FTIR Spectroscopy

Created in July 29, 2025

2025   ·   spectroscopy   FTIR   chemistry   analytical-techniques

1. Introduction to FTIR Spectroscopy

Fourier Transform Infrared (FTIR) spectroscopy is an analytical technique used to identify and characterize chemical compounds by measuring their absorption of infrared (IR) radiation. It provides a molecular fingerprint of the sample, allowing for qualitative and quantitative analysis.

Key Principles

  • Infrared Absorption: Molecules absorb IR light at specific frequencies corresponding to their vibrational modes (e.g., stretching, bending).
  • Interferometry: Instead of a dispersive element (like in traditional IR spectroscopy), FTIR uses an interferometer to modulate IR light, producing an interferogram.
  • Fourier Transformation: The interferogram is converted into a spectrum (absorbance vs. wavenumber, \(cm^{-1}\)) using a mathematical Fourier transform.

2. Step-by-Step Practical Example

Step 1: Sample Preparation

  • Solid Samples:
    • KBr Pellet Method: Mix ~1 mg of sample with 100 mg KBr, press into a transparent pellet.
    • ATR (Attenuated Total Reflectance): Place solid directly on ATR crystal (no preparation needed).
  • Liquid Samples:
    • Apply a thin film between two NaCl or KBr plates.
    • For volatile liquids, use a sealed liquid cell.
  • Gas Samples: Use a gas cell with IR-transparent windows.

Step 2: Instrument Setup

  1. Turn on the FTIR spectrometer and allow it to warm up (~15-30 min).
  2. Background Scan: Run a scan without the sample to account for atmospheric CO₂ and H₂O interference.
  3. Load the sample into the sample compartment (e.g., place KBr pellet in holder or ATR crystal).

Step 3: Data Acquisition

  1. Select scan parameters:
    • Wavenumber range: Typically 4000–400 cm⁻¹
    • Resolution: 4 cm⁻¹ (higher resolution for fine details)
    • Number of scans: 16–64 (averaging improves signal-to-noise ratio)
  2. Run the scan: The interferometer modulates IR light, and the detector records the interferogram.
  3. Fourier Transform: Software converts the interferogram into a spectrum.

Step 4: Data Analysis

  • Peak Identification: Compare observed peaks to reference spectra (e.g., O-H stretch ~3300 cm⁻¹, C=O stretch ~1700 cm⁻¹).
  • Quantitative Analysis: Use Beer-Lambert law (\(A = \epsilon \cdot c \cdot l\)) for concentration determination.

Step 5: Post-Measurement

  • Clean the sample holder (e.g., wipe ATR crystal with ethanol).
  • Store data and export spectra for further processing.

3. Example Application

Identifying an Unknown Polymer

  1. Obtain a spectrum of the polymer (e.g., polyethylene).
  2. Observe key peaks:
    • C-H stretch: ~2900 cm⁻¹
    • C-H bending: ~1460 cm⁻¹
  3. Match with a spectral library to confirm identity.

4. Advantages of FTIR

  • Fast acquisition (seconds per scan).
  • High sensitivity and resolution.
  • Minimal sample preparation (especially with ATR).

By following these steps, FTIR spectroscopy can be effectively used for material characterization in chemistry, pharmaceuticals, and materials science.