Contactless conductivity detection in capillary zone electrophoresis

Some basic performance parameters of a high-frequency contactless conductiv ity detection technique originally designed for capillary isotachophoresis (ITP) were evaluated from the point of view of its use in capillary zone el ectrophoresis (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 di fferences were also associated with different limits of detection (LODs) at tainable for the test analytes (inorganic anions) by the evaluated detector s. For the detector characterized by minimum values of noise/detection sens itivity ratios for the test anions, the LOD (a 200-nl sample injection volu me) ranged from 0.3 mu mol/l (sulfate) to 0.7 mu mol/l (fluoride) in the ca rrier electrolyte solution, which had a pH value of 3.5. These LOD values w ere two-eight times higher in comparison to those attainable by an on-colum n contact conductivity detector under otherwise identical working condition s. 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 extra cts of soil samples of high humic acid content (these samples are known to contaminate surfaces of the detection electrodes of the contact conductivit y detectors). Here, 1-3% RSD values in the determinations of chloride, nitr ate and sulfate could be achieved in large series of CZE runs, with no dist urbances linked with the performance of the contactless conductivity detect or even in situations when the sample matrix precipitated in the capillary. (C) 1999 Elsevier Science B.V. All rights reserved.