Fourier-Transform Infrared Spectroscopy, or FTIR, is an analytical technique used across many scientific fields to quickly identify and characterize chemical substances. The method works by exposing a sample to a beam of infrared light and measuring how the material absorbs specific wavelengths of that energy. Because every unique chemical structure absorbs infrared radiation in a distinct way, this absorption pattern serves as a chemical barcode for identification. The technique is valued for its speed, reliability, and non-destructive nature.
The Physics of Molecular Vibration
Atoms in a molecule are constantly moving, connected by chemical bonds that behave like miniature springs. These movements, known as molecular vibrations, can be categorized into two primary types: stretching, where the bond length increases and decreases along the bond axis, and bending, where the angle between two bonds changes. Every chemical bond possesses a natural, specific frequency at which it prefers to vibrate, determined by the mass of the connected atoms and the strength of the bond.
Infrared (IR) light is a form of electromagnetic radiation that carries energy corresponding precisely to these molecular vibrational frequencies. When a molecule is exposed to a wide range of IR frequencies, it will only absorb the energy that exactly matches one of its natural vibrational modes. This phenomenon, known as resonance, drives the FTIR process, causing the molecule to vibrate more intensely.
For a vibration to be detectable by FTIR, it must result in a change to the molecule’s dipole moment. The dipole moment is a measure of the separation of positive and negative charges within the molecule. Symmetrical vibrations, such as those in linear molecules, do not alter the charge distribution. They are considered “IR-inactive” because they cannot interact with the light’s electromagnetic field.
Conversely, asymmetrical vibrations cause the electrical center of the molecule to shift, making them “IR-active” and allowing the bond to absorb and register the infrared energy. Stronger bonds and lighter atoms generally vibrate at higher frequencies, requiring higher-energy infrared light for resonance.
Key Components of the Instrument
The physical analysis process begins with an infrared source, typically a heated ceramic element, which emits a broadband beam containing all the necessary wavelengths of infrared energy. This beam is directed toward the Michelson interferometer. Before reaching the sample, the light is split into two separate paths by a specialized component called a beam splitter.
The interferometer operates by sending one light beam to a fixed mirror and the other to a mirror that moves a precise, short distance. When the two beams recombine, they interfere with each other, creating a unique wave pattern called an interferogram. This single, complex signal contains intensity information for all wavelengths simultaneously, contributing to FTIR’s superior speed and sensitivity.
After the light has passed through the sample chamber, where the material selectively absorbs certain wavelengths, the remaining beam reaches a sensitive detector. The detector registers the changes in the intensity of the recombined beam as a function of the moving mirror’s position. The resulting interferogram is a complex measurement in the time domain, essentially a raw signal that is not yet interpretable as a chemical spectrum.
The instrument’s computer applies a complex mathematical operation to the raw interferogram signal. This mathematical process rapidly converts the time-based signal into the final, frequency-based spectrum, plotting the intensity of absorption against the specific infrared wavelengths.
Decoding the FTIR Spectrum
The final output of the FTIR analysis is a graph that plots the material’s interaction with the infrared light. The horizontal axis (x-axis) is represented by wavenumbers, a unit of frequency inversely proportional to the wavelength and directly proportional to the light’s energy. The vertical axis (y-axis) typically measures percent transmittance or absorbance, showing how much light passed through the sample or how much was absorbed at each wavenumber.
The spectrum is generally divided into two main areas for interpretation, beginning with the higher wavenumber region, which typically spans from about 4000 to 1500 cm⁻¹. This section is known as the functional group region, where peaks correspond to the vibrations of specific, common chemical groups. This region is used to determine the major types of functional groups present, such as alcohols, acids, or hydrocarbons.
For example, a strong, broad absorption peak around 3300 cm⁻¹ is an indicator of an O-H stretch, suggesting the presence of a hydroxyl group found in alcohols or water. Similarly, a sharp, intense peak near 1700 cm⁻¹ confirms the presence of a carbonyl group (C=O), which is characteristic of ketones, aldehydes, and esters.
The lower wavenumber section, below 1500 cm⁻¹, is called the fingerprint region and is the most unique and definitive part of the spectrum. This area contains complex patterns generated by the bending, wagging, and rocking vibrations of the entire molecular framework, which are sensitive to the molecule’s specific structure. The combination of these vibrations creates a pattern unique to that specific compound, much like a human fingerprint.
This fingerprint region allows for the definitive identification of an unknown material by comparing its pattern to a vast digital library of known reference spectra. Even two molecules with the same functional groups, such as different structural isomers, will produce distinctly different, non-matching patterns in this lower-frequency region.
Real-World Applications
FTIR spectroscopy is routinely employed in pharmaceutical manufacturing for rapid quality control and verification. It is used to confirm the identity of incoming raw materials, ensuring that the correct active ingredients and excipients are being used before they enter the production line. This fast, non-destructive check prevents costly manufacturing errors and ensures compliance with strict regulatory standards.
In materials science, the technique is invaluable for analyzing polymers and plastics, such as those used in packaging, construction materials, and textiles. By scanning a polymer sample, researchers can instantly determine its composition, identify additives like plasticizers, or detect degradation that may have occurred due to heat or UV exposure. This analysis helps manufacturers maintain product consistency and predict material performance over time.
Forensic scientists frequently utilize FTIR to identify minute quantities of trace evidence collected from crime scenes, such as fibers, paint chips, or unknown powders. The technique is also applied in art conservation to identify the chemical composition of pigments and binding media in historical artifacts or paintings without causing damage.
The technology also plays a significant role in environmental monitoring and safety. It can be used to analyze gaseous samples, helping to monitor emissions from industrial stacks or identify volatile organic compounds (VOCs) in the atmosphere. This capability allows for the precise and timely identification of pollutants in both air and water samples, aiding regulatory efforts to maintain environmental quality.