INFLUENCE OF ELUTION CONDITIONS ON HPLC RETENTION INDEX VALUES OF SELECTED ACIDIC AND BASIC DRUGS MEASURED IN THE 1-NITROALKANE SCALE

The retention indices (RI) of 62 acidic, neutral, and basic drugs were determined by HPLC under isocratic conditions (20, 40, and 60% acetonitrile-phosphate buffer, pH 3.2, containing 0.05% nonylamine) and with gradient elution (5-70% acetonitrile-phosphate buffer pH 3.2, containing 0.05% nonylamine). The RI values of acidic drugs and earlier-eluting basic drugs decreased distinctly with increasing concentrations of acetonitrile. For the later-eluting basic drugs, the opposite trend was observed. The RI data obtained with different isocratic conditions and with gradient elution were not transferable. The study shows the importance of careful standardization of elution conditions for interlaboratory comparison.