Molecular Forces and Chromatographic Selectivity

To choose an appropriate stationary phase it is necessary to select a substance with which the solutes are likely to interact strongly to provide significant retention. For example, if the solutes to be separated were predominantly dispersive, then a dispersive, hydrocarbon-like stationary phase would be appropriate. In GC, such a stationary phase might be a high molecular weight hydrocarbon such as squalane. The operating temperature would be chosen so that the kinetic energy of the dissolved solutes molecules was sufficiently high to provide adequate partial vapor pressure to permit each solute to be eluted through the column in a reasonable time.

Separations Based on Dispersive Interactions

Separations that are based exclusively on dispersive interactions in GC are obviously carried out on stationary phases that have non-polar or dispersive characteristics (typically hydrocarbons or one of the alkyl silicone polymers e.g. the methyl silicones). The chromatogram of a sample of unleaded gasoline that has been separated on a 50-m fused silica capillary column coated with 0.5 mm of a methylsilicone polymer is shown in figure 34.

The carrier gas was helium and the flow-rate about 20 ml/min. and the column was temperature programmed from 35^oC to 200^oC at 2^oC per min. As would be expected solute retention increases with molecular weight irrespective as to whether they are simple alkanes or aromatics with polarizable nuclei. As the stationary phase can exhibit no polar interactions, the interactions with the aromatics must be exclusively dispersive and, thus, directly related to their molar volume. As a consequence, they would fall in a similar molecular weight sequence as the paraffins.

Courtesy of Supelco Inc.

Figure 34. A Chromatogram of the Hydrocarbons in Unleaded Gasoline Using a Dispersive (Nonpolar) Stationary Phase