Systematic comparison of different functionality columns for a classical pharmaceutical problem.

The performance of five reversed-phase columns which included a standard C18 phase, a polar embedded phase (amide group), a polyethyleneglycol phase, a cyano phase, and a perfluorinated phase, all coming from the same manufacturer, have been studied. They were systematically compared with a test mixture containing basic, neutral and acidic compounds of very different polarities, as well as different functional groups (acetaminophen, phenylephrine hydrochloride or phenylpropanolamine hydrochloride, chlorpheniramine maleate, 4-aminophenol, 4-chloracetanilide and 4-nitrophenol) at three pH levels (2.5, 4.6 and 7.0) and three proportions of buffer/acetonitrile (80:20, 50:50 and 20:80, v/v). The results obtained have permitted our group to develop unique applications with these columns, as these compounds are not only a test mixture, due to their chemical characteristics, but they are also usually contained in pharmaceutical formulations for the relief of common cold symptoms and have been selected as a real-life case study. Moreover, after observing the reversed-phase and normal-phase-like characteristics for certain analytes on the perfluorinated phase, a systematic study was developed in this column to understand the chromatographic behaviour of these compounds at three pH (2.5, 4.6 and 7.0) and seven different organic proportions from 20 to 80% acetonitrile. The predominant electrostatic interactions observed on the perfluorinated phase could explain its special behaviour and the high retention at higher percentages of organic solvent, especially for amine compounds, which makes this column very advantageous in working with LC/MS. Different applications with volatile buffers, such as TFA at pH 2.5 and ammonium acetate at pH 4.6 and 7.0, were also considered. Some results have been related to parameters frequently employed for column description.