Simultaneous determination of small cations and anions by capillary electrophoresis

A capillary electrophoresis system for the simultaneous determination of small cations and anions has been developed. Cations and anions in a sample are separated in one single electrophoretic run using one capillary and just one detector. The sample is injected into the first end of the separation capillary and subsequently into the second end. When the high voltage is applied, the cations and the anions in the two injected sample portions start to migrate against each other toward the center of the capillary. The detection window is placed approximately in the middle of the capillary. A suitable electrolyte consists of 6 mM 4-aminopyridine, 2.7 mM H2CrO4, and 30 mu M cetyltrimethylammonium bromide at pH 8. The composition of the background electrolyte provides excellent separation of 22 small inorganic and organic anions and 1A and 2A class cations within 5 min. The overall repeatability of the migration times is less than 0.3%, and the repeatability of the peak area ranges from 1.7 to 5.5% (n=9) based on manual hydrodynamic injections with a duration of 40 s. The technique has been applied for the analysis of tap water, rainwater, and process water samples. The total time for an analysis of both cations and anions in water sample is less than 3 min. Milk and mud samples were also analyzed after off-line dialysis pretreatment.