In Gas Chromatography (GC), detectors play a crucial role in identifying and quantifying the components in a sample. Below are the constructions and working principles of three commonly used GC detectors:

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1. Flame Ionization Detector (FID)

Construction:

  • The FID consists of a burner that produces a flame, typically fueled by hydrogen and air.
  • The sample, after separation in the column, is introduced into this flame.
  • Electrodes are placed near the flame to collections formed in the combustion process.
  • A collector (usually a positively charged electrode) is positioned above the flame, while a negatively charged electrode is placed below the flame or at the base.

Working Principle:

  • Organic compounds eluting from the GC column are burned in the hydrogen-air flame.
  • During combustion, organic molecules are ionized, creating positively charged ions and electrons.
  • The ions are collected by the electrodes, generating an electrical current.
  • The current is proportional to the number of ions formed, which is directly related to the concentration of organic compounds in the sample.
  • FID is sensitive to hydrocarbons but does not respond to water, CO₂, or other gases that do not ionize in the flame.

Key Features:

  • Highly sensitive to hydrocarbons.
  • Detection limit: ~10⁻¹² g/sec for hydrocarbons.
  • Requires hydrogen gas for operation.

2. Thermal Conductivity Detector (TCD)

Construction:

  • TCD consists of a filament made of a conductive material (usually tungsten or platinum) placed in the gas stream.
  • The detector chamber has two cells: one contains the carrier gas, while the other contains the eluted gas sample from the column.
  • The filament is heated electrically and its resistance depends on the thermal conductivity of the surrounding gas.

Working Principle:

  • The thermal conductivity of the carrier gas (typically helium or hydrogen) is significantly higher than that of most organic compounds.
  • When the carrier gas alone passes over the filament, the heat is carried away efficiently, maintaining a stable filament temperature.
  • As the sample elutes from the column, it passes through the second chamber, reducing the thermal conductivity around the filament.
  • This causes a temperature increase in the filament, altering its resistance.
  • The change in resistance is measured as a voltage change, which is proportional to the concentration of the sample component in the gas stream.

Key Features:

  • Universal detector (responds to all gases, including organic and inorganic).
  • Less sensitive than FID (detection limit: ~10⁻⁶ g/mL).
  • Non-destructive and suitable for detecting permanent gases like O₂, N₂, and H₂.

3. Electron Capture Detector (ECD)

Construction:

  • The ECD consists of a radioactive source (commonly nickel-63) that emits beta particles (electrons).
  • The beta particles ionize the carrier gas (often nitrogen), producing free electrons.
  • A collector electrode captures these electrons, generating a measurable current.

Working Principle:

  • As the sample components elute from the GC column, they pass through the detector chamber where the beta particles ionize the carrier gas.
  • Electronegative compounds (such as halogens, nitriles, and peroxides) capture the free electrons, reducing the current.
  • The degree of electron capture is proportional to the concentration of the electronegative compounds in the sample.
  • The resulting decrease in current is recorded and used for quantitative analysis.

Key Features:

  • Highly sensitive to electronegative compounds (e.g., halogenated compounds).
  • Detection limit: ~10⁻¹⁵ g/sec, making it one of the most sensitive detectors.
  • Suitable for environmental analysis, especially for detecting pesticides, PCBs, and other halogenated compounds.