An electron capture detector (ECD) is a highly sensitive instrument capable of detecting sulfur hexafluoride (SF6) at concentrations below 1 ppmv. This sensitivity stems from SF6's high electron attachment coefficient, which refers to its strong ability to capture electrons. The free electrons available for attachment to SF6 molecules are generated by a radioactive source within the ECD. Typically, the ECD employs a radioactive emitter in the form of a metallic membrane coated with the radionuclide nickel.
When the detector is in operation, the electrons emitted by the radioactive source are accelerated by an electric field. These accelerated electrons then ionize the background gas, which is usually ambient air. As a result, a steady - state ionization current is established as ions and electrons are collected at the electrodes.
When SF6 is present in the air sample being analyzed, it reduces the number of free electrons in the system. This occurs because the electrons become attached to the SF6 molecules. The reduction in the ionization current is directly proportional to the concentration of SF6 in the sample. However, it should be noted that other molecules also possess a certain electron attachment coefficient, which means the detector is sensitive not only to SF6 but also to these other molecules.
In essence, the ECD functions as a flow rate detector. This is because the sensor pumps the gas sample through the electric field at a constant speed. Through calibration procedures, the flow rate data is internally converted into SF6 concentrations and is then recorded in parts per million by volume (ppmv).
The accompanying photo showcases an electron capture detector (ECD).
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