Principles and Practice of Chromatography - Molecular Forces and Chromatographic Selectivity > Separations Based on Polar Interactions > Page 31

Courtesy of Supelco Inc.

Figure 11 A Chromatogram of the Hydrocarbons Contained in Unleaded Gasoline Using a Dispersive (Nonpolar) Stationary Phase

 

Helium was employed as the carrier gas at a flowrate of 20 ml/min. The temperature of the column was programmed from 35oC to 200oC at 2oC per min. The solute retention increases with the molecular weight irrespective as to their being simple alkanes or aromatics with polarizable nuclei. This is because, interactions with the aromatics are purely dispersive and related to their molar volume. As a consequence, they fall in the expected molecular weight sequence for the paraffins.

It is seen that an excellent separation is obtained. To reduce the retention of the solutes the concentration of acetonitrile would need to be increased. This would increase the competitive dispersive interactions in the mobile phase and thus elute the solutes more rapidly. However, the column efficiency would remain the same and thus some resolution would be lost.

Separations Based on Polar Interactions

Two separations by GC of some airborne contaminants shown in figure 13 illustrate the different selectivity that can be obtained by using dispersive or polar stationary phases. GC separations demonstrate the effects of phase selectivity very elegantly as there are no significant interactions with the mobile phase and, therefore, retention differences can be exclusively attributed to the nature of the stationary phase.

Stationary Phase,             Stationary Phase

Polyethylene Glycol,             Carbopack (dispersive)