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There are new strategies under development that allow one to synthesize a continuous chromatographic bed without particle structure as “rigid rods” or “monoliths.” Such materials promise low-peak dispersion even at high-flow velocities.
Retention and Selectivity
Table 6 Physical Data on Silica Gel Materials for Analytical HPLC
Material name Mean pore Specific surface Specific pore
diameter (A) area (m2/g) volume (mL/g)
T .iChrosorh 60 500 0 75
(Merck) 100 300 1.0
LiChrospher 100-RP-18 300 0.85
(Merck) 100 400 1.25
100 RP-18 350 1.25
300 RP-18 80 1.0
Source: Ref. 46.
2. Silica Gels
Silica gels for chromatography are amorphous gels with a very polar surface. The main surface groups existing in materials that have not been heat-treated above 500°C are isolated free silanol groups, vicinal silanol groups that form hydrogen bonds with each other, and silanols that bind water by hydrogen bonding . These surface groups are illustrated in Fig. 6. The total concentration of free silanols in amorphous gels is about five groups per 100 A2 corre-sponding to about 8 /øþ³/m2 [47,48]. It varies, however, depending on the preparative procedure.
Silanol groups are weakly acidic with pKa values close to 5-6, depending on their structure (and on the composition of the surrounding mobile phase). They are strong adsorption sites for polar compounds, offering hydrogen-bonding, acid-base, and dipole-dipole interactions. Bare silica is the most common adsorbent used in normal (straight)-phase chromatography. The different surface groups shown in Fig. 6 are clearly not equivalent relative to the Gibbs energy of analyte adsorption. Silica surfaces are thus energetically heterogeneous. Further, traces of metals may be a source of chelating interactions. Controlling the silica material in this respect was decisive for improving energetic homogeneity.
The presence of adsorption sites with different adsorption free enthalpies causes the ad-sorption isotherms to significantly deviate from a linear configuration, even in the range of strong dilution. As a consequence, the resulting peak form becomes nonsymmetrical, exhibiting tailing in the presence of strong heterogeneity. Energetic heterogeneity of acidic silica materials might cause particular problems with basic analytes.
Silanol groups can easily be derivatized by proper chemical reactions to yield stable surface-modified adsorbents, as discussed later. The stability of silica gel toward acids and bases is limited. Aqueous mobile phases with a low content of organic modifier should have
, H H » H h * 'h
9 î 'O 0 0
² ²² ²²²
Figure 6 Illustration of the main surface groups of silica gels: (i) isolated free silanol group; (ii) vicinal silanol groups; (iii) silanols binding water. (According to Ref. 42.)
a pH in the range of 2.5-8.5. (Specially treated materials tolerate pH values up to 9; similarly at high organic modifier concentrations, the stability of silica is increased.)
3. Other Inorganic Materials
Alumina (A1203) is a polar, basic material that exhibits about six hydroxyl groups per 100 A2. Because of the Lewis-acidic sites of the A1 ion, unsaturated compounds are more strongly adsorbed onto alumina than on silica. The specific surface areas are between 100 and 200 m2/g, pore volumes about 0.2-0.3 mL/g, and mean pore diameters between 100 and 200 A . Alumina has been less frequently employed as adsorbent in analytical HPLC than has silica. It cannot be derivatized as easily as silica, but chemical modification of the surface can be achieved by coating the surface of the particle with polymeric layers, which will then be the material interacting with the analyte . With aqueous eluents, A1203 exhibits improved stability at higher pH values than does silica, and it offers weak cation-exchange properties.
Porous graphitized carbon (PGC)  is a hydrophobic material with a rigid surface structure. It is usually operated in a reversed-phase mode.
4. Organic Polymers
Organic polymeric support materials are most frequently used with attached functional groups appropriate for a special type of chromatography. Although these attached groups are the decisive interaction sites, the chemical structure of the support itself is of significant relevance, particularly for its hydrophobicity or hydrophilicity. Strongly hydrophobic support polymers, such as polystyrene-divinylbenzene, can be distinguished from hydrophilic materials, such as polyacrylamides, polymers of hydroxy-substituted acrylamides, and agarose (and, for low-pressure LC, also cellulose). The stability of these materials across a wide range of pH is excellent, which makes them excellently suited for ion-exchange chromatography, even under harsh conditions. The energetic heterogeneity of these materials is less troublesome than that of silica gels. An extensive discussion of the recent significant developments in polymeric support materials is given elsewhere .