Study on Separation Mechanism of Ephedrine and Pseudoephedrine in Borax Buffer by Capillary Electrophoresis

Background: In borax buffer, ephedrine (EPH) and pseudoephedrine (PSE) can be well separated (Rs>10) in capillary electrophoresis (CE) without adding any other compounds, but the specific mechanism remains unclear. Methods: Titration analysis, nuclear magnetic resonance analysis, mass spectrum analysis and molecular modeling were used to investigate the separation mechanism. Results: Tetrahydroxy borate (a hydrolyzate of borax or boric acid) could react with ephedrine and pseudoephedrine to form self-assembled complexes, but the two complexes had different stabilities and electrophoretic mobilities resulting from the different steric configurations of analytes. Conclusion: The complexation based on borax was responsible for the separation.