Elution of propranolol as an ion-pair complex by buffer solutions on C18-Silica

Propranolol (pK(a) = 9.4) was eluted on C-18-bonded Kromasil, equilibrated with buffer solutions of methanol and water (40/60, v/v) containing a constant concentration of a counteranion (12 mM). Nine different counter-anions were studied: Cl-, I-, NO3-, SO42-, CH3COO- , HOOCC2H4COO-, -OOCC2R4COO-, HOOCCOHCOOHCOO-, HOOCCOHCOO-COO-, and -OOCCOHCOO-COO-. The co-cation was K+ or Na+. Vacancy perturbations were measured on three concentration plateaus of propranolol hydrochloride, at 1.2, 12, and 24 mM, by injecting 100 muL of a pure mixture of methanol and water (40/60, v/v). Indirect detection of the solvent, the counteranion, the co-cation, and the chloride ion was carried out at 325 mn, a wavelength at which only propranolol responds. In a 1.2 mM propranolol hydrochloride solution, there is a 10-fold excess of counteranions and only a positive perturbation peak, due to the excluded co-cation and eluting before the column hold-up time, and a large vacancy peak, associated with propranolol, were recorded. Association between propranolol and the counteranion in excess determines the retention time of this second perturbation. The hydrophobicity of the complexes increases in the order Cl- < CH3COO- similar or equal to HOOCC2H4COO- < NO3 < I- < HOOCCOHCOOHCOO- < -OOCC2H4COO- < SO42- similar or equal to HOOCCOHCOO-COO- < -OOCCOHCOO-COO-. Propranolol retention is larger in the presence of the trivalent citrate anion than in that of the bivalent citrate, succinate, or sulfate anions. It is larger with these bivalent anions than with any monovalent anion. Equal concentration of propranolol hydrochloride and buffer in the mobile phase reveals five system peaks associated with the five components (solvent, counteranion, co-cation, chloride, propranolol molecules). In contrast with monovalent anions, bivalent anions (sulfate, succinate, citrate) or trivalent anions (citrate) cause a reversal of the elution order of the perturbation peaks of chloride anions and buffer molecules. This confirms a competition between chloride and buffer anions to form ion pairs with propranolol. The retention of the perturbation signal of the buffer increases with increasing anion charge because multivalent anions can bind to several molecules of propranolol. The perturbation measurements demonstrate the influence of the valence and hydrophobicity of the buffer on the retention of ionizable compounds. The inverse method allowed the derivation of the isotherm parameters from the overloaded band profiles of propranolol. These values confirm that adsorbate-adsorbate interactions increase with increasing valence of the anions.