Dual-channel capillary electrophoresis for simultaneous determination of cations and anions

• Original CE apparatus for simultaneous determination of cations and anions.
• Dual-channel CE separation controlled through a single high voltage source.
• Separation in short capillaries characterized by separation time less than 1 min.
• Sample and BGE solution flow controlled by piezoelectric micro-pumps.
• Miniaturized CE apparatus constructed on a board with dimensions of 20 × 25 cm.

An original electrophoresis apparatus for simultaneous rapid determination of cations and anions has been designed and tested. The separation part of the apparatus consists of two identical fused-silica capillaries, each with a length of 10.5 cm and inner diameter of 25 μm. The injection space is formed by the crossing of four channels in a plexiglass cross-piece. The capillaries pass through two opposing channels and their injection ends are located opposite one another at a distance of approx. 0.5 mm in the centre of the crossing point. The exit ends of the capillaries are placed in vessels containing the background electrolyte in which are immersed the electrodes of a high-voltage source. Contactless conductivity detectors with semi-cylindrical electrodes are located 2 cm from the exit ends of the capillaries. The injection part of the apparatus consists of two piezoelectric micro-pumps bringing the solution through another channel in the cross-piece to the injection ends of the capillary. During the injection, the sample is brought through one of them and is injected electrokinetically for a defined time. Then the sample zone is forced out of the injection space by a stream of background electrolyte from the second micro-pump. The timing of the injection process is computer-controlled. Thus the equipment can be considered to constitute electrophoresis in one capillary with injection into its centre. The use of short capillaries and miniature micro-pumps without other mechanical components enabled the construction of the apparatus on a board with dimensions of 20 × 25 cm. The proposed equipment was used to test simultaneous separation of a mixture of cations and anions, NH4+, K+, Ca2+, Mg2+, Sr2+, Ba2+, Cl−, NO3−, SO42−, ClO3− and F−, in BGE with composition 500 mM HAc + 20 mM Tris + 2 mM 18-crown-6 (pH 3.3). Baseline separation of all the components was achieved in time less than 1 min. Quantification of the content of nitrate nitrogen (determined as NO3−), ammoniacal nitrogen (determined as NH4+), K2O (determined as K+) and SO3 (determined as SO42−) was performed on a real-world sample of mineral fertiliser. The determined compositions differed from the declared contents by an amount of 0.5–5.6%; the RSD value expressing the repeatability of the determination was in the range 3.4–7.5%. The LOD values were in the range from 6.9 μM (K+) to 10.6 μM (NH4+).