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)

Courtesy of Supelco Inc.
Figure 13 The Separation of Some Airborne Contaminants by GC

The solutes that disclose the nature of the interactions that are taking place with the two stationary phases are methanol and benzene. On the polyethylene glycol stationary phase, methanol (B. Pt. 64.7oC, polar) and benzene (B. Pt. 80.1oC, polarizable) are eluted well after n-heptane (B. Pt. 98.4oC, dispersive) and 1-heptene (B. Pt. 93.6oC, dispersive and slightly polarizable). The strong polar interactions between the OH groups of the polyethylene glycol and both the OH groups of the methanol and the polarized nucleus of the benzene are far greater than the dispersive interactions with n-heptane and n-heptene. This difference occurs despite the greater molecular weights and higher boiling points of heptane and heptene. On the stationary phase Carbopack, methanol, weakly dispersive, is eluted almost at the dead volume while the more dispersive solutes are extensively retained.