Conductivity detection for conventional and miniaturised capillary electrophoresis systems

Abstract

Since the introduction of capillary electrophoresis (CE), conductivity detection has been an attractive means of detection. No additional chemical properties are required for detection, and no loss in sensitivity is expected when miniaturising the detector to scale with narrow-bore capillaries or even to the microchip format. Integration of conductivity and CE, however, involves a challenging combination of engineering issues. In conductivity detection the resistance of the solution is most frequently measured in an alternating current (AC) circuit. The influence of capacitors both in series and in parallel with the solution resistance should be minimised during conductivity measurements. For contact conductivity measurements, the positioning and alignment of the detection electrodes is crucial. A contact conductivity detector for CE has been commercially available, but was withdrawn from the market. Microfabrication technology enables integration and precise alignment of electrodes, resulting in the popularity of conductivity detection in microfluidic devices. In contactless conductivity detection, the alignment of the electrodes with respect to the capillary is less crucial. Contactless conductivity detection (CCD) was introduced in capillary CE, and similar electronics have been applied for CCD using planar electrodes in microfluidic devices. A contactless conductivity detector for capillaries has been commercialised recently. In this review, different approaches towards conductivity detection in capillaries and chip-based CE are discussed. In contrast to previous reviews, the focus of the present review is on the technological developments and challenges in conductivity detection in CE.

Portable capillary-based (non-chip) capillary electrophoresis

“…C 4 D as a third option for ECD was introduced in coupling with CE by Zemann [116] and da Silva [117] in 1998. Its main strengths are contactless operation, high sensitivity, ease of miniaturization and quasi-universal detection [118]. Its main weakness is the need to have a significant difference between the conductivity (mobility) of the de-tected ion and the background electrolyte (co-ion mobility).…”

Bioanalytical profile of the l-arginine/nitric oxide pathway and its evaluation by capillary electrophoresis

“…It is very attractive for developing integrated and microscale CE platforms since it is well suited for microfabrication technologies, does not require chemical modification of analytes, and its miniaturization is usually not compromised, but has positive effects on sensitivity and selectivity (for a comprehensive review see ref. [294]). Conductivity detectors fall into two major categories:…”

“…After a prototype of a contactless conductivity detector was designed for capillary ITP [301] and a theoretical treatment of contactless conductivity measurement was proposed [302,303], it became increasingly employed in HPLC and HPCE analyses [294]. Slightly different CCD techniques were proposed for HPCE in 1998 [304,305] and subsequently improved (for reviews see refs.…”

“…In addition to the rapid progress of CZE/CCD technology for fused silica capillaries, several on-chip platforms have been proposed (for recent reviews see refs. [294,[308][309][310]323]). In summary, CCD is a very potent and promising technique for both fused silica capillary-and micro-chip-based CZE platforms; however, applications focusing on profiling of L-arginine/NO pathway in biological samples have yet to be developed.…”

Direct determination of valproic acid in biological fluids by capillary electrophoresis with contactless conductivity detection

“…However, as VPA lacks a strong chromophore, indirect methods had to be used in order to enable its quantification by UV-absorption [36,38,39] or laser induced fluorescence [37], the standard detection techniques available on commercial capillary electrophoresis instruments. Contactless conductivity detection (for recent reviews see [40,41]) is better suited for such analytes than the indirect optical methods because of the poor detection limits of the latter. Indeed, a few studies on the application of this new detection technique to the analysis of blood samples have already been reported.…”