The Mechanism of Chromatographic Retention - Retention and Exclusion > Blended Silica for Exclusion Chromatography > Page 66

The advantages of high column efficiencies in exclusion chromatography are clearly seen. All the solutes are clearly resolved despite having molecular weight differences of only two methylene groups. It is also clear that two solutes differing by only one methylene group would be clearly discerned from one another. The molecular weight of decyl benzene is 218 and, thus, one methylene group would represent a differential of only 6.4% of the molecular weight. As a result of limited column inlet pressures, high efficiency columns must be long and small in diameter. Such columns must be packed in relatively short lengths (1 m) and joined. As a consequence, high efficiency columns are difficult and expensive to build.

Blended Silica for Exclusion Chromatography

Natural silica gels appear only capable of separating solutes having a limited range of molecular diameters. Silica gels with a wide range of pore sizes, suitable for the separation of solutes having a large range of molecular weights are prepared by blending different silica gels. Northrop et al (17) examined the exclusion properties of a number of silica gels all having a particle size of 10m They choose two specific gels, one with a mean pore diameter of 80 and the other with a mean pore diameter of 500 and blended them to provide an exclusion packing that had a molecular weight range extending from 100 to about 1,000,000. The packing was intended for use in the separation of high molecular weight materials of biological origin. The exclusion curves obtained for the two silica gels and the blend having 38% 80 silica and 62% 500 silica are shown in figure 31.



Figure 31. Graph of Exclusion Volume against Molecular Weight