Ion chromatography on chelating stationary phases: separation of alkali metals

Analytical 250×4.6 mm I.D. columns packed with iminodiacetic acid (IDA) derivatised silica were used to separate alkali metal ions and the ammonium ion in combination with non-suppressed conductivity detection. The addition of 2.5–10 mmol/l of the macrocyclic crown ether 18-crown-6 to the nitric acid eluent resulted in a change in the elution order and a significant improvement in the resolution between potassium and ammonium because of selective complexation of potassium. However, the admixture of 15-crown-5 did not improve the resolution of lithium and sodium, although 15-crown-5 is a selective complexing agent for sodium. Retention and resolution of lithium, sodium, ammonium, cesium, rubidium and potassium ions increased at lower temperatures down to 1°C. The simultaneous separation of alkali and transition metals under isocratic conditions was achieved with an eluent comprising 10 mmol/l 18-crown-6, 1.5 mmol/l dipicolinic acid, and 1.9 mmol/l nitric acid. The chromatographic system enabled the quantitation of alkali metal ions with detection limits in the low ppb range and excellent linearity. Finally, the applicability of the method was approved by quantitation of sodium, ammonium and potassium in different water samples.