Contactless conductivity detection in capillary zone electrophoresis

Some basic performance parameters of a high-frequency contactless conductivity detection technique originally designed for capillary isotachophoresis (ITP) were evaluated from the point of view of its use in capillary zone electrophoresis (CZE). These parameters included the response characteristic (a response of the detector to changes of the specific conductivity in the on-column detection cell), short-term noise and the detection sensitivity. A series of seven detectors, provided with capillary tubes (300 mu m I.D.) that were made either of PTFE (six detectors) or fused-silica tone detector), was taken into the evaluation. A certain scatter of the noise/detection sensitivity ratios of the tested detectors (attributable to differences in the on-column detection cells of the same construction) was found. These differences were also associated with different limits of detection (LODs) attainable for the test analytes (inorganic anions) by the evaluated detectors. For the detector characterized by minimum values of noise/detection sensitivity ratios for the test anions, the LOD (a 200-nl sample injection volume) ranged from 0.3 mu mol/l (sulfate) to 0.7 mu mol/l (fluoride) in the carrier electrolyte solution, which had a pH value of 3.5. These LOD values were two-eight times higher in comparison to those attainable by an on-column contact conductivity detector under otherwise identical working conditions. A lower sensitivity of the contactless conductivity detector can be more than compensated for in practical situations by its operational robustness. These features were proved effective in the CZE analyses of aqueous extracts of soil samples of high humic acid content (these samples are known to contaminate surfaces of the detection electrodes of the contact conductivity detectors). Here, 1-3% RSD values in the determinations of chloride, nitrate and sulfate could be achieved in large series of CZE runs, with no disturbances linked with the performance of the contactless conductivity detector even in situations when the sample matrix precipitated in the capillary. (C) 1999 Elsevier Science B.V. All rights reserved.