Determination of thyroxine in urine by cathodic stripping square-wave voltammetry

Abstract

A square-wave polarographic method for the determination of the amino acid thyroxine in urine, after isolation from the matrix on a C₁₈ Sep-Pak cartridge and subsequent elution through a column packed with Amberlite IRA-400 anionic resin, was developed. The optimal working conditions for the application of the method were found to be pH 10, provided by a carbonate buffer, and a potential of –0.690 V during 10 min. The equilibration time was applied during 5 s, and potential scans were performed at a square-wave amplitude of 100 mV and a scan rate of 300 mV s^–1, with a square-wave frequency of 100 Hz. The measuring-system response was linear over the thyroxine concentration range from 10 to 60 ng ml^–1 and the detection limit achieved was 2.71 ng ml^–1. The relative error and relative standard deviation obtained were 1.18 and 4.67%, respectively.