Non-invasive infrared spectroscopic techniques for the characterization of medicinal plants and their constituents

Near- and mid-infrared spectroscopy (NIRS; 10.000–4.000cm^-1; MIRS; 4.000–400cm^-1) are non-invasive spectroscopic tools enabling a fast qualitative and quantitative characterization of medicinal plants and their constituents down to the ppm-level. Treatment of spectra recorded with chemometrical and multivariate approaches allows determining chemical (e.g. secondary plant metabolites, leading compounds) and physical parameters (e.g. water, alcohol content) simultaneously by one single measurement lasting only a few seconds.

Liquid plant extracts are investigated in the transflection mode at thermostated conditions using light-fibre optics, dried parts of plant (flowers, leaves, roots) also in the reflection mode using a sample desk. For the quantitative analysis of secondary metabolites including 3',4',5'-trimethoxyflavone in Flos Primulae veris, hypericin and hyperforin in St. John's Wort, etheric oils in Achillea species, a reference method based on liquid chromatography (LC), gas chromatography (GC) or capillary electrophoresis (CE) is applied. Qualitative cluster analysis not only allows identifying different parts of a plant but also enables to distinguish different species, which is essential also in traditional Chinese medicine (TCM). For the investigation of active ingredients distribution within a plant part imaging spectroscopy with a resolution down to 5µm combined with hierarchical cluster analysis (HCA) offers an immense potential as a novel screening tool.

In the present contribution the main advantages of the novel quality control IRS tool in medicinal plant analysis are pointed out and discussed in detail by several applications.