Principles and Practice of Chromatography - Molecular Forces and Chromatographic Selectivity > Separations Based on Dispersive Interactions > Page 30

Molecular Forces and Chromatographic Selectivity

To chose a suitable stationary phase for a particular separation it is necessary to select a substance with which the solutes will interact relatively strongly. If the solutes to be separated are predominantly dispersive, then a hydrocarbon-like stationary phase would be appropriate, which, in GC, 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 for each and thus permit elution in a reasonable time.

Interactions in the mobile phase are extremely weak in GC, (5) and are not employed to influence selectivity. In LC, an appropriate dispersive stationary phase might be a bonded phase with a long aliphatic chain. To ensure that the selectivity resided predominantly in the stationary phase, a complementary polar and weakly dispersive mobile phase would be used. In LC, it is usual to allow one type of interaction to dominate in the stationary phase while a different type of interaction remains controlling in the mobile phase.

Separations Based on Dispersive Interactions

Separations based solely on dispersive interactions in GC must employ a nonpolar 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.