Hydrophobicity‐aided potentiometric detection of catecholamines, beta‐agonists, and beta‐blockers in a mixed‐solvent capillary electrophoresis system

Abstract

A series of cationic drug-like substances with distinct basicity, hydrogen-bonding ability, and hydrophobicity, including three catecholamines, two beta-agonists, and thirteen beta-blockers, was successfully detected in a capillary electrophoresis system using an end-capillary coupled potentiometric sensor consisting of a PVC-based liquid membrane deposited directly on a 100 mum diameter copper rod. The electrophoretic separation was performed on a 72 cm x 75 microm id uncoated fused-silica capillary with an acidic background electrolyte containing phosphoric acid in a water-acetonitrile mixture, pH* 2.8. Samples were injected electrokinetically at 5.0 kV for 10 s and a running voltage of 19.5 kV was applied. Excluding the bufuralol/practolol pair, baseline separation of all substances was achieved in the developed CE system within 9 minutes. A linear relationship (R(2) 0.8752) between the sensitivity of the applied potentiometric detector and the parameter log P characterising the hydrophobicity of the analytes was demonstrated. The best observable limits of detection (LODs) were obtained for the highly hydrophobic substances, i. e. bufuralol (8.10 x 10(-8) M injected concentration, S/N = 3), propranolol, alprenolol, and clenbuterol (ca. 1.10 x 10(-7) M). In the case of hydrophilic catecholamines and carbuterol their LODs with potentiometric detection were lowered by a factor of almost one thousand, reaching a value of 6.6 x 10(-5) M.

Fundamentals of electrochemical detection techniques for CE and MCE

“…If a membrane contains an ion of charge opposite to the analyte ions, instead of a ionophore, then the selectivity is simply governed by the lipophilicity of the analyte ions, which in turn is related to their hydration energies [12]. Such electrodes, termed Hofmeister electrodes, have been employed for detection of inorganic cations [13][14][15] and anions [14,16,17], but as the highest response is obtained for the more lipophilic ions it works best for organic ions [18][19][20]. This is illustrated in Fig.…”