Semi-micro ion chromatography of iodide in seawater

Large sample volume injections including both on-column analyte focusing and on-column matrix elimination techniques were examined for semi-micro ion chromatography of trace iodide (I⁻) in seawater. A semi-microcolumn (35×1 mm I.D.) packed with styrene–divinylbenzene copolymer with high anion-exchange capacity and a mobile phase of 0.03 M NaClO₄+0.5 M NaCl+5 mM sodium phosphate buffer, pH 6.0, was used. Iodide in seawater was effectively concentrated on the column by both electrostatic and hydrophobic interactions and was eluted without peak broadening. ClO₄⁻ (NaClO₄) in the mobile phase was effective for the elution of iodide and Cl⁻ (NaCl) for both the concentration of iodide (I⁻) with hydrophobicity and the removal of interference by the major anions. An excess of major anions in seawater did not disturb the detection of iodide at UV 226 nm. The relative standard deviations for successive injections of 5 and 1 μg/l I⁻ (2 ml of 35‰ artificial seawater) were 1.5 and 5.8% (n=5, each), respectively. The slope of calibration curve (by peak area) using the semi-microcolumn was ∼2.8-times higher than that for a conventional column with the same resin (150×4.6 mm I.D.) The present method had a detection limit of 0.2 μg/l I⁻ for 2 ml of 35‰ artificial seawater and was successfully applied to seawater samples.