Diffuse reflectance near-infrared spectra

Fig. 3 Diffuse reflectance near-infrared spectra of several pharmaceutical compounds (A) Comparison of the spectra of three different monosaccharides measured as crystalline powders (for clarity, the spectra of fructose and galactose are offset spectral resolution 32cm ). (B) Comparison of spectra of pure caffeine and paracetamol (offset, spectral resolution 32 cm ).

The scope of the present chapter will be exclusively concerned with investigations of diffuse reflectance work performed in the UV/VIS region of the spectrum, and with colors that can be perceived by the human eye. Much work has been conducted in the near-infrared region of the spectrum, but that aspect will be covered elsewhere in this book. 

Fig. 2 Comparison of near-infrared spectra of crystalline anhydrous glucose. Spectrum measured in transmittance using the KBr pellet technique [spectral resolution 64cm intensity data in absorbance (top trace) and diffuse reflectance spectrum using a spectral resolution of 32 cm and intensity data transformed to —log R) (bottom trace)].

Because a diffuse reflectance spectrum is scatter-dependent, information on mean particle sizes is also obtainable, which is a parameter of great importance in powder technology. An approach using near-infrared spectrometry combined with multivariate calibration has been presented by Ilary, Martens, and Isaksson. Included was a spectrum standardization by multiplicative scatter correction (see later). 

G J. Kemeny and D. L. Wetzel, Moisture Study of a Lively Near-Infrared Diffuse Reflectance Spectrum, Paper 117, FACSS 13 th Annual Meeting, September 28 to Octobers, 1986, SL Lonis, MO. 

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. 

---