Fourier Transform Infrared (FTIR) spectroscopy is a powerful analytical technique used to identify and characterize chemical compounds based on their molecular vibrations. It operates on the principle of infrared absorption, where molecules absorb specific frequencies of infrared light corresponding to their vibrational energy levels. The core of FTIR lies in interferometry, which allows simultaneous measurement of all wavelengths, improving speed and sensitivity compared to traditional dispersive infrared spectroscopy.

The working principle of FTIR relies on the Michelson interferometer, a key component that generates an interferogram. The interferometer consists of a beam splitter, a fixed mirror, and a moving mirror. Infrared light from the source is directed toward the beam splitter, which divides the beam into two paths: one reflecting toward the fixed mirror and the other transmitting toward the moving mirror. The two beams recombine at the beam splitter after reflecting off their respective mirrors, creating an interference pattern. The moving mirror introduces an optical path difference, generating constructive and destructive interference as it travels. The resulting interferogram encodes all infrared frequencies as a function of the mirror displacement. A detector captures this signal, and a Fourier transform mathematically converts it into a spectrum of intensity versus wavenumber (cm⁻¹).

Molecular vibrations detectable by FTIR include stretching and bending modes. Stretching vibrations involve changes in bond lengths, such as symmetric and asymmetric stretches in groups like CH₂ or OH. Bending vibrations involve changes in bond angles, including scissoring, rocking, wagging, and twisting motions. These vibrations occur at specific frequencies, producing characteristic absorption bands in the infrared spectrum. For example, carbonyl groups (C=O) typically absorb around 1700 cm⁻¹, while hydroxyl groups (O-H) appear near 3300 cm⁻¹. The position, shape, and intensity of these bands provide information about molecular structure, functional groups, and intermolecular interactions.

FTIR spectroscopy can be performed in different measurement modes, each suited for specific sample types and analytical needs. Transmission mode is the most straightforward, where infrared light passes through a thin sample, and the transmitted light is measured. This mode is ideal for homogeneous liquids, gases, or thin solid films. Reflectance modes include specular and diffuse reflectance, where light reflects off the sample surface. Specular reflectance is used for smooth surfaces, while diffuse reflectance is suitable for powders or rough surfaces, often coupled with techniques like DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy).

Attenuated Total Reflectance (ATR) is a widely used sampling technique that requires minimal sample preparation. In ATR, the infrared beam enters a high-refractive-index crystal (e.g., diamond or germanium) at an angle greater than the critical angle, creating an evanescent wave that penetrates a few micrometers into the sample. The evanescent wave interacts with the sample, and the attenuated reflected light is detected. ATR is particularly useful for analyzing thick, opaque, or strongly absorbing samples, such as polymers, biological tissues, or viscous liquids. Unlike transmission mode, ATR does not require thin samples and is less affected by scattering effects.

The advantages of FTIR over dispersive spectroscopy include higher signal-to-noise ratio, faster data acquisition, and better wavelength accuracy due to the use of a laser reference for mirror positioning. The multiplex advantage allows all frequencies to be measured simultaneously, while the throughput advantage results from the absence of narrow slits that limit light in dispersive instruments. These features make FTIR highly sensitive and capable of detecting trace concentrations in complex matrices.

Applications of FTIR span various fields, including pharmaceuticals, polymers, environmental science, and forensics. In pharmaceuticals, it is used for drug formulation analysis and quality control. Polymer scientists rely on FTIR to study degradation, cross-linking, and additive composition. Environmental monitoring employs FTIR for detecting pollutants in air and water. Forensic analysis uses FTIR to identify unknown substances in criminal investigations.

The interpretation of FTIR spectra involves comparing observed absorption bands with reference databases or known standards. Peak assignments are based on established correlations between vibrational frequencies and molecular structures. For instance, alkanes show C-H stretches near 3000 cm⁻¹, while aromatic rings exhibit characteristic peaks between 1600 and 1450 cm⁻¹. Quantitative analysis is possible using Beer-Lambert law, where absorbance is proportional to concentration, provided calibration standards are available.

Limitations of FTIR include interference from water vapor and CO₂ in the atmosphere, which can obscure sample signals. Proper background subtraction and purging with dry air or nitrogen mitigate these effects. Samples with strong absorption may require dilution or alternative sampling techniques like ATR to avoid saturation. Additionally, FTIR cannot detect homonuclear diatomic molecules (e.g., N₂ or O₂) due to their lack of a dipole moment change during vibration.

In summary, FTIR spectroscopy is a versatile and non-destructive technique for molecular analysis. Its foundation in interferometry and the Michelson interferometer enables rapid, high-resolution measurements across a broad spectral range. By detecting vibrational modes, FTIR provides detailed chemical fingerprints useful for identification and characterization. The choice between transmission, reflectance, and ATR modes depends on sample properties and analytical requirements. With its wide applicability and robust performance, FTIR remains a cornerstone of modern analytical chemistry.