DETERMINATION OF THYROXINE IN URINE BY CATHODIC STRIPPING SQUARE-WAVE VOLTAMMETRY

A square-wave polarographic method for the determination of the amino acid thyroxine in urine, after isolation from the matrix on a C-18 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.