Nickel–titanium alloy electrodes for stable amperometric detection of underivatized amino acids in anion-exchange chromatography

Nickel–titanium (Ni–Ti) alloy electrode was used as an electrochemical detector for the analysis of underivatized amino acids in flow systems. In strong alkaline solution, an oxide film on the Ni–Ti alloy electrode surface exhibited a high catalytic activity toward the oxidation of amino acids. Cyclic voltammetry experiments confirmed that electrogenerated Ni(III)O(OH) functioned as the key redox mediator associated with the oxidation of the amine group in amino acids. The electrochemical behavior of the Ni–Ti electrode in alkaline medium was very similar to the Ni electrode. However, the oxide film was found to be much stable on Ni–Ti than on Ni. Consequently, the Ni–Ti alloy electrode exhibited an excellent stability for constant-potential amperometric detection of amino acids in flow systems. For example, the relative standard deviation (R.S.D.) for the repetitive 100 injections of 50 μM (1.2 nmol) glycine over 10 h was less than 1%. It was postulated that the presence of Ti in the alloy stabilizes the microstructure of oxide layer on the electrode surface. The sensitivities of amino acids at the electrode were different, depending on their chemical structures. The detection limits obtained in a range from 0.9 pmol for arginine to 90.2 pmol for leucine and isoleucine. The Ni–Ti alloy electrodes have been demonstrated to be very suitable for the amperometric detection of underivatized amino acids in anion-exchange chromatography.