Evaluation of microchip capillary electrophoresis with external contactless conductivity detection for the determination of major inorganic ions and lithium in serum and urine samples

The determination of inorganic ions in clinical samples in less than 90 seconds was demonstrated for microchip capillary electrophoresis using capacitively coupled contactless conductivity detection ((CD)-D-4). Bare electrophoresis chips were used in combination with external electrodes which were part of the chip holder. In order to achieve the required selectivity and sensitivity, an optimization of the electrode layout was carried out. Limits of detection (LOD) of 1 mu M for K+, 1.5 mu M for Ca2+, 3 mu M for Na+, 1.75 mu M for Mg2+ and 7.5 mu M for Li+ were achieved. The determination of inorganic cations (NH4+, K+, Na+, Ca2+, Mg2+) and anions (Cl-, NO3-, SO42-, phosphate) in blood serum and urine samples was possible in one common electrolyte solution containing 15 mM L-arginine, 10.75 mM maleic acid and 1.5 mM 18-crown-6 at pH 5.90 by simply switching the separation voltage from positive to negative polarity. Lithium, present at significant levels when used for therapeutic purposes, can also be determined in blood serum using a slightly modified background electrolyte solution.