Electron Capture Detector
The first option for environmental measurements, offers excellent performance in the determination of PCBs, pesticides, and other halogenated organic compounds.
The invention of the ECD (for GC) is attributed to Lovelock, based on his publication in 1961. It is a selective detector that provides very high sensitivity for those compounds that “capture electrons.” These compounds include halogenated materials like pesticides and consequently, one of its primary uses is in pesticide residue analysis.
Inert, very dry nitrogen is used as the carrier gas, or as the make-up gas with capillary columns. A radioactive source, 63Ni emits a soft β particle which ionizes N2. This ionization produces a high, steady background current of electrons. With no sample there is a high background current. When electronegative samples elute from the column (halogenated compounds, explosives, nitro groups, aromatic compounds and highly conjugated organic compounds) they capture electrons and reduce the high background current. Nowadays, most of the ECDs operate in constant current and pulsed variable frequency mode: when the standing current decreases, the electronic circuit applies a pulse voltage to a certain frequency to maintain a constant current. The detector response corresponds to the frequency change. This mode offers improved sensitivity and dynamic range compared to the original DC mode .
A schematic of a typical ECD is shown in Figure above. 63Ni is shown as the beta emitter. The carrier gas used for the ECD can be helium, but then with a capillary column make-up gas of nitrogen is necessary for proper operation of the ECD.
The ECD is one of the most easily contaminated detectors and is adversely affected by both traces of oxygen and water. Ultrapure, dry gases, freedom from leaks, and clean samples are necessary. Evidence of detector contamination is usually a noisy baseline or peaks that have small negative dips before and after each peak. Cleaning the radioactive foil can sometimes be accomplished by operation of the detector with hydrogen carrier gas at 350°C to burn off organic impurities. It is best to remove the column for this cleaning procedure.