Near-Infrared Spectroscopy in Laboratory and Process Analysis
Process Instrumental Methods
Published Online: 17 DEC 2012
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Alcalà, M., Blanco, M., Menezes, J. C., Felizardo, P. M., Garrido, A., Pérez, D., Zamora, E., Pasquini, C. and Romañach, R. J. 2012. Near-Infrared Spectroscopy in Laboratory and Process Analysis. Encyclopedia of Analytical Chemistry.
- Published Online: 17 DEC 2012
Near-infrared (NIR) is a spectroscopic method based on the absorption of light in the wavelength region between 700 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 predominantly from vibrations of C–H, O–H, and N–H.(1)
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 the following equation:
where I is the signal intensity from the sample relative to the signal intensity from a reference, I0.
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, 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 setup, ɛ and l are constants. Letting a constant, k = (ɛl)−1, then (Equation 3)
A principal advantage of NIR over mid-IR for process applications is the ability to use fiber optics to transmit light between the instrument and the sample over considerable distance with NIR. This permits the instrument, a potential source of ignition, to be placed in safe areas where 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 NIR is the characteristics of the spectra, which are typically composed of broad, overlapping peaks in comparison to IR spectra. This often requires the use of sophisticated data analysis methods that place additional demands on vendors to provide stable NIR instruments and upon manufacturers and end-users to have highly trained staff, often at the PhD level, for calibration model development, implementation, and maintenance.