Understanding capacitively coupled contactless conductivity detection in capillary and microchip electrophoresis. Part 2. Peak shape, stray capacitance, noise, and actual electronics

Although simple equivalent circuits have been used to explain the basic functioning of a capacitively coupled contactless conductivity detector ((CD)-D-4), more sophisticated models are required to take into account the effects of the spatial non-homogeneity of the solution conductivity as the electrophoretic zones pass inside the detector. The overshooting phenomenon observed in real electropherograms may be explained by modeling the coupling of the electrodes with the inner capillary with a network of resistors and capacitors and its dependence with the stray capacitance becomes evident. An even more detailed model of the cell based on electrostatics allows one to calculate the stray capacitances. For the typical geometries and materials, this capacitance is on the order of a few to hundreds of femtofarads. It was possible to demonstrate that the ground plane, sometimes used, reduces the capacitance, but does not eliminate it completely. Possible noise sources are also discussed. The electrode tightness minimizes a possible source of mechanical noise due to variation of the coupling capacitances. Thermal control should also be ensured; the calculations showed that a temperature fluctuation as low as 7 x 10(-3)degrees C induces artifacts as high as the limit of quantification of K+ in a typical electrophoretic condition, for which the technique has one of its highest sensitivities.