General properties of stationary phases

The particle size distribution should be as narrow as possible (with a ratio of diameters of the smallest and the largest particles of 1 1.5 or 1 2, as already mentioned). This is because the smallest particles determine the column permeability and the largest particles fix the plate number. Small particles produce a high flow resistance and large particles are responsible for a high degree of band broadening. For this reason, a particle size analysis accompanies many products. 

The properties of the packing materials may vary from batch to batch of the same product, so a large amount of one batch should be bought for series analysis over a long period of time. 

Various types of stationary phases are in use Porous particles, nonporous particles of small diameter, porous layer beads, perfusive particles, and monolithic materials. 

This is the usual type of HPLC stationary phase. These materials are 1.8, 3.0, 3.5, 5.0 or 10 gm in size. As a rule of thumb, their performance, i.e. the plate number per unit length, doubles each time from 10 to 5 and 3 gm, whereas the pressure drop increases each time by a factor of four. Their internal structure is fully porous and can best be compared with the appearance of a sponge (however, in contrast to a sponge, the structure is very rigid). Within the pores the mobile phase (and the analytes) does not flow but moves only by diffusion. 

These are large particles with a diameter in the 30 gm range which allows to pack them dry. They consist of a nonporous core (e.g. from glass) which is covered with a 1 -3 gm layer of a chromatographically active material. Porous layer beads (PLBs) are rarely used nowadays but can be found in guard columns or as repair material for deteriorated columns with collapsed packing. 

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