Prediction of zone patterns in capillary zone electrophoresis with conductivity detection - Concept of the zone conductivity diagram

Conductivity detection (CD) is one of the few universal detection principles for capillary electrophoresis (important, e.g., in the detection of poorly absorbing compounds). Because of the availability of commercial instrumentation for capillary zone electrophoresis (CZE) with sensitive conductivity detection, CD in CZE became more widely used hi the past few years. For the successful utilization of CD, both qualitative and quantitative aspects of the CD signal have to be recognized and understood. This requires a detailed knowledge of how the conductivity changes along an analyte zone depending on (i) the ionic mobility and dissociation constant of the analyte, (ii) the parameters of the background electrolyte used and (iii) the concentration of the analyte in its zone. This contribution is aimed at characterizing the basic properties of CD in CZE. Based on a simple theoretical treatment, the key parameters controlling the sign and magnitude of the CD signal are revealed. The concept of the limiting molar conductivity response defined as the limiting slope of the analyte zone conductivity vs. analyte concentration dependence at infinite concentration, is established. This quantity can easily be calculated for any given background electrolyte and analyte and can successfully serve for predicting the character of a CD pattern, It is shown that the sign of the limiting molar conductivity response determines the sign of the CD signal and its magnitude relates to the magnitude of the CD signal. Using a pK vs. ionic mobility coordinate system, a map of limiting molar conductivity response values can be calculated and plotted for a given CZE system. Such a diagram can be used to predict sign and magnitude of the CD response of any analyte of known ionic mobility and pK. Experimental data obtained with two commercial conductivity detectors (LKB 2127 Tachophor and AT Unicam Crystal 1000 CE) were found to agree well with theoretical predictions.