Abstract
The authors developed a capillary zone electrophoresis (CZE) method employing transient isotachophoresis (tITP) for the direct determination of inorganic ammonium (NH4+) in seawater. In this system, sodium ions (Na+) naturally present in seawater and those present in the background electrolyte (BGE) served as the leading ion (LI）, whereas tris(hydroxymethyl)aminomethane (Tris), subsequently introduced into the capillary, acted as the terminating ion (TI). At pH values higher than 9.75, ammonium nitrogen exists predominantly as molecular ammonia (NH3), which exhibits strong UV absorption. This property enabled direct UV detection of NH3 at 190 nm using BGE adjusted to pH 10.0. To enhance the separation efficiency, 0.03 ％ w/v hydroxypropyl methylcellulose (HPMC) was added to the BGE. The method achieved a limit of detection (LOD, S/N ＝ 3) of 1.6 mg/L and a limit of quantification (LOQ, S/N ＝ 10) of 5.4 mg/L for NH4+-N (as N). The relative standard deviations (RSD, n ＝ 10) for peak area, peak height, and migration time were, respectively, 3.4, 2.8, and 1.6 ％. A linear calibration curve was obtained for the peak area over the tested concentration range, with a correlation coefficient of 0.9982. The method was applied to ascertain the NH4+ concentrations in seawater samples collected from a marine landfill: a final waste disposal site constructed on reclaimed land. The CZE results obtained using the standard addition and single-point calibration methods showed good agreement with those obtained using the indophenol blue method, thereby demonstrating the applicability of the proposed CZE method for environmental monitoring. However, the proposed method is affected by the Na+ concentration in seawater. When the Na+ concentration is not known in advance, the standard addition method must be used. In addition, the standard addition method requires a longer analysis time than the external calibration method.
