Lesson 10 - Detectors in GC

Advanced Instrumental Analysis

  • Course: Advanced Instrumental Analysis (LS5013FP)

  • Lesson: Detectors in Gas Chromatography (GC)

Page 1: Introduction

  • Topic: Detectors in GC

Page 2: Types of GC Detectors

  • Types of Detectors:

    • Flame Ionisation Detector (FID)

    • Thermal Conductivity Detector (TCD)

    • Electron Capture Detector (ECD)

    • Nitrogen Phosphorus Detector (NPD)

Page 3: Learning Objectives

  • After this lesson, students will be able to:

    • List types of detectors used in GC

    • Select appropriate detectors for given samples

Page 4: Detector Types and Features

  • Detector Type and Support Gases:

    • Flame Ionisation Detector (FID):

      • Support gases: Hydrogen, Air

      • Selectivity: Most organic compounds

    • Thermal Conductivity Detector (TCD):

      • Support gas: Helium or Hydrogen

      • Selectivity: Universal

    • Nitrogen Phosphorus Detector (NPD):

      • Support gases: Hydrogen, Air

      • Selectivity: Nitrogen, Phosphorus

    • Electron Capture Detector (ECD):

      • Support gas: Nitrogen (carrier)

      • Selectivity: Halides, nitrates, nitriles, peroxides, organometallics

    • Flame Photometric Detector (FPD):

      • Support gases: Hydrogen, Air, Oxygen

      • Selectivity: Sulphur, Phosphorus, Tin, Boron, Arsenic, Germanium, Selenium, Chromium

Page 5: Detector Comparison

  • Comparison of Detectors:

    • Selectivity:

      • ECD (selective)

      • TCD (universal)

    • Destructiveness:

      • FID (destructive)

      • TCD, ECD (non-destructive)

Page 6: Flame Ionisation Detector (FID) Principles

  • Working Principle:

    • Column effluent is burned in a small oxy-hydrogen flame, producing ions.

    • Collected ions form a small current, which is the signal.

    • Requires high purity of hydrogen and air to minimize ionization from impurities.

Page 7: FID Operating Conditions

  • Temperature Management:

    • Detectors operate at 125ºC to avoid condensation.

    • Typical operating temperature for FID: 250ºC or hotter.

Page 8: FID Response Characteristics

  • Response to Organic Compounds:

    • FID responds to all organic compounds that burn in the oxy-hydrogen flame.

    • Signal proportional to carbon content - "equal per carbon rule."

Page 9: Advantages of FID

  • Advantages of FID:

    • Good sensitivity

    • Large linearity (up to 106)

    • Simple to operate

    • Ruggedness and adaptability to various column sizes

Page 10: FID Characteristics

  • Characteristics:

    • Minimum Detectable Quantity (MDQ): 10^-11 g (~50 ppb)

    • Responds to organic compounds only

    • Excellent linearity: 10^6

    • High stability, minimal effect from flow/temperature

    • Maximum temperature: 400ºC

    • Carrier gases: Nitrogen or Helium

Page 11: Thermal Conductivity Detector (TCD) Overview

  • General Features of TCD:

    • Suitable for detecting both organic and inorganic analytes (e.g., H2O, CO, CO2, H2).

    • Measures the thermal conductivity of the analyte compared to carrier gas.

Page 12: How TCD Works

  • Working Mechanism:

    • Pure carrier gas flows over two cells in a bridge circuit.

    • Temperature changes due to alterations in thermal conductivity are sensed as changes in resistance.

Page 13: TCD Signal Processing

  • Signal Processing:

    • Voltage drop across a voltage divider relays information to a recorder.

    • The system returns to balance after analyte elution.

Page 14: TCD Sensitivity and Operation

  • Sensitivity Considerations:

    • Higher heating current increases sensitivity but reduces filament life due to oxidation.

    • Requires oxygen-free carrier gas and leak-free systems for optimal performance.

Page 15: Advantages of TCD

  • Advantages of TCD:

    • Ruggedness

    • Universal detection capability for all compounds with different thermal conductivities.

    • Moderate sensitivity

Page 16: TCD Characteristics

  • Characteristics:

    • MDQ: 10^-9 g (~10 ppm)

    • Response: all compounds

    • Linearity: 10^4

    • Good stability

    • Maximum temperature: 400ºC

    • Carrier gas: Helium

Page 17: Electron Capture Detector (ECD)

  • Operation Principle:

    • Ionization-type detector based on detecting decreased ionization levels.

    • Uses radioactive 63Ni to produce electrons that generate a standing current.

Page 18: ECD Current Measurement

  • Current Dynamics:

    • Electrons captured by electronegative analytes lead to a decrease in current, creating a negative peak.

Page 19: Drawbacks of ECD

  • Limitations of ECD:

    • Requires a radioactive source, which may need a license.

    • Highly susceptible to contamination by oxygen and water.

Page 20: ECD Operating Requirements

  • Essential Parameters:

    • Requires ultrapure, dry gases.

    • Evidence of contamination typically shows as a noisy baseline or small negative dips in peaks.

Page 21: ECD Characteristics

  • Characteristics:

    • MDQ: 10^-9 g to 10^-12 g

    • Very selective response

    • Linearity: 10^3 to 10^4

    • Fair stability

    • Carrier gas: Nitrogen

Page 22: Nitrogen Phosphorus Detector (NPD)

  • General Overview:

    • Known as Thermionic Ionization Detector (TID) or Flame Thermionic Detector (FTD).

    • Very responsive to nitrogen and phosphorus-containing compounds.

Page 23: NPD Functionality

  • Sensor Details:

    • Comprises a rubidium or cesium bead within a heating coil.

    • The heated bead emits electrons via thermionic emission, generating an ion current.

Page 24: NPD Mechanism of Action

  • Detection Mechanism for NPD:

    • When analytes with nitrogen or phosphorus are present, they are adsorbed on the bead.

    • This process enhances electron emission, increasing anode current.

    • Sensitivity: 10^-12 g/ml for phosphorus, 10^-11 g/ml for nitrogen.

Page 25: Conclusion

  • Type of Detectors Summary:

    • Various detectors available for GC systems, with FID and TCD being the most common.

    • Different detectors respond differently to various analytes.