initial studies of radial compression were carried out with a controlled compression pressure, applied either to preparative columns packed with irregularly shaped particles with a particle size of about 80/rni or analytical columns packed with spherical lO- tm particles (12). 

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