Bonded Phases - The Retention Properties of Bulk and Brush Phases
The Retention Properties of Bulk and Brush Phases
It was shown that by determining the carbon content of the reverse phase and taking into account the surface area of the parent silica, the surface coverage and thus, the extent of coverage could be determined. It would now be interesting to consider how the silica gel loading (the amount of bonded phase placed on the silica surface) as indicated by the carbon content of the reverse phase effects its retentive properties.
The retention volume of a solute (V'r ) is given by the following equation,
V'r = KVS or V'r = KAS
where (K) is the distribution coefficient of the solute between the two phases,
(VS) is the effective volume of stationary phase in the column,
and (AS) is the effective surface area of the packing.
The two equations are given to illustrate that the reverse phase system may be considered as a liquid/liquid or liquid/solid distribution system where,
VS = dfAS
and (df) is the effective film thickness of the bonded material together with any solvent that may also be adsorbed on its surface
It is seen that the retention volume of a solute will depend solely on the volume, or surface area, of the bonded material that, providing all the bonded phase is available for solute interaction, will be proportional to the carbon content of the phase. In addition, solute retention need not necessarily be related simply to the surface area but can also depend on the thickness of the stationary phase. Scott and Kucera (27) examined a series of commercially available reverse phases and determined the carbon content of each phase and the retention volume of a series of solutes on columns packed with each adsorbent. The results obtained are shown in table 2.
Table 2. The Properties of Five Different Commercial Reverse Phases
Table 2 shows that the E. M. Labs products are prepared from base silica s having very different surface areas. As the pore size tends to vary inversely to the surface area, it is also clear that the reverse phase having the longest chain length is bonded to the silica with the largest pore size.