The theory of NIR, according to our wise friend Wikipedia is:
Nearinfrared spectroscopy (NIRS) is a spectroscopic method that uses the nearinfrared region of the electromagnetic spectrum (from about 700 nm to 2500 nm). Typical applications include pharmaceutical, medical diagnostics (including blood sugar and pulse oximetry), food and agrochemical quality control, and combustion research, as well as research in functional neuroimaging, sports medicine & science, elite sports training, ergonomics, rehabilitation, neonatal research, brain computer interface, urology (bladder contraction), and neurology (neurovascular coupling).
Nearinfrared spectroscopy is based on molecular overtone and combination vibrations.
Such transitions are forbidden by the selection rules of quantum mechanics. As a result, the molar absorptivity in the nearIR region is typically quite small. One advantage is that NIR can typically penetrate much farther into a sample than mid infrared radiation.
Nearinfrared spectroscopy is, therefore, not a particularly sensitive technique, but it can be very useful in probing bulk material with little or no sample preparation.
The molecular overtone and combination bands seen in the nearIR are typically very broad, leading to complex spectra; it can be difficult to assign specific features to specific chemical components. Multivariate (multiple variables) calibration techniques (e.g., principal components analysis, partial least squares, or artificial neural networks) are often employed to extract the desired chemical information. Careful development of a set of calibration samples and application of multivariate calibration techniques is essential for nearinfrared analytical methods.
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