Introduction
FT-IR spectroscopy has been a workhorse technique for materials analysis in the lab for over 70 years. An IR spectrum represents a fingerprint of a sample with absorption peaks which correspond to the frequencies of vibrations between the bonds of the atoms making up the material. Because each different material is a unique combination of atoms, no two compounds produce the same IR spectrum. Therefore, IR spectroscopy can result in a positive identification (qualitative analysis) of every different kind of material. In addition, the size of the peaks in the spectrum is a direct indication of the amount of material present. With intuitive software algorithms, infrared is an excellent tool for quantitative analysis.
FT-IR Introduction | Continuous Scan Interferometry | Step-Scan Interferometry | Polarization Modulation FT-IR Spectroscopy | High-Resolution Spectral Range Extension of FT-IR
Principles of FT-IR: The Michelson Interferometer
Invented more than 100 years ago, the two-beam Michelson interferometer is still the heart of most modern FT-IR spectrometers. It consists of a fixed mirror, a moving mirror and a beamsplitter, as illustrated in Figure 1.1. The beamsplitter is a laminate material that reflects and transmits light equally. The collimated IR beam from the source is partially transmitted to the moving mirror and partially reflected to the fixed mirror by the beamsplitter. The two IR beams are then reflected back to the beamsplitter by the mirrors. The detector (D) then sees the transmitted beam from the fixed mirror and reflected beam from the moving mirror simultaneously.
Figure 1.1: Optical diagram of a classic Michelson interferometer, which consists of a fixed mirror, a moving mirror and a beamsplitter.
The two combined beams interfere constructively or destructively depending on the wavelength of the light (or frequency in wavenumbers) and the optical path difference introduced by the moving mirror. The latter is referred to as retardation, δ (cm). To obtain an interferogram, I(δ), the detector signal is digitized and recorded as a function of retardation.
The interferogram I(δ) is a simple sinusoidal wave when a monochromatic source is used, as shown in Figure 1.2. For a continuum (or polychromatic) source, I(δ) is a superposition of sinusoidal waves for IR light at all wavenumbers σ. At zero path difference (ZPD) or zero optical retardation, all the sinusoidal waves are totally constructive, producing a centerburst on the interferogram.
Figure 1.2: Sinusoidal and center-burst interferograms for monochromatic and continuum light sources, respectively.
FT-IR Introduction | Continuous Scan Interferometry | Step-Scan Interferometry | Polarization Modulation FT-IR Spectroscopy | High-Resolution Spectral Range Extension of FT-IR | 
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