The Direct Electrochemical Detection of Amino Acids at a Platinum Electrode in an Alkaline Chromatographic Effluent

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Abstract
Most organic compounds including amino acids do not produce reversible or even quasi-reversible anodic waves at a Pt electrode under conditions of conventional cyclic voltammetry. Amperometric detection of these compounds at a constant electrode potential is not successful because of the accumulation of adsorbed reaction products and/or an oxide film at the electrode surface. However, a Pt electrode surface is cleaned quite effectively of adsorbed organic molecules and radicals simultaneously with the anodic formation of the oxide radicals simultaneously with the anodic formation of the oxide layer. This oxidation of adsorbed organic species is concluded to be electrocatalyzed by PtOH formed as the first step in the production of the oxide layer (PtO). A pulsed-potential waveform applied at a frequency of .apprx. 1 Hz provides direct amperometric detection of adsorbed amino acids at a Pt electrode. Satisfactory analytical precision (i.e., < 3% relative standard deviation) results because the waveform reproducibly generates the catalytically active surface state at the Pt electrode. Both primary and secondary amino acids are determined with satisfactory detection limits: e.g., .apprx. 13 ng for glycine, 7 ng for phenylamine and 23 ng for hydroxyproline in 50-.mu.l samples. Analytical response probably depends on the adsorption isotherm of the amino acid being detected. Hence, the calibration plot of 1/Ipeak vs. 1/CD is linear for low surface coverages. Results are shown for amperometric detection of a synthetic mixture of amino acids by anion-exchange chromatography using NaOH as the eluent and supporting electrolyte.