a.c. Electron-capture detection

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Journal of Chromatography A





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Theoretical arguments nonwithstanding, it is possible to drive electron capture detectors with alternating current (a.c.). This study examines the basic features of such detectors, in particular their current and response profiles. It does so over a 10-106 Hz range of sinusoidal and rectangular a.c. polarization, and compares the results with corresponding data from conventional, unipolar regimes. In contrast to the authors' own expectations, the a.c. driven detector is capable of collecting nearly all, and not just half, of the generated electrons on the counter- electrode. On the lower and higher frequency slopes of this curren level (called the "plateau") are situated two response maxima. According to speculations advanced in this study, the position of the former maximum is determined primarily by the cation-electron recombination rate, but the precise nature of its mechanism cannot be further defined without knowledge on recombination and neutralization rate constants. The position of the latter maximum is governed by the onset of electron oscillation, and its mechanism relies, predominantly if not exclusively, on a migrating negative space charge. A self-consistend interpretation is advanced for the various features of the current and response profiles, and experimental evidence is provided from corresponding unipolar pulsing regimes in support of this interpretations. Four 63Ni detectors, two commercial and two lab-made, were used in this study. All four showed similar behaviour but, depending on their constrution, the relative sizes of the two response maxima (hence, speculatively, the relative contributions of the two response mechanisms) varied widely. The a.c. mode produced approximately the same calibrations curves and detection limits as optimized unipolar regimes, and there are reasons to expect that it might work particularly well with analytes that have relatively low electron-capture cross sections. It is also suggested that "constant current" operation should be possible in the a.c. mode. © 1987.