Electron-capture detector with positive response (increase in current)

Even though the electron-capture detector is based on the removal of electrons by analyte molecules, and hence responds to these normally by a decrease in cell current, it is possible to utilize the space charge effect, i.e. its own response mechanism, to make the detector signal the passage of analyte by an increase in current (”positive“ response).

A prototype electron-capture detector was designed to produce such positive response. It was built in three-chamber construction, using a bifurcated current flow to two anodes. One of these is intercepted by analyte molecules, the other is not. The two are coupled with each other, and with cation-electron recombination, via the spatially heterogeneous nitrogen plasma. The anode exposed to gas chromatographic column effluent produces (conventional) negative response, the anode bathed in pure carrier gas provides positive response.

The positive electron-capture response is similar in magnitude to its twin negative response, and to typical response from conventional electron-capture detectors. As is the case with response, noise in the two channels is also coupled through plasma perturbation, and that limits detection to a present minimum of 0.1 pg lindane per second. The importance of this detector resides less in its practical performance, which is unexceptional, than in the fact that it exists at all — and the implications this has for the confirmation of its space-charged based mechanism.