NIR Technology

In NIR spectroscopy, the unknown substance is illuminated with a broad-spectrum (many wavelengths or frequencies) of near infrared light, which can be absorbed, transmitted, reflected or scattered by the sample of interest. The illumination is typically in the wavelength range of 0.8 to 2.5 microns (800 to 2500nm). The light intensity as a function of wavelength is measured before and after interacting with the sample, and the diffuse reflectance, a combination of absorbance and scattering, caused by the sample is calculated.

Light is absorbed in varying amount by the sample at particular frequencies corresponding to the combinations and overtones of vibrational frequencies of some bonds of the molecules in the sample. Specifically, the bond vibrations between oxygen and hydrogen (OH), carbon and hydrogen (CH), and nitrogen and hydrogen (NH) result in NIR absorbance bands.

The bands seen in the NIR are typically very broad, leading to spectra that are more complex to interpret than FTIR spectra; it can be difficult to assign specific features to specific chemical components. Careful development of a set of calibration samples and application of multivariate calibration techniques is essential for near-infrared analytical methods.

NIR can typically penetrate much further into a sample than FTIR, and unlike Raman, is not affected by fluorescence. Thus, although NIR spectroscopy is not as chemically specific as Raman or FTIR, it can be very useful in probing bulk material with little or no sample preparation.

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