Near‐Infrared Spectroscopy in Natural Product Research

The properties of natural products are related to certain classes of compounds such as alkaloids, flavonoids, and essential oils. Traditionally, separation techniques including thin-layer chromatography (TLC), liquid chromatography (LC), gas chromatography (GC), and capillary electrophoresis (CE) even hyphenated to mass spectrometry (MS) were used for the elucidation and qualitative and quantitative analyses of individual compounds. In the food industry, spectroscopic investigations using infrared radiation have been used to monitor and evaluate the composition and quality since the early 1960s. During the last four decades, near-infrared (NIR) spectroscopy (800–2500 nm; 12,500–4000 cm−1) has become one of the most attractive and used methods for analysis for the following reasons: It represents a noninvasive analytical tool allowing a fast and simultaneous qualitative and quantitative characterization of natural products and their constituents. In this contribution, the principle, technique, and methodology are described followed by a discussion of quantitative and qualitative application possibilities. Finally, some hints to helpful regulatory issues and advantages and limitations are critically evaluated. Keywords: near-infrared spectroscopy; natural product; medicinal plant; transflection; diffuse reflection; multivariate analysis; cluster analysis