HYPERCOULOMETRY OF THE 2ND KIND IN ELECTRON-CAPTURE DETECTION

Hypercoulometry in electron-capture detectors (ECDs) has been repeatedly described in the literature and has been attributed to the space charge effect of analyte-derived anions migrating to the anode beyond the radioactive plasma [W. A. Aue and S. Kapila, J. Chromatogr., 188 (1980) 1]. However, the existence of a different kind ofhypercoulometry has recently been suggested by experiments using clean pulse conditions in a small-volume electron-capture detector [K. W. M. Siu, G. J. Gardner and S. S. Berman, J. Chromatogr., 330 (1985) 87]. The present study provides a speculative explanation of this "second kind" of hypercoulometry by relying on computer-aided simulation, and by using apparent rate constants from a measurement of ECD steady states under unipolar and bipolar drive conditions [K. W. M. Siu, S. S. Berman and W. A. Aue, J. Chromatogr., 408 (1987) 53]. Two salient features of this hypercoulometric response (vis-a-vis conventional ECD mechanisms) are that the rate constant for anion-cation neutralization is significantly smaller than the one for electron-cation recombination and that overall electrical neutrality prevails. As one of the consequences, steady state takes a much longer time to reach than in conventional model systems. The simulation characterizes "hypercoulometry of the second kind" as the charge effect of non-collected, analyte-derived anions in or near the radioactive plasma, which causes a larger cation concentration and hence a higher electron-cation recombination rate. Given certain simplifying assumptions, the effect can be approximated to a large extent by conventional, i.e. "stirred reactor" type kinetic modelling. The model is capable of producing strongly hypercoulometric response profiles that agree well with the experimental profiles measured earlier in the same detector [K. W. M. Siu, G. J. Gardner and S. S. Berman, J. Chromatogr., 330 (1985) 87]. Even when hypercoulometry is not observed, the internal detector processes remain the same. It is therefore suggested that a large proportion of hypocoulometric response in well-performing detectors also results from a protracted presence of anions and, in turn, an increase in the electron-cation recombination rate. © 1990.