Gas Chromatography : Module 8

Types of Gas Chromatography Detectors

”If you want to increase your success rate, double your failure rate”

— Thomas Watson, Sr. founder of IBM

The Gas Chromatography detector is capable of establishing both the identity and concentration of eluting components in the carrier gas stream. Before going further into the types of detectors it is essential to understand the nature of detectors and their desirable characteristics.

  • Non-selective

o Responds to all compounds present in carrier gas stream except the carrier gas itself

  • Selective

o Responds to range of compounds with a common physical or chemical characteristic

  • Specific

o Responds to a single specific compound only

Detectors can also be grouped into concentration or mass flow detectors

Concentration Dependent

The response of such Gas Chromatography detectors is proportional to the concentration of the solute in the detector such as TCD. Dilution of sample with makeup gas will lower detector response.

Mass Flow Dependent

Signal is dependent on the rate at which solute molecules enter the detector such as FID. Response of such detectors is not affected by makeup gas flow rate changes.

Desirable characteristics of detectors :

  •  Reproducible response to changes in eluent composition in carrier gas stream
  •  High sensitivity 
  •  Large linear dynamic range 
  •  Low noise 
  •  Small volume to avoid peak broadening and resultant loss of resolution 
  •  Preferably non – destructive

Common Gas Chromatography detectors

Flame Ionization Detector (FID)

Flame ionization Detector

  •  Mass sensitive detector 
  •  Response depends on conducting power of ions or electrons produced on burning of organic compounds in the flame 
  •  Selective detector but sample detected must be combustible 
  •  Large linear dynamic range (107)
  •  No response to inorganic and permanent gases such as CO, CO2, NH3, CS2, N2, etc.
  • It is the most widely used detector in Gas Chromatography

Thermal Conductivity Detector (TCD)

  •  Non-destructive universal detector 
  •  Response depends on the thermal conductivity difference between the carrier gas and the eluted components 
  •  Wide dynamic range (107 – % to ppm levels) 
  •  Responds also to inorganic gases such as CO, CO2, NH3, CS2, N2, etc.

Electron Capture Detector (ECD)

  •  Operation based on absorption of β – rays emitted by radioactive source Ni63. Electrophores absorb the β – rays thereby reducing the current in the detector 
  •  Specific detector, non-destructive in nature
  •  Linear dynamic range about 105 
  •  Widely used in environmental analysis e.g. organochlorine pesticide

Nitrogen Phosphorous detector (NPD)

  •  Ions migrate to the collector electron creating a current proportional to sample concentration 
  •  Responds selectively to most organic compounds that contain phosphorus or nitrogen down to picogram levels 
  •  Mass flow dependent detector 
  •  Linear range about 106 
  •  Useful for analysis of drugs and pesticides containing phosphorus

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