Above the integrating sphere

Thermo Scientific Antaris™ FT-NIR analyzers offer diffuse reflectance capabilities. Many important application areas for Fourier transform near-infrared (FT-NIR) analysis involve the acquisition of diffuse reflectance spectra directly from powdered or solid samples. The diffuse reflectance module of the Antaris system is based on an optimized internal integrating sphere concept.

The choice of resolution may not be important for samples with broad peaks, but many of the active pharmaceutical materials have spectra with sharp peaks. It is important to remember that while higher resolution may improve the specificity of the analysis with an FT-NIR system, the spectral noise also increases with improved resolution. For many applications, a resolution of 4cm-1 or 8 cm-1 provides the best compromise between sensitivity and specificity. Four major performance areas having significant effects on an analysis are 1) wavelength accuracy, 2) instrument sensitivity, 3) instrument stability and 4) spectral resolution. In this report, we will describe a series of tests performed on the Antaris Solid Sampling System to verify key performance features that are important in a number of reflectance applications.

The analyzer has been designed to provide a large, open area around the protective sapphire window so special accessories or large samples can be easily positioned at the focal point above the integrating sphere. A major component of the diffuse reflectance sampling system is the gold-coated integrating sphere that collects the reflected energy from a sample placed on the integrating sphere window. An important feature of this design is the computer-controlled diffuse gold reference reflector, which can be automatically positioned in front of the window to acquire a background spectrum. This internal positioning not only eliminates damage to the reference but also allows background spectra to be acquired without moving the sample.

We developed the Antaris FT NIR System (Figure 1) to provide an easy way to rapidly analyze solid and powdered materials. Because the major reason for the increased popularity of FT-NIR spectroscopy is the large number of samples that can be rapidly analyzed or identified with little or no sample preparation. High-quality spectra can be acquired directly from solid samples and from powders in glass or plastic containers by diffuse reflectance spectroscopy.