Separations Based on Polar Interactions

Two GC separations of some airborne contaminants are shown in figure 36 and illustrate the different selectivity that can be obtained by using dispersive or polar stationary phases. In GC, there are no significant interactions with the mobile phase and, therefore, the separations shown in figure 36 demonstrate the effects of phase selectivity very elegantly as retention differences can be positively attributed to the nature of the interactions with the stationary phase. The solutes that most effectively disclose the nature of the interactions with the stationary phase in the two mixtures are methanol and benzene. On the polyethylene glycol stationary phase, methanol (B. Pt. 64.7^oC, polar) and benzene (B. Pt. 80.1^oC, polarizable) are eluted well after n-heptane (bp. 98.4^oC, dispersive) and 1-heptene (bp. 93.6^oC, dispersive and slightly polarizable).

Stationary Phase, Stationary Phase

Polyethylene Glycol Carbopack (dispersive)

Courtesy of Supelco Inc.

Figure 36. The Separation of Some Airborne Contaminants by GC

The strong polar interactions between the OH groups of the polyethylene glycol and both the OH groups of the methanol and, to a lesser extent the polarized nucleus of the benzene are far greater than the dispersive interactions with heptane and 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.