Bonded Phases - Bulk Type Bonded Phases
There is some experimental evidence supporting the rigid nature of the structure but more work on identifying the precise nature of stationary phase formation would be welcome. The diagrams given demonstrate the chemistry that is taking place and not the actual physical nature of the reacting surface. The surface is not likely to be geometrically organized and will be very disorganized and random in nature. After the reaction is complete, the bonded phase should be well washed with solvent and "capped" with trimethylchlorsilane or hexamethyldisilazane in the usual manner.
Using appropriate organic chlorsilane reagents, polar or polarizable groups such as nitriles or aromatic rings can be bonded to the silica to provide stationary phases covering a wide range of polarities.
Bonded ion exchange media have also been synthesized and one example, prepared by Halasz, has already been discussed. The more common, contemporary form of ion exchange media, however, is prepared from cross-linked polystyrene beads. Ion exchange characteristics can be given to the polystyrene beads, for example, by sulphonation. In general the ion exchange resin columns are less efficient and slower than the bonded phase ion exchange columns. However, resin beads are far more stable at extremes of pH that can be very important in ion exchange chromatography.
Another interesting type of carbon stationary phase that can be made indirectly from silica gel was developed by Knox (11). The basic material is prepared by filling the pores of appropriately sized silica particles with a suitable plastic monomer and allowing the material to polymerize in situ. Excess of monomer is then removed and the product (i.e. silica particles filled with polymer) is then carbonized by heating at elevated temperatures. The resulting carbonized material consisted of a silica gel matrix within which, a form of charcoal has been generated. The silica can then be removed by treatment with a strong alkali or, alternatively, hydrofluoric acid forming, perhaps, what could be considered as a true "reverse phase". It would actually be a "reverse phase" in the true sense, where the pores are where the primary particles of silica existed and the original pores now being replaced by a solid matrix. In this form, the product is far too active for use in chromatography. The adsorption isotherms of solutes being adsorbed on the charcoal surface would be highly non-linear and, as a result, give asymmetric chromatographic peaks. As a consequence, the carbon must be graphitized at extremely high temperatures usually by exposure to argon plasma. Under this treatment, the highly active adsorption sites on the carbon are eliminated. The product, treated in this way, displays linear adsorption isotherms with most solutes and consequently, when used as a stationary phase, produces symmetrical elution peaks. Due to the complexity of its manufacture, the material is expensive but its areas of application and general efficacy are now well established. This stationary phase behaves as a reverse phase in the sense that its interactions with solute and solvent molecules are almost exclusively dispersive in nature. Whether it's performance relative to that of a conventional reverse phase merits the greater cost, is still somewhat open to discussion.