Effect of the counter-anion type and concentration on the liquid chromatography retention of beta-blockers

Analysis of beta-blockers, basic pharmaceutical compounds with pKa values greater than 8.5, in reversed-phase HPLC can sometimes be challenging in terms of selection of the mobile phase pH, buffer concentration, and acidic modifier. The effect of the type and concentration of various mobile phase additives on the reversed-phase HPLC retention of these compounds was studied. HPLC analysis was performed at a mobile phase pH of 3 ensuring the protonation of the beta-blockers. It was found that at increasing perchlorate anion concentration at a constant mobile phase pH the retention factor for all beta-blocker compounds studied increased to varying degrees. The relative increase in the retention was attributed to ion interaction with the anionic mobile phase additive. Similar trends were observed when other types of inorganic salts such as NaH2PO4, NaPF6, NaBF4, and CF3CO2Na were employed. Differences in selectivity of the beta-blockers were obtained at a constant pH and an equimolar concentration of the different additives throughout the whole concentration range studied.