An example of the use of induced dipoles to separate polarizable substances is afforded by the analysis of some aromatic and nitroaromatic hydrocarbons by LC using silica gel as the stationary phase.

Courtesy of Supelco Inc.

Figure 14 The Separation Aromatic and Nitro-Aromatic Hydrocarbons

A small-bore column 25 cm long and 1 mm I.D. was employed, packed with silica gel having a particle diameter of 10 m. The mobile phase was n-hexane at a flow-rate 50 ml per min. The solutes of interest are naphthalene and pyrene, the first two peaks. The two solutes are well separated and, as they have no permanent dipole, and as dispersive interactions with the silica gel are weak, they are selectively retained almost exclusively by induced dipole interactions. These interactions occur between the strong dipoles of the silanol groups on the silica gel surface and the induced dipoles on the aromatic nucleus resulting from their proximity to the silanol groups. To ensure that polar interactions dominate in the stationary phase the mobile phase consists of the dispersive solvent n-hexane. An interesting example of polar selectivity by hydroxyl groups on the surface of native silica is shown by the analysis of Darvocet® and its generic equivalent in figure 15.

Courtesy of Supelco Inc.

Figure 15 The Analysis of Acetaminophen Formulations

Darvocet® is an acetaminophen product in which it is the active ingredient. Other substances are present which also contain polar groups and thus, the sample lends itself to separation on the polar stationary phase, silica gel. The analysis was completed in less than 4 minutes using a short column 3.3 cm long and 4.6 mm in diameter packed with silica (particle size of 3 m). The column appears to be significantly overloaded, but the impurities are well still separated from the main component and a substance in the generic formulation that was not present in the Darvocet® clearly indicated. The mobile phase was 98.5% dichloromethane with 1.5% v/v of methanol containing 3.3% ammonium hydroxide. Although, the ammoniacal methanol helped to decrease extreme polar activity from especially active adsorption sites on the silica surface, the overall interaction of the solutes with the stationary phase was predominately polar. In contrast solute interactions with the methylene dichloride in the mobile phase would be almost exclusively dispersive.