Electron capture detector constant voltage

Varian Model 2KX) gas chromatograph equipped with a Ni electron-capture detector (8.5 mCi Ni beta source), operating with a constant polarizing voltage (d.c. mode) and a Varian Model A-25 1-mV recorder Column... 

Apparatus. An upstream flow control valve maintains a constant flow through a packed column. Liquid samples are charged with an Osage linear slide valve. A Jarrell Ash 26-755 electron capture detector is thermostatted in a 26-750 oven. Drycell batteries deliver a constant d.c. voltage to the detector. A Gyra Electronics E-302... 

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]. 

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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