Separation of Hydrophobic, Positively Chargeable Substances by Capillary Electrophoresis

Abstract

The electrokinetic separation behavior of positively chargeable substances with various hydrophobicities over the pH range 2-11 is described, using plain aqueous buffers, binary media with organic solvents, and systems with surfactants at concentrations between 0.0005 and 75 mM or other buffer additives. Examples studied include dextromethorphan, methadone, ketoconazole, itraconazole, amiodarone, and desethylamiodarone, compounds that play an important role in pharmacotherapy. Electrokinetic chromatography systems comprising dynamically formed micelles or ionic buffer additives that can form ion pairs with the solutes are shown to be unsuitable for the separation of highly hydrophobic, positively chargeable compounds, including the latter three drugs investigated. However, capillary zone electrophoresis (CZE) in binary systems with organic solvents (40-80%, v/v) at a buffer pH near or lower the solute's pK a values or CZE at low pH in the presence of an electrically neutral complexing agent, such as β-cyclodextrin, can be employed effectively. Data presented illustrate that pK a values in binary systems with organic solvents that are more basic than water are smaller than those in aqueous media. Furthermore, somewhat less hydrophobic and positively chargeable compounds can be sufficiently dissolved in aqueous buffer at pH < pK a and can therefore be analyzed by CZE without any buffer additives.