The Mechanism of Chromatographic Retention - Retention and Exclusion > Silica Gels with Different Exclusion Properties > Page 65

At the other extreme Partisil 10 would be ideal for separating substances having relatively small molecular diameters e.g. from 5 to about 100, solute having molecular diameters greater than 100 being crowded together and unresolved.

Although, a very useful technique for separating substances on the basis of molecular size, exclusion chromatography suffers from the disadvantage of having very limited 'peak capacity'. All the solutes to be separated must be eluted between the excluded column volume and the fully permeating column volume. Thus, all the peaks in the chromatogram must be eluted in perhaps only 1 or 2 ml of mobile phase. Consequently, for multi-component mixtures to be successfully separated, the column must have a very high efficiency.

An example of a column 10 m long, having an efficiency of 250,000 theoretical plates separating some aromatic hydrocarbons is given in figure 30.

Column, length 10m, I.D. mm, mobile phase tetrahydrofuran, flow-rate 30ml/min, adsorbent Partisil 10. Solutes benzene, ethyl benzene, butyl benzene, hexyl benzene, octyl benzene and decyl benzene.

Figure 30. Chromatogram of a Series of Alkyl Benzenes Separated on a Column having an Efficiency of 250,000 Theoretical Plates