Fourier-Transform Infrared Spectroscopy (FTIR) is an analytical technique that allows scientists to identify the chemical composition of materials. The method relies on the principle that nearly every molecule absorbs specific frequencies of infrared light in a unique pattern. This absorption creates a characteristic “molecular fingerprint” used to identify a compound (solid, liquid, or gas). FTIR quickly measures absorption across a wide range of infrared frequencies simultaneously, providing a fast and specific analysis of the sample’s structure.
Molecular Foundation: Interaction of Infrared Light
Infrared spectroscopy is possible because molecules are constantly in motion, with atoms connected by bonds that behave like tiny springs. When a molecule absorbs infrared radiation, the energy causes these bonds to vibrate by stretching, bending, or rocking. The specific frequency of light absorbed corresponds directly to the natural vibrational frequency of its chemical bonds.
For absorption to occur, the vibration must cause a change in the molecule’s net dipole moment—an uneven distribution of electrical charge. For example, the symmetric stretching of carbon dioxide is “IR inactive,” but its asymmetric stretch does, making it absorb infrared light. The resulting spectrum maps these vibrational energy transitions, revealing the presence of different chemical bonds.
Instrumental Design: The Role of the Interferometer
The initial step in FTIR is generating a broadband beam of infrared light from a glowing black-body source. This light passes through the heart of the instrument: a Michelson Interferometer, composed of a beam splitter, a fixed mirror, and a moving mirror.
The beam splitter divides the incoming infrared beam into two paths: one transmitted toward the fixed mirror, the other reflected toward the moving mirror. Both beams reflect back and recombine. The moving mirror travels a short, precise distance, continuously changing the path length relative to the fixed mirror.
When the beams recombine, the difference in travel distance (the optical path difference) causes them to interfere (constructively or destructively). This process encodes the entire range of infrared frequencies simultaneously. The recombined beam then passes through the sample compartment before reaching the detector.
Data Generation: From Light Signal to Interferogram
The detector measures the intensity of the light that emerges from the interferometer and passes through the sample. Since the moving mirror is constantly scanning, the light hitting the detector is a complex, oscillating signal called an interferogram.
The interferogram plots light intensity versus the position of the moving mirror (the optical path difference). When the mirrors are equidistant, the path difference is zero, and all wavelengths constructively interfere, creating a sharp, central peak known as the centerburst. This raw signal contains information about all infrared frequencies that interacted with the sample, but this time-domain signal is not yet easily interpretable for identifying chemical structures.
Data Processing: The Fourier Transformation
To convert the unreadable interferogram into a useful spectrum, a specialized mathematical algorithm known as the Fourier Transform (FT) is applied rapidly by the spectrometer’s computer system. The Fourier Transform takes the time-domain signal (the interferogram) and mathematically separates it into its individual frequency components.
The FT is necessary because the interferogram encodes all infrared frequencies simultaneously as a single, complex wave pattern. The mathematical function deconvolutes this pattern to reveal the intensity of each specific frequency component. The result is the final infrared spectrum, where the \(x\)-axis represents frequency and the \(y\)-axis represents intensity. This process converts the raw data based on mirror displacement into a spectrum based on wavenumber (inverse centimeters, \(\text{cm}^{-1}\)), which relates directly to the energy of molecular vibrations.
Spectral Analysis: Interpreting the Final Output
The final output of the FTIR process is the infrared spectrum, a graph plotting transmittance or absorbance against wavenumber. The \(x\)-axis indicates the frequency of the absorbed light, which relates directly to the energy of the molecular vibration. The \(y\)-axis shows the percentage of light transmitted or the degree of light absorbed by the sample.
Peaks appear at specific wavenumbers and are used to identify the functional groups present. For example, a strong, broad peak between 3200 and \(3600\ \text{cm}^{-1}\) indicates an O-H (hydroxyl) stretching vibration, while a sharp peak near \(1700\ \text{cm}^{-1}\) signals a C=O (carbonyl) group. The region below \(1500\ \text{cm}^{-1}\) is the “fingerprint region.” This area contains complex absorption patterns unique to the molecule’s structure, allowing for definitive identification by comparing its pattern to a database of known spectra.