Infrared spectroscopy operates on the principle that molecules absorb frequencies that are characteristic of their structure. These absorptions occur at resonant frequencies, i.e. the frequency of the absorbed radiation matches the vibrational frequency. The energies are affected by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling. FTIR is an excellent method for identifying the “chemical family” of a substance.
Attenuated Total Reflectance (ATR) can be used to analyze insoluble or multi-layer samples.
How FTIR works – the infrared spectrum of a sample is recorded by passing a beam of infrared light through a sample. When the frequency of the IR is the same as the vibrational frequency of a bond or collection of bonds, absorption occurs. Measurement of the transmitted light reveals how much energy was absorbed at each frequency (or wavelength).
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Instrumentation Available
Key Attributes
- Signal Detected: Infrared absorption
- Elements Detected: Molecular functional groups
- Detection Limits: 1-10 wt % (quantification of known components); 5-20 % (identification of unknown components)
- ATR Depth Resolution: 6-2.5 µm
- Imaging: Yes
- Lateral Resolution: > 15-50 µm
Strengths
- Identification of organic functional groups and specific organic compounds
- Spectral libraries can be used to identify compounds and mixtures
- Sample analyzed at atmospheric pressure
- Quantitative with appropriate standards and uniform sample thicknesses
- Complementary to Raman spectroscopy
- No vacuum required
Limitations
- Limited surface sensitivity (typical limit of detection is a film thickness of 25 nm)
- Only specific inorganic species give an FTIR spectrum
- Standards required for quantitative analysis
- Cannot analyze glass substrates due to absorption
- Difficult to analyze samples in water due to absorption
- Metals cannot be analyzed by FTIR due to reflection
Applications
- Characterization and identification of complex mixtures of materials, including gases, liquids and solids
- Identification of organic contaminants (e.g. particles, residues) on the macro and micro scales
- Quantification of materials
Additional Reading
Wikipedia
