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Theory of FT-NIR Spectroscopy
Infrared electromagnetic radiation is a widely established technique to probe the chemical composition of materials.
The infrared spectrum is divided into 3 zones; near- (14000-4000 cm-1), mid- (4000-400 cm-1) and far-infrared (400-10 cm-1).
The mid-infrared (MIR) region is used to analyse the fundamental vibrations of molecules and is strongly absorbed so materials have to be analysed as thin films or in small path length cells (e.g. milk analysis).
Near-infrared (NIR) spectroscopy is based on molecular overtone and combination vibrations, which are forbidden by the selection rules of quantum mechanics. This means that NIR can penetrate much further into materials than MIR. This makes NIR very useful in probing bulk material with little or no sample preparation.
Because NIR probes the overtone and combination bands the spectra are usually very complex. Individual bands can not be assigned to specific features as with MIR. This means multiple wavelength (multivariate) calibration techniques are used to extract structural information. The design of powerful software packages, such as PLSplus/IQ, allows users with minimal chemometric experience the opportunity to generate and maintain their own calibration models without relying on general models from a third party that are not specific to their materials.
The increased processing power of computers has allowed the introduction of Fourier Transform (FT) infrared analysers. Prior to this technology instruments either had to either use filters to look at the absorption of specific wavelengths or use diffraction gratings to scan through the wavelengths and measure the changing absorptions. FT technology uses interferometers that allow all the information at all wavelengths to be collected simultaneously. This means much more information can be collected in a shorter time.
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