Electron capture detector standing current

High purity nitrogen gas is used for direct current electron capture detectors. This gas should be filtered and dry. An exhausted drier will allow passage of water and reduce the standing current. 

Application of a fixed or pulsed potential to the ionization chamber allows collection of the thermal electrons establishing the standing (or baseline) current. When an electron-capturing compound enters the ionization chamber, thermal electrons are removed by formation of negative ions. The increased rate of neutralization of these ions by positive ions, or their reduced drift velocity during collection of the thermal electrons, is responsible for the detector signal. In contrast to the other ionization detectors a decrease in the detector standing current proportional to solute concentration is measured rather than an increase in the number of ions or electrons produced. 

The electrons formed by this combined process result in a high standing current (about 10 a) when collected by a positive electrode. When an electronegative analyte is eluted from the column and enters the detector, it captures some of the free electrons and the standing current is decreased giving a negative peak ... 

The detector can be made to function in two ways either a constant potential is applied across the sensor electrodes (the DC mode) or a pulsed potential is used (the pulsed mode). In the DC mode, a constant electrode potential of a few volts is employed that is just sufficient to collect all the electrons that are produced and provide a small standing current. If an electron capturing molecule (for example a molecule containing a halogen atom which has only seven electrons in its outer shell) enters the sensor, the electrons are captured by the molecules and the molecules become charged. The mobility of the captured electrons are much reduced compared with the free electrons and, furthermore, are more likely to be neutralized by collision with any positive ions that are also generated. As a consequence, the electrode current falls dramatically. 

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... 

In Equation 6.16, Is is the standing current, I the current measured when electronabsorbing species are in the detector at concentration c, and A is a constant characteristic to the cell and the species present (also known as the electron-capture coefficient). 

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