Q1(f). Flame ionisation detector

Answer in brief

FID measures the ion current generated when organic compounds burn in a hydrogen–air flame. CO₂ and H₂O give essentially no useful response.

Key points

• Column effluent enters the flame; carbon-containing fragments generate charged species during combustion.
• A potential between burner and collector draws the ions to an electrode. The resulting small current is amplified and plotted against retention time.
• Response is approximately proportional to the mass flow of reduced organic carbon over a broad range. It is destructive and very sensitive for hydrocarbons.
• Fully oxidised carbon compounds such as CO₂ and inorganic substances such as water have negligible normal FID response.