Types of Bonded Phase There are three basic types of bonded phases, the brush phases, the bulk phases and the oligomeric phases. The different phases are produced by the use of the mono, di- and tri- substituted silanes respectively in the bonding process, e.g. the monochloro, dichloro and tricoloro silanes. The monochlorosilanes, for instance octyldimethyl-chlorosilane, react with the hydroxyl groups on the silica surface to produce dimethyloctylsilyl chains attached to the silica. Brush Phases The alkyl chains are thought to stand out from the surface like bristles of a brush,

procedure can be used as that in the synthesis of oligomeric phases and the material can be alternatively treated with water and the trichlorosilane reagent. Layers of bonded phase are built up on the surface but, in this case, due to the tricoloro function of the reagent, extensive cross-linking occurs. As a result of the polymerization process, the stationary phase has a chemically cross-linked, multi-layer character and, consequently, is termed a "bulk" phase. The "bulk" phases are almost as popular as the "brush" phases as they tend to have a higher carbon content (more organic material bonded to the surface) and thus provide a little greater retention and selectivity. "Bulk" phases have about the same stability to aqueous solvents and pH as the "brush" phases.   Using appropriate organic chlorosilanes, polar or polarizable groups such as nitriles or aromatic rings can be bonded to the silica to provide stationary phases

of entry and consequently the proximity of interaction which, in turn, will determine the energy of interaction and the magnitude of the retention. Finally, the fifth group contains the cyclodextrin based materials that control retention in a similar manner to that previously described for GC. In LC, the cyclodextrin stationary phases are bonded to a support such as silica and are prepared using similar techniques to those for making reverse phases. The more recent and most effective stationary phases are without doubt those based on the macrocyclic glycopeptides and the cyclodextrins

Liquid Chromatography Stationary Phases Traditionally the stationary phase used in LC has been silica gel which separates solutes largely on the basis of polarity, although, due to its unique structure, silica gel also exhibits strong exclusion characteristics. The bonded phases were introduced to provide a material that would separate solutes by dispersive interactions and also to provide some semie polar stationary phases. The bonded phases were also based on silica gel. More recently, polymeric stationary phases were

Between 'Brush' and 'Bulk' Reverse Phases and Aqueous Solvents The interactions between aqueous solvents and brush reverse phases differ very significantly from those with a  bulk reverse phase at very low concentrations of solvent. This difference has been investigated by a number of workers (25-27) and the basic difference between the two types of phase are shown in the curves relating retention volume of methanol to the concentration of methanol in the mobile phase in figure 35. The phases shown are the RP-18 brush, reverse phase manufactured by E. M. Laboratories, which had a C18 (dimethyloctadecyl) chain and ODS-3 a bulk reverse phase which had a C18 (octadecyl) chain and was manufactured by Whatman Inc. The curves relating retention volume with solvent composition for the two phases show very different behavior patterns. The ODS-3 bulk reverse phase behaves in the expected manner, as the concentration of methanol increases the retention volume of the ethanol

High Temperature Stationary Phases   A capillary column contains very little stationary phase and it follows that the partial pressure of the coated material must be extremely low, otherwise it will be vaporized and rapidly lost. The chromatographic effect of loss of stationary phase will be a continues reduction in retention times until finally all the solutes will be eluted at the dead volume. As a consequence, it is extremely difficult to find stationary phases that have sufficiently low vapor pressures to allow them to be used much above 250 ûC. The stationary phases that have the highest operating temperatures are, without doubt, the carborane/silicones. Typically, Dexsil 300 carborane methyl silicone, a largely dispersive stationary phase, can operate satisfactorily at 540oC whereas, the weakly polar Dexsil 400 carborane/methyl phenyl silicone is much less thermally stable and can only be used at 450oC. A diagram of the carborane structure is