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(adjusted to pH 7 with 85% phosphoric acid)60% acetonitrile and 15% methanol Flow-Rate 2 ml/min. Detector UV adsorption at 215 nm Sample Volume 100ml     An example of the use of native silica is in the analysis of Darvocet® and its generic equivalent formulation. The separation obtained is shown in figure 44. Darvocet® is an acetaminophen product of which acetaminophen itself is the active ingredient together with other weakly polar substances present. Consequently, the mixture lends itself to separation on 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. The silica packing had a particle size of 3 m providing a maximum efficiency of about 5,500 theoretical plates.   Courtesy of Supelco Inc.   Figure 44 The Analysis of Acetaminophen Formulations

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

was shown to be present in the generic formulation that was not in the Darvocet®. The mobile phase was 98.5% dichloromethane with 1.5% v/v of methanol containing 3.3% ammonium hydroxide. The ammoniacal methanol deactivated the silica gel but the interaction of the solutes with the stationary phase would still be polar in nature. In contrast solute interactions with the methylene dichloride would be exclusively dispersive.   Courtesy of Supelco Inc.   Figure 59. The Analysis of Acetaminophen Formulations   Ion chromatography can be used in a number of novel ways and employing the appropriate conditions can even be used to separate mixtures where the components are not ionic or do not normally  produce interactive ions in aqueous solution. An example of this type of separation is the analysis of saccharide mixtures using ion exchange interactions. An illustration of such a separation is given in figure 60. The  saccharides are reacted with a borate with which

the stationary phase would become almost exclusively dispersive. This is an example where a complex phase system was necessary because there was limited column efficiency available. It is likely that a column with intrinsically more efficiency might achieve the separation with a much simpler solvent system and a more straightforward solvent program. An example of the use of native silica is given by for the analysis of Darvocet® and its generic equivalent formulation. Darvocet® is an acetaminophen product in which the active ingredient (and other material in the medicine) are weakly polar and, consequently, lend themselves to separation on a strongly polar stationary phase such as silica gel. The analysis is depicted in figure 59. The analysis is completed in less than 4 minutes using a short column 3.3 cm long and 4.6 mm in diameter. The silica packing had a particle size of 3 m providing a maximum efficiency of about 5,500 theoretical plates