EFFECT OF THE CONCENTRATION OF 18-CROWN-6 ADDED TO THE ELECTROLYTE UPON THE SEPARATION OF AMMONIUM, ALKALI AND ALKALINE-EARTH CATIONS BY CAPILLARY ELECTROPHORESIS

Capillary zone electrophoresis with indirect ultraviolet absorption detection was carried out to determine ammonium, alkali and alkaline-earth cations using a background electrolyte buffer containing 10 mM imidazole (pH 4.5) and 18-crown-6 as complexing agent. The optimization of the experimental parameters (18-crown-6 concentration, pH, temperature, running voltage) was studied. The crown ether concentration appeared to be a very convenient parameter to monitor the selectivity of the cations during the analysis of complex matrix aqueous samples. Adding 2.5 mM 18-crown-6 to the 10 mM imidazole electrolyte system (pH 4.5) generally gave the maximum resolution of ammonium and potassium cations; nevertheless, it was possible to shift the potassium peak in the electropherogram by adding up to 300 mM 18-crown-6. The stability constant of 18-crown-6 complexes with four cations (K+, NH4+, Ba2+, Sr2+) has also been experimentally determined by capillary electrophoresis; it was confirmed that K+ has a stability complex constant with 18-crown-6 (log K-s=2.1) higher than NH4+ (log K-s=1.01). Simultaneous quantitation of sodium, calcium, potassium, ammonium, magnesium and lithium has been performed in such optimized separation conditions.