Principles and Practice of Chromatography - Molecular Forces and Chromatographic Selectivity > Separations Based on Polar Interactions > Page 33
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.
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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.