The Mechanism of Chromatographic Retention - Retention and Exclusion > Silica Gels with Different Exclusion Properties > Page 62

Exclusion-Advantages and Disadvantages

The ability of silica gel to act as an exclusion media has both advantages and disadvantages. On the one hand it can be used, with benefit, to separate substances on the basis of molecular size. On the other hand it renders the mechanism of retention in normal or reverse phase separations complex and difficult to interpret. Retention will depend, not merely on the nature of the molecular interactions between solute and stationary phase but also on the molecular size of the solute molecule, as this will determine the amount of stationary phase with which it can interact. As the range of pore diameters of different silica gels 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 in relatively small elution volumes. In exclusion chromatography, all the peaks will be contained in a mobile phase volume equivalent to that of the total pore volume of the column. Consequently, the column volume itself must be large and a column 25 cm long and 4.6 mm I.D. is a practical size to obtain results having adequate accuracy. The sample volume used should be 0.5 to 1.0 ml in volume and the detector should have a low sensor volume (cf. 2-4 ml). A mobile phase that would be suitable for exclusion measurements with silica gel is tetrahydrofuran (THF) which will completely de-activate the silica gel so retention will depend mostly on exclusion. Scott and Kucera (16) studied a series of commercially available silica gels and the results they obtained are shown as curves relating pore volume to pore diameter in figure 28.