Sampling for Near-Infrared Spectrometry

Another type of transflectance spectroscopy that is commonly used for measurements of near-infrared spectra falls somewhere between specular reflection and diffuse reflection spectroscopy (see Chapter 16). Because overtone and combination bands are weak, anomalous dispersion is usually negligibly small in the near infrared. It is quite common for the NIR spectra of liquids to be measured by transflection using the same optical geometry used for diffuse reflection. In this case, the sample is loaded in a cell whose rear face is a ceramic disk. Radiation that has passed through the sample is scattered back through the sample, emerges from the window, and is focused onto the detector. This is a particularly useful way of measuring the spectrum of turbid samples, such as milk. 

In summary, the measurement of specular reflection spectra allows a very wide variety of samples, from partial monolayers on metallic and dielectric substrates to bulk liquids and polymer films, to be characterized. 

Golden, Fourier transform infrared reflection-absorption spectroscopy, in Fourier Transform Infrared Spectroscopy Applications to Chemical Systems, J. R. Ferraro and L. J. Basile, Eds., Academic Press, Orlando, IL, 1985, Vol. 4, p. 315. 

Milosevic, Harrick Scientific Corporation, personal communication to P, R. Griffiths (2003). 

Born and E. Wolf Principles of Optics Electromagnetic Theory of Propagation, Interference and Diffraction of Light, 6th ed., Cambridge University Press, Cambridge, 1998. 

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