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

Retention and Exclusion All stationary phases based on silica gel exhibit exclusion properties.  Polystyrene stationary phases are broadly similar in physical form but their exclusion properties tend to be less significant than those of silica gel. The exclusion properties of silica gel arise from its method of formation and so the processes involved will help reveal the physical nature of the gel. Silica gel is an amorphous, highly porous, partially hydrated form of silica  made from the two most abundant elements in the earth's crust, silicon  and oxygen. More than 55%w/w of the earth's surface consists of either silica (silicon dioxide) or silicates (silica combined with metal oxides). Silica found naturally is not

The Preparation of Spherical Silica Gel Spherical particles of silica can be prepared by spraying a neutralized silicate solution (the colloidal silica sol.) into fine droplets before gelling has taken place and subsequently drying the droplets in a stream of hot air. It has also been shown possible to disperse a silica sol in the form of an emulsion in a suitable organic solvent where the droplets gel in spherical form (12). Unfortunately, details for the preparation of spherical silica have tended to be kept very confidential for commercial reasons and so information is a little sparse. One of the first methods reported was that of Le Page et al (13,14). A stable silica sol. (generated at low pH so that gelling does not take place) is passed through a non aqueous solvent in such a manner as to produce droplets. These droplets rapidly solidify and are then filtered off, dried, and heated to 400˚C to 800˚C to form a rigid xerogel. The structure of the resulting

may vary significantly, retention data obtained on one silica gel cannot be compared directly with that obtained on another, even if the same phase systems are employed. Silica Gels with Different Exclusion Properties The size of the primary particles can be changed, either by the method of manufacture or by the subsequent treatment of the gel, consequently, silica gels can be synthesized with a particular range of pore diameters to suit a specific application. Alternatively, standard silica gels can be blended together to give a mixed gel with definitive exclusion properties for specific separations. The exclusion properties of a silica gel cannot be obtained with sufficient accuracy for chromatographic use from nitrogen adsorption data or mercury porosity tests. It is necessary to determine the range of pore diameters and pore volume of a silica gel by a special experimental procedure that is designed to obtain accurate retention volume measurements for solutes eluted

The Structure of Silica Gel The matrix of the primary silica gel particle consists of a core of silicon atoms joined together with oxygen atoms by siloxane bonds (silicon-oxygen-silicon bonds). On the surface of each primary particle some residual, uncondensed hydroxyl groups from the original polymeric silicic acid remain. These residual hydroxyl groups confer upon silica gel its polar properties. These hydroxyl groups react with the silane reagents to form bonded phases. The silica surface is quite complex and contains more than one type of hydroxyl group, strongly bound or 'chemically' adsorbed water and loosely bound or 'physically adsorbed' water. There are three types of hydroxyl group. The first is a single hydroxyl group attached to a silicon atom which has three siloxane bonds joining it to the gel matrix. The second is one of two hydroxyl groups attached to the same silicon atom which, in turn, is joined to the matrix by only two siloxane bonds. These twin

and the onset of gel formation and, consequently, the mechanical strength of the gel.    After the gel has first set, it is a very soft and is usually transferred to vats or trays where it is then allowed to stand for a number of days. During this period, condensation between the primary particles continues to take place and the gel shrinks and exudes salinated water. This shrinking process accompanied by saline elimination is called sinerisis and eventually a firm, almost rigid gel is produced which is called the hydrogel. The compact hydrogel is then well washed under controlled conditions to eliminate the last of the sodium chloride and then heated for a few hours at 120oC. The resulting product is a hard amorphous mass called the xerogel. The xerogel, ground and graded is the material that is used for packing LC columns and manufacturing bonded phases. The product, prepared in this way, is called irregular silica gel, to differentiate it from spherical silica gel