Used to detect chemical compounds in sample substances, an electron capture detector generally emits radioactive beta particles and gas, such as nitrogen, into a mixture. These particles typically collide with and are carried by gas molecules, while collisions scatter electrons which then move toward a positively charged electrode. A current is generated by this process, and when the electrons are recaptured, the current between two electrodes is decreased. Compounds in the sample are usually detected by the change in electrical current. Invented in the 1950s, the electron capture detector is often used to detect halogen, nitro or nitril based compounds, as well as carbon and metal bonded chemicals.
Modern electron capture detectors in the 21st century sometimes use plasma to generate an electron flow. Most devices operate either by applying a constant direct current (DC) potential or a pulsed potential. With DC mode operation, enough voltage is introduced to collect the electrons that are emitted. A constant current is produced and typically falls when molecules begin to collide. The pattern of change is used for detecting compounds.
Pulsed mode operation generally blocks the path of most negatively charged molecules. When the pulse is in off mode, then the electrons react with the gas. Materials can be detected through the motion of their particles, which is influenced by the timing, frequency, and amplitude of the pulses. Nitrogen and halogen are often used in an electron capture detector because relatively few electrons are held by their atoms. The substances can therefore more readily capture electrons flowing free through the system.
Electron capture detectors are often used in gas chromatography, and they were first developed to be used in that field. Still used in 21st century, these detectors generally need to be handled with care. Permission by regional authorities is often needed to acquire and use an electron capture detector. Records of inventory, test results from the manufacturer, and a list of specifications sometimes have to be submitted for safety review.
A profession with a permit may be the only person allowed to install, test, or remove an electron capture detector. Regulations can also dictate that caution labels are visible on the device or in the room it is stored. There are often requirements that flammable substances not be stored near the detector. Since it typically has radioactive material, specific procedures for disposing of the instrument may also need to be followed.