Combined HPLC-DAD–MS, HPLC–MSn and NMR spectroscopy for quality control of plant extracts: The case of a commercial blend sold as dietary supplement

•HPLC-DAD–MS, HPLC–MSn and NMR analyses revealed the fraudulous content of a dietary supplement.•Rauwolfia sp. alkaloids (ajmaline, reserpine) were found.•Reserpine was in the therapeutic range and responsible for the collapses of the patients. The efficiency of 1D and 2D NMR spectroscopy along with HPLC-DAD–MS analyses in characterising the content of a dietary supplement is demonstrated. Experiments directly performed on a lyophilised sample of a commercial product gave details on the quality control of the product. The lack of the marker constituents of some of the declared plant species (Crataegus oxyacantha, Olea europea, Capsella bursa-pastoris and Fumaria officinalis) and the presence of banned adulterants, responsible for the strong antihypertensive effect of the supplement were established. The analyses proved the presence of indole alkaloids belonging to the group of Rauwolfia sp., such as ajmaline, reserpine and yohimbine. Quantitative HPLC analysis showed that the content of reserpine in the product was in the therapeutic range and therefore responsible for the collapses of the patients.