Liquid Chromatography - The Synthesis of Bonded Phases
The Synthesis of Bonded Phases
The particle size will be determined by the nature of the separation and will vary with the complexity of the mixture and the instrument characteristics. For example a difficult separation will require high efficiencies and thus a small particle diameter (e.g. 3 mm). If however, the available column pressure is limited then the particle diameter made need to be relatively large (e.g. 10 mm) to provide adequate flow rate.
The choice of the pore size and surface area of the silica is more complex. The surface area of a silica tends to vary inversely as the pore size, so the larger the pore size the smaller the surface area. Thus, when one is defined the other, to some extent, is also specified. The most efficient bonded phase has the maximum surface coverage. It is understood, that due to stearic hindrance from the bonded moiety itself, only a proportion of the silanol groups can be bonded and there is little that can be done to avoid this problem. However, there are other reasons for incomplete silanization of the silica. Incomplete silanization can result from the reagent molecule being excluded from the smaller pores of the silica. Exclusion can be a particular problem when bonding relatively large molecular weight materials such as long chain hydrocarbons onto the silica surface. It is therefore, important to choose a silica gel that has a relatively large pore size (e.g., a mean pore diameter of 150Å) which may limit the surface area to between 150 and 250 sq.m per gram and thus, reduce the retentive capacity of the stationary phase. Nevertheless, this will ensure that the vast majority of the pores will be accessible to the silanizing reagent. Care must be taken to ensure that the pore size is not chosen to be so large that the silica gel is mechanically weak and collapses under pressure during packing or even during normal use. An alternative approach is to subject the silica to hydrothermal treatment. Hydrothermal treatment removes silica from the surface of large pores and deposits it on the smaller pores, which eventually become blocked and thus, impermeable. This would remove the small pores, and consequently possible sites for future erosion and make the large pores even larger and more accessible to the silanizing reagents.