Serum inorganic iodide determined by paired-ion reversed-phase HPLC with electrochemical detection

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 now 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 1200 mu L aliquot of the ultrafiltrate is concentrated by a factor of 10 to a volume of 120 mu L in a centrifugal vacuum concentrator (Speed-Vac). A 100 mu L 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 0.004 mu mol/L. The recovery of iodide during concentration of the ultrafiltrate tested with I-123 was 102.2%.