Determination of Na+, K+, Mg2+ and Ca2+ in mixtures of seawater and formation water by capillary electrophoresis

A method has been developed for the determination of Na+, K+, Mg2+ and Ca2+ in mixtures of seawater and formation water from oil wells in the North Sea by capillary electrophoresis with indirect UV detection. The carrier electrolyte contained 5.0 mM 4-methylbenzylamine (UVCAT-1), 6.5 mM alpha-hydroxyisobutyric acid (HIBA), 6.2 mM 18-crown-6 and 25% (v/v) methanol. Baseline separation was achieved for concentrations below 70 ppm Na+, 20 ppm K+, 50 ppm Mg2+, 30 ppm Ca2+, 10 ppm Sr2+ and 10 ppm Ba2+. The limits of detection were 0.14 ppm Na+, 0.15 ppm K+, 0.08 ppm Mg2+, 0.08 ppm Ca2+, 0.18 ppm Sr2+ and 0.15 ppm Ba2+. The accuracy was checked by analysis of a reference water sample. The precision was better than 3.0%.