Q1(h). Electron-capture selectivity

Answer in brief

It responds strongly to electron-attaching molecules, especially halogenated compounds, and weakly to many hydrocarbons.

Key points

• A beta-emitting source ionises the carrier gas and establishes a standing electron current.
• Electronegative analytes capture electrons, reducing the current; the decrease gives the analytical signal.
• Electron affinity and capture cross-section vary strongly among compounds. Halogenated pesticides and related molecules often give very sensitive responses.
• Selectivity is chemical rather than universal: response factors require calibration, and the detector does not identify a compound by itself.