Use of ion-pairing reagent for improving iodine speciation analysis in seaweed by pressure-driven capillary electrophoresis and ultraviolet detection

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This study achieved resolution improvement for iodine speciation in the presence of an ion-pairing reagent by a pressure-driven capillary electrophoresis (CE) system. Addition of 0.01mM tetrabutyl ammonium hydroxide (TBAH) as the ion-pairing reagent into the electrophoretic buffer resulted in the complete separation of four iodine species (I−, IO3−, mono-iodothyrosine-MIT and di-iodothyrosine-DIT), because of the electrostatic interaction between TBAH and the negatively charged analytes. A +16kV separation voltage was applied along the separation capillary (50μm i.d., 80cm total and 60cm effective) with the inlet grounded. The detection wavelength was fixed at 210nm, and the pressure-driven flow rate was set at 0.12mLmin−1 with an injected volume of 2μL. The optimal electrolyte consisted of 2mM borate, 2mM TBAH and 80% methanol with pH adjusted to 8.5. Baseline separation of iodine species was achieved within 7min. The detection limits for I−, IO3−, MIT and DIT were 0.052, 0.040, 0.032 and 0.025mgL−1, respectively. The relative standard deviations of peak heights and areas were all below 3% for 5mgL−1 and 5% for 1mgL−1. Application of the proposed method was demonstrated by speciation analysis of iodine in two seaweed samples. The developed method offered satisfactory recoveries in the 91–99% range and good precisions (<5%). Good agreement between the determined values by the proposed CE method and the HPLC-ICP-MS method was also obtained. All results proved its great potential in routine analysis of iodine speciation in environmental, food and biological samples.