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

• Baseline separation was achieved by adding an ion-pairing reagent (TBAH).
• Resolution improvement resulted from the electrostatic interaction between TBAH and negatively charged iodine species.
• Rapid separation of four iodine species was obtained within 7.0 min.
• Iodine speciation in seaweed was analyzed.

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 0.01 mM 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 +16 kV separation voltage was applied along the separation capillary (50 μm i.d., 80 cm total and 60 cm effective) with the inlet grounded. The detection wavelength was fixed at 210 nm, and the pressure-driven flow rate was set at 0.12 mL min−1 with an injected volume of 2 μL. The optimal electrolyte consisted of 2 mM borate, 2 mM TBAH and 80% methanol with pH adjusted to 8.5. Baseline separation of iodine species was achieved within 7 min. The detection limits for I−, IO3−, MIT and DIT were 0.052, 0.040, 0.032 and 0.025 mg L−1, respectively. The relative standard deviations of peak heights and areas were all below 3% for 5 mg L−1 and 5% for 1 mg L−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.