Principles and Practice of Chromatography - Molecular Forces and Chromatographic Selectivity > Separations Based on Polar Interactions > Page 34
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 specially 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.