Serum Inorganic Iodide Determined by Paired-Ion Reversed-Phase Hplc with Electrochemical Detection

Abstract

We propose an automated method for the analysis of serum inorganic iodide (SII), using paired-ion reversed phase HPLC with electrochemical detection and a silver working electrode. Assay conditions include a flow rate of 1.0 mL/min and an operating potential of 0.10 V. The retention time for iodide is 5.3 min. Sample preparation consists in protein removal by ultrafiltration and concentration of the ultrafiltrate, because of the very low levels of SII, especially in iodine deficient areas. Ultrafiltration separation is achieved by pouring 2mL of a serum sample into a filter cup (membrane cutoff: 5kD) and then using a centrifugal force of 14000G over 90 min. A 1200μL aliquot of the ultrafiltrate is concentrated by a factor of 10 to a volume of 120μL in a centrifugal vacuum concentrator (Speed-Vac). A 100μL aliquot of the concentrate is injected into the HPLC. Due to concentration the detection threshold (signal-to-noise ratio of 3) could be lowered to 0.004μmol/L. The recovery of iodide during concentration of the ultrafiltrate tested with 123I was 102.2%. The within and between-run precision (CV) for a serum sample with 0.04μmol/L are 1.9% and 4.2%, respectively. For comparison with a standard method based on the isotope dilution principle serum samples from 26 patients who underwent thyroid scintigraphy with 123I were measured by both HPLC(y) and the standard technique(x). The data obtained show a high correlation (r=0.99; y=0.94x + 0.007; Syx=0.0179). Levels of SII typical of an iodine deficient area are measured in sera from 27 patients with low urinary iodine excretion (44μmol/molCrea): SII=0.023 μmol/L (mean); range: 0.01–0.036μmol/L.