Bonded Phases - Introduction to Bonded Phases
Introduction to Bonded Phases
Although the use of silica gel as a stationary phase actually made the renaissance of liquid chromatography possible, today silica gel, per se is not now used so often. There are several reasons for this. The selectivity obtainable from silica gel is predominantly based on polar interactions and in order to separate non-polar materials and materials of biological origin (where the demand for effective LC separation techniques is, perhaps, the greatest) a different type of stationary phase selectivity is essential. If attempts are made to separate materials of biological origin on silica gel, as the components of the sample will be highly polar, themselves, retention on the basis of polarity may be large, but the selectivity is often poor. This is due to the overall polarity of the individual sample components often being very similar. Silica gel, when used as a stationary phase demands a fairly wide range of solvents to be available as the mobile phase and this increases both the complexity of the analytical procedure and the solvent disposal costs. Water de-activates silica rapidly to such an extent that it exhibits very weak interactions with most substances during the separation process and, as a consequence they are only slightly retained and thus, poorly resolved. Another serious problem with silica as a stationary phase is that it is significantly soluble in water and, thus, if aqueous solvents are used as the mobile phase, the column performance will gradually deteriorate due to the dissolution of the silica. This will be particularly serious if the pH of the mobile phase is outside the range of 4.0 - 8.0. Consequently, it became obvious relatively early in the renaissance of LC that it was necessary to develop a stationary phase that could provide significant dispersive interactions with a solute that would be unaffected and insoluble in aqueous mobile phases. It follows that the preparation of reverse phases was the first bonding process to be investigated
Today there is a large number of bonded phases available with an extensive range of polarities but, of these, the reverse phases are by far the most commonly used in modern LC. It should be explained that the term reverse phase, though conventional and generally adopted, it has no precise meaning. The term reversed phase originated from the work of Martin and Synge when they were examining different phase systems. Martin and Synge replaced a phase system of water as the stationary phase and a hydrocarbon as the mobile phase by a hydrocarbon as the stationary phase and water as the mobile phase. In fact this can be called a phase reversal and the alternative stationary phase was called the reverse phase. Today, reverse phase is a term confined to stationary phases that are exclusively dispersive in interactive character and, thus, retain solutes exclusively by dispersive interactions.