Plastic Film Ring-Disk Carbon Electrodes for the Indirect Amperometric Detection of Underivatized Amino Acids in Liquid Chromatography

An indirect amperometric detection of underivatized amino acids has been developed based on the use of a plastic film ring-disk carbon electrode in liquid chromatography. Bromide present in the postcolumn addition reagent is oxidized to bromine at the upstream (disk) electrode, and is subsequently detected at the downstream (ring) electrode. Most of the underivatized amino acids that are electroinactive under conventional amperometric conditions can react rapidly with the electrogenerated bromine. The concentration of amino acids can therefore be indirectly determined by continuously mon-itoring the reduction current of bromine. The signal monitored at the downstream electrode was largely dependent on the pH of the mobile phase, the generation and collection potentials, and the bromide concentration in the mobile phase. Under the optimized conditions, the detection limits of the 0.5 µM level were obtained for a number of amino acids with a linear correlation coefficient of 0.990 - 0.998 over the concentration range 1 - 100 µM. The primary advantage of this approach lies in the capability that it affords a simple and sensitive determination of underivatized amino acids. In addition, it was almost unaffected by fouling effects normally arising from the anodic oxidation of the sulfur-containing amino acid.