Infrared (IR) Spectroscopy Configuration

Key Components and Terms

• Source

  • Commonly black body radiation (black body radiators)
  • Temperature range: 1500 K to 200 K
  • Wavelengths: Max ~5500 wave numbers (1.7 to 2 micrometer)
  • Spectrum range: 1 to 15 micrometers (700 to 10,000 wave numbers)
  • Types of black body radiators:
    • Nernst glower: Rare earth oxides; 1220 K to 2220 K
    • Globar: Shorter wavelengths; 1300 to 1500 K; needs water cooling
    • High-pressure mercury arc: Far IR region (>50 micrometers)

• Wavelength Selector

  • Dispersive: For double beam, uses reflection grading
  • Non-Dispersive: Uses filter or filter wedge

Sample Types

• Liquids, Solids, Gases
  • Cells/Windows: Alkali metal halide (e.g., KBr, NaCl)
  • Considerations: Range of transparency, cost, solubility, reactivity
  • Solids: Can be mixed with KBr and pressed into a pellet or ground with heavy hydrocarbon oil
  • Gases: Require long path length for sensitivity

Solvents

• Choose based on IR absorption and reactivity
• Avoid water and alcohols due to strong absorption bands

Sample Holder

• Consists of plates, gaskets, windows, and spacers
• Path Length: Important for minimizing solvent absorption

Path Length Determination

• Use constructive and destructive interference to determine path length
• Formula: n * λ = 2b (where b is path length)*

Detectors

• Types:
  • Thermal: Temperature detection; poor sensitivity; slow
  • TGS (Triglycine sulfate): Pyroelectric effect; fast; common for Fourier Transform IR
  • Photoconductive: Semiconductor-based; slow; nitrogen cooling reduces thermal noise

Instrumentation Schematic

• Details beam path: IR source → sample compartment → mirrors and beam splitter → detector
• Fourier Transform Interferometry: Used for enhanced analysis

Fourier Transform Advantages

• Signal-to-noise enhancement
• Rapid scanning
• Multiplex Advantage: Simultaneous measurement of resolution elements
• Throughput Advantage: Less optical attenuation, increased signal
• Precise calibration: Laser reference aids precise wave number calibration

IR Spectrum

• Wave numbers vs. Transmittance
• Regions:
  • Group Frequency: 4000 to 1600 (specific to groups)
  • Fingerprint: Unique to compound; differences in molecular structure

These notes summarize the key concepts and configuration details of IR spectrometry, providing a comprehensive overview of its components, sample handling, and advantages in measurement and analysis.