Near-Infrared Spectroscopy in Process Analysis
Process Instrumental Methods
Published Online: 15 SEP 2006
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Goldman, D. S. 2006. Near-Infrared Spectroscopy in Process Analysis. Encyclopedia of Analytical Chemistry.
- Published Online: 15 SEP 2006
Near infrared (NIR) is a spectroscopic method based upon the absorption of light in the wavelength region between 700 nm and 2500 nm due to vibrations of molecular functional groups in the sample. These spectral features arise from overtones and combinations of the fundamental molecular vibrations of these groups that occur in the infrared (IR) at longer wavelengths. NIR is rich with features arising from vibrations of CH, OH and NH.
NIR spectra are acquired by determining the absorbance, A, of light over a continuous range of wavelengths in spectrophotometers or at several specific wavelengths in fixed-filter photometers. Absorbance is defined by Equation (1):
where I is the signal from the sample relative to the signal from a reference, Io. The reference can be an empty transmission sample cell for liquids, or it can be a broadband spectral reflector, such as a white ceramic, for reflectance measurements of solids.
Absorbance is usually linearly related to concentration, following Beer's Law (Equation 2):
where ɛ represents the molar extinction coefficient and C is the concentration of the species, and l is the optical pathlength of light through the sample. For a particular species and fixed experimental set-up, ɛ and l are constants. Letting a constant, k = (ɛl)−1, then (Equation 3):
A principal advantage of process NIR over IR is the ability to use fiber optics to transmit light between the instrument and the sample over considerable distance. This permits the instrument, a potential source of ignition, to be placed in safe areas where no flammable vapors exist. Only the fiber optic probe in contact with the sample and the optical fibers that connect to the instrument need be placed in the hazardous location. A disadvantage of process NIR is the characteristics of the spectra, which are typically comprised of broad, overlapping peaks in comparison to IR spectra. This often requires the use of sophisticated data analysis methods that place additional demands upon vendors to provide stable NIR instruments and upon manufacturers to have highly-trained staff, often at the PhD level, for calibration model development, implementation and maintenance.