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•White Paper: Fundamental Infrared Spectroscopy

Principle of Infrared Spectroscopy

Spectroscopy is the study of the interaction of electromagnetic radiation with a chemical substance. The nature of the interaction depends upon the properties of the substance. When radiation passes through a sample (solid, liquid or gas), certain frequencies of the radiation are absorbed by the molecules of the substance leading to the molecular vibrations. The frequencies of absorbed radiation are unique for each molecule which provide the characteristics of a substance.

The electromagnetic spectrum covers an immense range of wavelengths. The infrared regions are classified as follows:

• Near Infrared: 12500 to 4000cm-1 (0.8 to 2.5um)
• Mid Infrared: 4000 to 200cm-1 (2.5 to 50um)
• Far Infrared: 200 to 12.5cm-1 (50 to 800um)

Either the wavelength (?m) or wavenumber (cm-1) is used to measure the position of an infrared absorption. The absorption bands for most organic and inorganic compounds are found in the infrared region. The table of characteristic IR absorptions of functional groups can be found inmost chemistry text books. Any absorption band can be characterized by two parameters: the wavelength at which maximum absorption occurs and the intensity of absorption at this wavelength. In an absorption spectrum, the y-axis measures the intensity of the band which is proportional to the number of molecules observed. This principle consequently leads to quantitative analysis.

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