Understanding Capacitively Coupled Contactless Conductivity Detection in Capillary and Microchip Electrophoresis. Part 2. Peak Shape, Stray Capacitance, Noise, and Actual Electronics

Abstract

Although simple equivalent circuits have been used to explain the basic functioning of a capacitively coupled contactless conductivity detector (C 4 D), 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 Â 10 À3 8C induces artifacts as high as the limit of quantification of K þ in a typical electrophoretic condition, for which the technique has one of its highest sensitivities.

Evaluation of photografted charged sites within polymer monoliths in capillary columns using contactless conductivity detection

“…However, C 4 D can be recorded along the entire length of a column non-invasively, with the recorded response directly proportional to the sum of conducting elements within the , virtual electrode area' [22][23][24][25][26][27]. The presence of non-conducting monolithic phase filled with a background solution, housed within a fused silica capillary of uniform character, will provide a conductive response proportional to the fluid volume.…”

“…The principles of C 4 D have been well studied [22][23][24][25][26][27], and the electronic circuitry has been found to be simple and of low cost. The advantage of C 4 D over conventional conductivity detection for chromatography is that the cell design is extremely robust and there is a lack of physical contact with the eluent.…”

Following the lipase catalyzed enantioselective hydrolysis of amino acid esters with capillary electrophoresis using contactless conductivity detection

“…3,4 The method has been termed C 4 D for ''capacitively coupled contactless conductivity detection.'' Its fundamentals have been studied extensively [5][6][7][8] and at least two commercial devices are now available and can be retrofitted to existing CE-instrumentation. Recent reviews on applications of C 4 D are also available.…”

A continuous wavelet transform algorithm for peak detection

“…Contactless conductivity detection (CCD), one of the electrochemical techniques, has attracted much attention in recent years (e.g. [9][10][11][12][13][14][15][16][17][18][19][20]). For miniaturized analytical system applications, this type of sensor has the unique advantage of lower chip fabrication costs, easier system integration and reduction of interference of high CE voltage with sensor signals.…”