Introduction of a spectrophotometric method for salivary iodine determination on microplate based on Sandell-Kolthoff reaction

Abstract

Background. Iodine is an essential element for the synthesis of thyroid hormones. Therefore, a reliable marker of iodine supply is important. Iodine is predominantly excreted via kidneys, but also via salivary glands. Our aim was to introduce a new and simple method for determination of salivary iodine concentration (SLIC).

Materials and methods. Self-prepared chemicals and standards for Sandell-Kolthoff reaction on microplate with ammonium persulfate (AP) in the range 0-400µg/L were used. Suitability of water-based standards (WBS) and artificial saliva-based standards (ASS) for standard curve were tested. We followed standards for method validation, defined concentration of used AP and compared our results with Inductively Coupled Plasma Mass Spectrometry (ICP-MS).

Results. WBS gave more reliable results than ASS as an underestimation of iodine concentration was found for ASS. LoB was 6.5µg/L, LoD 12.0µg/L, therefore analytical range was 12-400µg/L. Intra- and inter-assay imprecisions at iodine concentrations, namely 20, 100, 165, and 350µg/L were 5.1, 5.7, 5.3, and 9.4%, respectively, and 5.3, 7.2, 9.5, and 14.4%, respectively. Suitable molarity of AP was 1.0mol/L and showed no difference to 1.5mol/L (P values for samples with concentration 40, 100, and 150µg/L, were 0.761, 0.085, and 0.275, respectively), whereas there was a significant change using 0.5mol/L (P<0.001). Saliva samples could be diluted up to 1:8. There was no interference of thiocyanate and caffeine up to 193.5mg/L. Our original method was comparable to ICP-MS. Spaerman coefficient was 0.989 (95% CI: 0.984-0.993).

Conclusions. The new method for SLIC determination is in excellent agreement with ICP-MS and easy-to-use.