A Gas Chromatography-Electron Capture Detector (GC-ECD) system is a highly sensitive and selective analytical instrument used for the detection and quantification of electronegative chemical species, particularly halogenated compounds, at extremely low (trace) levels. It is a specialized tool for environmental and forensic analysis. In a GC-ECD system, volatile and semi-volatile compounds are first separated by a gas chromatograph. After separation, the compounds enter the Electron Capture Detector (ECD). The ECD contains a radioactive beta particle emitter, typically Nickel-63, which ionizes a carrier gas (e.g., nitrogen or argon-methane), creating a constant background current of free electrons. When an electronegative compound elutes from the column and enters the detector, it captures some of these free electrons, causing a measurable decrease in the background current. This decrease is proportional to the concentration of the electron-capturing analyte. The ECD is renowned for its exceptional sensitivity, often being 10 to 1000 times more sensitive than a Flame Ionization Detector (FID) for specific compounds, with detection limits as low as 5 femtograms per second. GC-ECD is primarily applied in environmental analysis for the ultra-trace detection of pesticides, herbicides, polychlorinated biphenyls (PCBs), and other persistent organic pollutants in water, soil, and air samples. It is also used in food safety for analyzing pesticide residues, in pharmaceutical analysis for detecting trace amounts of drugs, and in forensic analysis for identifying toxic substances. Its high selectivity for compounds containing halogens (e.g., chlorine, bromine), as well as certain nitrogen and oxygen compounds, makes it ideal for targeted analyses where high specificity is required. While GC-ECD offers unparalleled sensitivity for its target analytes, it typically has a more limited linear range compared to other detectors and may require specialized GC columns and carrier gases for optimal performance. Due to the presence of a radioactive source, users of GC-ECD systems may need to adhere to specific radiation safety procedures and regulations.