Comparison of different contactless conductivity detectors for the determination of small inorganic ions by capillary electrophoresis

The analytical performance parameters of four different contactless conductivity detectors for capillary electrophoresis were determined. These detectors were designed either with a miniature cell for versatility, with high voltage excitation for high sensitivity or with a battery power supply for field application. One of the units is commercially available. The plate numbers and reproducibility of peak areas (typically about 2%) were very similar, indicating that these parameters are generally limited by the separation and injection procedures rather than the detectors. The linear dynamic ranges are better than 2 orders of magnitude for all detectors and the correlation coefficients were also almost identical. Significant differences were found with regard to the detection limits. For the detector with a miniature cell, which lacks an in-cell amplifier, the detection limit was typically 1.5 mu M for the inorganic ions tested (K+, Ca2+, Na+, Mg2+, Li+, Cl-, NO3 and SO42-), while with the other 3 detectors this parameter was as low as about 0.1 mu M. The use of buffer solutions with relatively high background conductivity was found to lead to detection limits which were up to one order of magnitude higher. The extent of deterioration of this parameter with buffer conductivity was found to be related to the excitation voltage and was most pronounced for the high voltage detector.