POLARIZATION, RELAXATION AND UNRESTRICTEDLY LINEAR-RESPONSE IN A BIPOLAR, CONSTANT-FREQUENCY ELECTRON-CAPTURE DETECTOR

To investigate bipolar constant-frequency regimes in the electron-capt ure detector (ECD), a ''tripulser'' was built. The tripulser was able to generate unit sequences of up to three pulses, individually defined as to width, amplitude and relative position, with 600 ns to 1 s and 0 to 250 V definition ranges. On a commercial Ni-63 two-chamber ECD (T racer), the high-frequency region of bipolar pulsing (ca. 10 to 100 kH z) was explored. The detector showed clear polarization-relaxation (PR ) effects within time spans (on the order of 10(-5) s) that were comme nsurate with the theoretical mobility of electrons. Speculative eviden ce was found to suggest that PR kinetics, as driven by particular bipo lar pulse sequences, resulted in changes to the (heterogeneous) charge d-particle distribution and effectively allowed higher than usual conc entrations of electrons (and cations) to exist in the ECD. Based on th is evidence, a bipolar, constant-frequency drive was developed that, w hen tested on the Tracer ECD, showed good analytical performance. Most important (and in contrast to the behavior of any other unipolar cons tant-frequency mode) the bipolar (Tracer) ECD yielded strictly linear calibration curves -starting from the detection limit (5.10(-18) mol/s of alpha-1,2,3,4,5,6-hexachlorocyclohexane at S/N-p-p = 2), over thre e orders of magnitude, all the way to an amount of analyte that totall y exhausted the baseline current.