Separations Based on Dispersive Interactions

Separations based solely on dispersive interactions in GC must employ a non-polar stationary phase such as a hydrocarbon or an alkyl silicone polymer. The separation of unleaded gasoline separated on a 50-m fused silica capillary column coated with 0.5 mm film of a methysilicone polymer is shown in figure 11.

Courtesy of Supelco Inc.

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

Helium was employed as the carrier gas at a flow-rate of 20 ml/min. The temperature of the column was programmed from 35oC to 200oC at 2oC per min. The retention of the solutes increases with the molecular weight irrespective of them 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. An example of the use of dispersive interactions in LC is afforded by the separation of some benzodiazepines on a reverse-phase column shown in figure 12. The column, 15 cm long, 4.6 mm I.D., contained a bonded reversed phase (dispersive phase) having aliphatic chains 8 carbon atoms long. The mobile phase was a mixture of acetonitrile and an aqueous solution of potassium dihydrogen phosphate and triethylamine to act as a buffer (pH 3.15). The retentive interactions with the stationary phase are solely dispersive. In contrast, the interactive character of the mobile phase was largely polar but accompanied by some dispersive interactions with the acetonitrile.

Courtesy of Supelco Inc.

Figure 12 The Separation of Eight Benzodiazepines

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.