Gas Chromatography: Detectors, Advantages, Disadvantages, and Applications

Introduction

• This lecture covers detectors used in gas chromatography (GC), advantages, disadvantages, and applications.
• Previous videos covered theory, sample preparation, derivatization, injectors, columns, and temperature programming.

Ideal Detector Characteristics

1. Good Sensitivity
   • Detect trace amounts of solute.
   • Flame Ionization Detector (FID): up to picogram/femtogram levels.
   • Thermal Conductivity Detector (TCD): up to nanogram levels.
2. Stability and Reproducibility
   • Consistent results on repeated injections.
3. Short Response Time
   • Quick detection of samples.
4. Non-Destructive Nature
   • Should not destroy sample structure.
5. High Temperature Operation
   • Suitable for high-temperature GC operations.

Common Detectors in GC

1. Thermal Conductivity Detector (TCD)
   • Non-destructive.
2. Flame Ionization Detector (FID)
   • Destructive.
3. Electron Capture Detector (ECD)
   • Non-destructive.
4. Nitrogen Phosphorus Detector (NPD)
   • Destructive.
5. Mass Spectrometer Detector (MS)
   • Destructive.

Detector Details

Thermal Conductivity Detector (TCD)

• Construction & Working
  • Metal block with cavities holding tungsten filaments.
  • Filaments form a Wheatstone bridge and are cooled by carrier gas.
  • Reduced thermal conductivity increases filament temperature and decreases current, detected by sensors.
• Principle
  • Based on differences in thermal conductivity between carrier gas and solute.
• Advantages
  • Simple, linear response, detects both organic and inorganic solutes, non-destructive.
• Disadvantages
  • Low sensitivity.
• Applications
  • Universal detector, widely used despite low sensitivity.

Flame Ionization Detector (FID)

• Construction & Working
  • Platinum jet, hydrogen, and air create a flame that ionizes solute molecules.
  • Current between electrodes increases with solute concentration.
• Principle
  • Based on electrical conductivity difference with solute ions.
• Advantages
  • High sensitivity, linear response, insensitive to water/oxygen.
• Disadvantages
  • Destructive, cannot detect certain compounds.
• Applications
  • Detects virtually all organic compounds, useful in pollution studies.

Electron Capture Detector (ECD)

• Construction & Working
  • Radioactive isotope (Nickel-63) coated electrode.
  • Solutes capture electrons, reducing current.
• Principle
  • Difference in electron capture by solutes.
• Advantages
  • Extremely sensitive, non-destructive.
• Disadvantages
  • Insensitive to certain functional groups.
• Applications
  • Environmental testing, especially for PCBs and organochlorine pesticides.

Nitrogen Phosphorus Detector (NPD)

• Construction & Working
  • Similar to FID but with a rubidium silicate bead.
  • Sensitive to nitrogen and phosphorus compounds.
• Advantages
  • Highly sensitive to nitrogen and phosphorus.
• Disadvantages
  • Not universal, destructive.
• Applications
  • Food, forensic, and environmental analysis.

Mass Spectrometer Detector (MS)

• Principle & Working
  • Measures mass-to-charge ratio of ions from samples.
  • Requires vacuum, interfaces with GC using a transfer line.
• Advantages
  • Fast, qualitative and quantitative analysis, highly sensitive.
• Disadvantages
  • Expensive.

Overall Advantages and Disadvantages of GC

Advantages

• Highly efficient, sharp reproducible peaks, sensitive detectors.

Disadvantages

• Limited to thermally stable and volatile compounds, most detectors are destructive.

Applications of Gas Chromatography

Qualitative Analysis

• Identification of Unknown Compounds
  • Compare retention times with standards under identical conditions.
• Purity and Impurity Detection
  • Additional peaks in sample chromatogram indicate impurities.

Quantitative Analysis

• Based on Peak Height/Area
  • Peak height/area proportional to concentration.
  • Peak area: height × width at half-height.

Additional Applications

• Diagnostic purposes, environmental monitoring, food analysis, pharmaceutical industry, cosmetology.

Conclusion

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