photo from Lamont-Doherty Earth Observatory website(just for display)
An electron capture detector(ECD) is used to detect SF6 with high sensitivity below 1 ppmv. This device makes use of the high electron attachment coefficient of SF6, i.e. the ability to capture electrons. The free electrons to be attached to the SF6 molecules are created by a radioactive source. The ECD uses a radioactive emitter, typically, a metallic membrane coated with the radionuclide nickel. The emitted electrons are accelerated in an electric field and ionize the background gas, normally ambient air. Ions and electrons are collected at the electrodes resulting in a steady-state ionization current.
The presence of SF6 besides air reduces the number of free electrons, as they are now attached to the SF6 molecules. The decrease in the ionization current is proportional to the SF6 concentration. However, other molecules also have a certain electron attachment coefficient. the detector is also sensitive to those molecules.
In principle, this device is a flow rate detector, since the sensor is pumping the gas at a constant speed through the electric field. By calibration, this flow rate is internally converted into SF6 concentrations and recorded as ppmv.
The photo shows an ECD.