Silica gels polar surface-modified

The stationary phase in LC is a fine granular solid such as silica gel. It can be used as such (mainly for nonpolar compounds), or the granules can be modified by a surface-bonded coating that changes (reverses) the polarity of the gel. A very small selection of stationary phases is listed in Table 35.2. 

Nevertheless, silica gel is the material of choice for the production of the vast majority of LC stationary phases. Due to the reactive character of the hydroxyl groups on the surface of silica gel, various organic groups can be bonded to the surface using standard silicon chemistry. Consequently, the silica gel surface can be modified to encompass the complete range of interactive properties necessary for LC ranging from the highly polar to almost completely dispersive. 

These silica gels retain analytes through interaction between polar groups silica gel itself or the surface-modified polar silica gels shown in Figure 15.10 may be used. 

Silica gel 60, the most versatile and most frequently used TLC sorbent, was taken as a basis. The mean particle size of this sorbent was optimized simultaneously, the particle size distribution was brought to within as narrow limits as possible (L- ) The sorbent material thus obtained was used t prepare HPTLC pre-coated plates silica gel 60, being followed subsequently by the development of other sorbents for processing into HPTLC pre-coated layers. The materials chosen were largely "reversed phase" sorbents, that is to say, chemically modified silica gels with a non-polar surface (, 8), as well as microcrystalline cellulose ( )""... 

To remedy this situation and reduce the often excessive gel s polarity, the silanol groups are exploited in order to provide sites of covalent bonding for organic molecules. Bonded silica gel, modified in this way, behave as a liquid in that the separation mechanism now depends on the partition coefficient instead of adsorption coefficient. These covalently bonded phases, whose polarity can be easily adjusted, constitute the bases of the reversed phase polarity partition chromatography or RP-HPLC, used in the majority of HPLC separations. Two types of syntheses lead to monomeric or polymeric bonded surfaces ... 

The recently introduced hydrophilic modified precoated plates have broadened the spectrum of selectivity available for TLC. These plates are referred to as HPTLC-NH2 precoated plates, in which > -aminopropyl groups are bonded to the surface of a silica-gel skeleton, and HPTLC-CN precoated plates. with cyanopropyl moieties as the functional groups. In terms of polarity the two hydrophilic modifications can be classified as follows Normal Si02 > NH2 > CN > RP. Since the surface NHv and CN groups are of medium polarity, no wetting problems are encountered with the corresponding plates even at the maximum level... 

Reversed-phase separation of carotenoids on octadecyl-bonded silica provides a mild and sensitive method for the TLC analysis of chloroplast pigments. The separation can be explained on the basis of partition between the mobile phase and the hydrophobic surface of the modified silica gel layer, but the separation mechanism is not fully understood. Less polar compounds such as the carotenes are strongly held according to their lipophilic nature. The retention of the xanthophylls is determined mainly by the nature and the number of the oxygenated substituents. Representative Ry values as found for some carotenoids with this system are shown in Table 7. Solvent systems 2 and 3 are most suitable for general separations, while system I gives a better separation of the more polar compounds. 

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