Probing the response mechanism of the thermionic detector by resonance enhanced ionization spectroscopy

The thermionic detector is a sensitive, selective device used in gas chromatography. It responds selectively towards nitrogen- and phosphorus-containing organic compounds with detection limits in the picogram range. The detector is of great importance for the measurement of trace levels of drugs, pesticides and herbicides in biological matrices and the environment. There is, however, some dispute in the literature regarding the detector’s response mechanism (1,2,3). The detector is based on a hydrogen-air diffusion flame. Two electrodes polarise the flame with a potential difference of about 200 V and the current through the flame is measured using an electrometer amplifier. The selectivity of the system relies on the presence of a ceramic bead in the flame doped with an alkali metal, usually Rb. In the presence of nitrogen- and phosphorus-containing organics, CN−CN− and PO−PO− anions are formed, yielding a current which is the measured response. It has been suggested (1) that this selective response arises from a charge transfer reaction between the Rb excited states and CN•CN• or PO•PO• and PO•2PO2• radicals. Using an AlGaAs diode laser, the Rb excited state population can be modulated and the influence on detector current monitored. The Rb resonance-enhanced ionization signal, laser-induced fluorescence and emission signals have all been used to probe the response mechanism of the detector. The results demonstrate that in the gas phase, the above charge transfer reaction plays little if any role in detector response.