Electron capture detector pulsed constant frequency

Gas Chromatograph -- A Varian 6000 equipped with two constant-current/pulsed-frequency electron capture detectors, a 30-m x 0.53-mm ID DB-5 fused-silica open-tubular column (1.5-/xm film thickness), and a 30-m x 0.53-mm ID DB-1701 fused-silica open-tubular column (1.0-/im film thickness), both connected to a press-fit Y-shaped fused-silica inlet splitter (Restek Corporation, Bellefonte, Pennsylvania), was used to analyze for the nitroaromatic compounds. The columns were temperature-programmed from 120°C (1.0-min hold) to 200°C (1-min hold) at 3°C/min, then to 250°C (4-min hold) at 8°C/min injector temperature 250°C detector temperature 320°C helium carrier gas 6 mL/min nitrogen makeup gas 20 mL/min. 

Recent EC detectors have pulsed-voltage power supplies that maintain a constant current. With no sample, the pulse frequency is very low as the sample enters the detector, the frequency increases to offset the current loss due to the electron-capturing species. The pulse frequency is proportional to the sample concentration and can be used for quantitative analysis. 

The beta-rays emitted from the cathode ionize the carrier gas, thereby liberating electrons. If a pulsed voltage is applied to the electrode in the cell, these electrons are captured, so producing an electric current. If electrophilic molecules are introduced into the cell, these absorb electrons and become negatively ionized. The electron density in the detector therefore decreases, so that a smaller number of electrons are captured at each pulse. The total number of electrons captured per unit of time (i.e. the current) can be kept constant by increasing the pulse frequency when the number of electrons decreases. The pulse frequency is then proportional to the concentration of the electrophilic molecules passing through the detector [8]. 

Pulsed variable frequency with constant current mode is considered the superior method and is used by most manufacturers of ECDs. A preselected level of current is required. When an electron-capturing solute enters the detector, the standing current decreases the electronic circuit is adjusted to a frequency of pulsing to maintain a constant current. The detector response is in the frequency... 

Electron capture is more effective, the slower the electrons move. For this reason, sensitive ECDs are operated using a pulsed DC voltage. By changing the pulse frequency, the current generated by the electrons is kept constant. The pulse frequency thus becomes the actual detector signal. 

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