Summary of Instrumentation for IR spectroscopy (CHE)

Summary of "Instrumentation for IR Spectroscopy (CHE)"

This instructional video module covers the fundamentals of sample preparation and instrumentation used in infrared (IR) spectroscopy, focusing on both traditional dispersive IR spectrometers and modern Fourier Transform Infrared (FTIR) spectrometers. It also includes a practical guide for operating an FTIR instrument.

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Main Ideas and Concepts

1. Learning Objectives

• Understand how to prepare samples for IR spectral analysis of organic compounds.
• Comprehend the working principles of IR spectrophotometers.
• Gain hands-on experience in recording IR spectra.

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2. Sample Preparation for IR Spectroscopy

IR spectra can be recorded from samples in various physical states: solid, liquid, gas, or solution. Proper sample preparation is crucial because water and moisture can interfere with IR measurements.

• General considerations:
  • Samples must be dry (water absorbs strongly near 3710 and 1630 cm⁻¹).
  • Use moisture-free cell materials (e.g., sodium chloride, Potassium Bromide).
• Solid samples:
  • Pressed disc method:
    • Mix finely ground solid with powdered Potassium Bromide (KBr).
    • Press into a translucent pellet.
    • KBr is IR transparent (4000–650 cm⁻¹) but hygroscopic (absorbs moisture).
  • Mull (paste) method:
    • Mix solid with an oily mulling agent (e.g., Nujol).
    • Place thin film between NaCl plates.
    • Disadvantage: mulling agents have interfering absorption bands.
  • Film method:
    • Dissolve solid in a non-hygroscopic solvent (e.g., Methylene Chloride).
    • Deposit a drop on KBr or NaCl plates; evaporate solvent to form a thin film.
    • Film thickness must be controlled for optimal light transmission.
• Liquid samples:
  • Use neat liquids or solutions.
  • Place a drop between two salt plates (NaCl or KBr).
  • Plates are IR transparent but fragile and water-soluble.
  • Spectra obtained without solvent are called "neat spectra."
• Gaseous samples:
  • Introduce gas into a special long-path-length cell (usually ≥10 cm).
  • Cells have NaCl windows.
  • Vapor phase technique is limited by low vapor pressure of most organics.
• Note: Different sample states produce slightly different spectral features.

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3. Instrumentation for IR Spectroscopy

A. Dispersive IR Spectrophotometer

• Components:
  • Radiation source (e.g., Nernst filament, silicon carbide globe, nichrome coil).
  • Sample and reference cells (made of alkali halides like KCl or NaCl).
  • Monochromator (uses rotating chopper and diffraction grating to select wavelengths).
  • Detector (thermal or photon detectors).
  • Amplifier and recorder.
• Operation:
  • Source emits continuous IR radiation.
  • Beam split into sample and reference paths.
  • Monochromator isolates specific frequencies sequentially.
  • Detector converts IR radiation intensity differences into electrical signals.
  • Recorder plots % transmittance vs. wavenumber (cm⁻¹).
• Detectors:
  • Thermal detectors: measure heating effect; use thermocouples.
  • Photon detectors: rely on semiconductor excitation; more sensitive.

B. Fourier Transform Infrared (FTIR) Spectrometer

• Advantages:
  • Faster and more sensitive than dispersive instruments.
  • Simultaneously measures all frequencies.
  • Widely used in modern labs.
• Components:
  • IR source.
  • Interferometer (commonly Michelson Interferometer) with:
    • Beam splitter.
    • Fixed mirror.
    • Moving mirror (key to producing optical path difference).
  • Sample compartment.
  • Detector.
  • Analog-to-digital converter and computer.
• Operation:
  • Source beam split into two paths, reflected by mirrors.
  • Moving mirror changes path length, creating constructive and destructive interference.
  • Interference pattern (interferogram) contains all spectral information.
  • Interferogram passes through sample; detector records transmitted intensity.
  • Computer performs Fourier transform to convert time-domain interferogram into frequency-domain IR spectrum.
• Michelson Interferometer details:
  • Moving mirror’s position controls interference pattern.
  • Maxima occur when path difference is integral multiples of wavelength.
  • Minima occur when path difference corresponds to destructive interference.

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4. Hands-on Operation of FTIR Spectrometer

• Step 1: Prepare the sample using appropriate method (solid, liquid, gas).
• Step 2: Obtain background interferogram (includes atmospheric gases like CO₂ and water vapor).
• Step 3:

Category

Educational

Video