Liquid Chromatography - Liquid Chromatography Stationary Phase 1

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 introduced to provide materials that were insoluble in water and that were stable at extremes of pH.

Silica Gel

The Preparation of Irregular Silica Gel

Silica gel is manufactured by releasing silicic acid from a strong solution of sodium silicate by hydrochloric acid. (Sodium silicate is prepared by heating sand at a high temperature in contact with caustic soda or sodium carbonate).

Initially, silicic acid is released,

Na2SiO3 +H2O + 2HCl = Si(OH)4 + 2NaCl

and then the free acid quickly starts to condense with itself with the elimination of water to form dimers, trimers and eventually polymeric silicic acid. The polymer grows, initially forming polymer aggregates and then polymer spheres, a few Angstrom in diameter. These polymeric spheres are called primary silica particles. These primary particles continue to grow until, at a particular size, the surface silanol groups on adjacent primary polymer particles, condense with the elimination of water. This condensation causes the primary particles to adhere to one another and at this stage the solution begins to gel. During this process, the primary particles of silica gel will have diameters ranging from a few Angstrom to many thousands of Angstrom depending on the conditions of formation.