Qualitative characterization of Desmodium adscendens constituents by high-performance liquid chromatography-diode array ultraviolet-electrospray ionization multistage mass spectrometry

The many effects of the African medicinal herb Desmodium adscendens were studied in the 1980s and 1990s. In spite of this, a comprehensive analytical protocol for the quality control of its constituents (soyasaponins, alkaloids and flavonoids) has not yet been formulated and reported. This study deals with the optimization of extraction conditions from the plant and qualitative identification of the constituents by HPLC–diode array UV and multistage mass spectrometry. Plant constituents were extracted from leaves by liquid–liquid and solid matrix dispersion extraction. Separation was achieved via RP-C18 liquid chromatography with UV and MSn detection and mass spectrometry analysis was conducted by electrospray ionization ion trap or orbitrap mass spectrometry. High resolution mass spectrometry (HRMS) was used for structural identification of active molecules relating to soyasaponins and alkaloids. The flavonoid fragmentations were preliminarily studied by HRMS in order to accurately characterize the more common neutral losses. However, the high number of isomeric species induced us to make recourse to a more extended chromatographic separation in order to enable useful tandem mass spectrometry and ultraviolet spectral interpretation to propose a reasonable chemical classification of these polyphenols. 35 compounds of this class were identified herein with respect to the five reported in literature. In this way we made up a comprehensive protocol for the qualitative analysis of the high complexity content of this plant. This result paves the way for both reliable quality control of potential phytochemical medicaments and possible future systematic clinical studies.