Manipulation of separation selectivity of inorganic anions in electrostatic ion chromatography by the use of mixed cationic–zwitterionic micelles as the column coating solution

This paper describes an electrostatic ion chromatographic system in which the separation selectivity for inorganic anions, especially for sulfate and phosphate, could be manipulated by altering the molar ratio of the zwitterionic and cationic surfactants in the column coating solution used to prepare the stationary phase. The zwitterionic surfactant used for this study was 3-(N,N-dimethyltetradecylammonio)propanesulfonate (Zwittergent-3-14) and the cationic surfactant was tetradecyltrimethylammonium (TTA). Using a reversed-phase C₁₈ column (250×4.6 mm I.D.) coated with 10/10 (mM/mM) of TTA/Zwittergent-3-14 mixed micelles as the stationary phase and either NaHCO₃ or Na₂CO₃ aqueous solution as the eluent, together with suppressed conductivity detection, baseline separation of seven model inorganic anions was obtained. The elution order for those anions was found to be F⁻4²⁻−4²⁻2⁻−3⁻. Under the same conditions but using 1/10 (mM/mM) of TTA/Zwittergent-3-14 mixed micelles as the column coating solution, the elution order for these model ions was F⁻4²⁻4²⁻−2⁻−3⁻. The early elution of phosphate and sulfate is a unique attribute of this system. Detection limits for F⁻, HPO₄²⁻, Cl⁻, SO₄²⁻, NO₂⁻, Br⁻ and NO₃⁻ (S/N=3, sample injection volume 100 μl) were 0.11, 0.12, 0.12, 0.18, 0.49, 0.49, 0.52 μM, respectively.