Fourier-transform infrared spectroscopy

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FTIR Spectrometer with ATR accessory

Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid, or gas. An FTIR spectrometer simultaneously collects high-resolution spectral data over a wide spectral range. This confers a significant advantage over a dispersive spectroscopy technique, which measures intensity over a narrow range of wavelengths at a time.

Principles of FTIR

FTIR works on the principle of interferometry. The core component of an FTIR spectrometer is the interferometer, which modulates the infrared light. The most common type of interferometer used in FTIR is the Michelson interferometer.

Interferogram produced by an FTIR spectrometer

Michelson Interferometer

The Michelson interferometer consists of a beam splitter, a fixed mirror, and a moving mirror. The beam splitter divides the incoming infrared beam into two paths: one path reflects off the fixed mirror, and the other reflects off the moving mirror. The two beams recombine at the beam splitter, creating an interference pattern known as an interferogram.

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Diagram of a Michelson Interferometer

Interferogram

The interferogram is a signal that contains all the infrared spectral information of the sample. It is a plot of light intensity versus the position of the moving mirror. The interferogram is then subjected to a Fourier transform to convert it into a spectrum.

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Digitisation of the interferogram

Fourier Transform

The Fourier transform is a mathematical process that transforms the raw data from the interferogram into a spectrum. This transformation allows the separation of the individual frequencies of light absorbed by the sample, providing a detailed spectrum that can be analyzed to determine the sample's composition.

Advantages of FTIR

FTIR spectroscopy offers several advantages over traditional dispersive infrared spectroscopy:

• Speed: FTIR can collect data for all wavelengths simultaneously, making it much faster.
• Sensitivity: The technique is more sensitive due to the Jacquinot advantage, which results from the higher throughput of the interferometer.
• Precision: The use of a laser as an internal calibration standard provides high precision in wavelength measurement.

Applications

FTIR spectroscopy is widely used in various fields, including:

• Chemistry: For identifying chemical compounds and studying chemical reactions.
• Pharmaceuticals: In the analysis of drug formulations and quality control.
• Environmental Science: For monitoring air and water quality by detecting pollutants.
• Materials Science: In the characterization of polymers and other materials.

Related pages

• Infrared spectroscopy
• Michelson interferometer
• Fourier transform
• Spectroscopy