A Rapid Method for the Determination of Potassium Iodide in Ophthalmic Formulations by Indirect Derivatization with 4-Hydroxybenzoic Acid Using UHPLC–DAD and MS/MS Detection

A rapid ultra-high-performance liquid chromatography method with diode-array (DAD) and tandem mass spectrometric (MS/MS) detection was developed and, for the first time, applied for the determination of potassium iodide in ophthalmic formulations. The approach is based on an indirect derivatization reaction in which iodide is oxidized by hydrogen peroxide to electrophilic iodine species that react with 4-hydroxybenzoic acid, forming 3-iodo-4-hydroxybenzoic acid as a stable and quantifiable product. Key reaction parameters, including oxidant concentration, temperature, and incubation time, were optimized to ensure selective mono-iodination and consistent analytical response. Chromatographic separation was performed on a C18-DE RP column using gradient elution with aqueous ammonium acetate and acetonitrile, while MS/MS detection was carried out in MRM mode under ESI(−) conditions. Both UHPLC–DAD and UHPLC–MS/MS methods were validated according to ICH Q2(R2), demonstrating linearity with r2 ≥ 0.998, recovery values of 97.5–107.1%, and intraday/interday RSD values up to 3.7%. UHPLC–MS/MS provided higher sensitivity (LOD 37.7 ng/mL; LOQ 114 ng/mL), whereas UHPLC–DAD reached LOD and LOQ values of 24.9 and 75.4 µg/mL. Comparative analysis showed that DAD is suitable for routine quantification, while MS/MS allows lower detection limits and improved selectivity. The developed method offers a practical and reliable tool for the quality control of potassium iodide in ophthalmic formulations.