Determination of anthraquinones in Frangula alnus by Supercritical Fluid Chromatography

Anthraquinones are the major bioactive compounds in alder buckthorn, a plant with a long history of use for the treatment of obstipation in European and Asian traditional medicine. As these metabolites are also associated with toxic side effects, reliable analytical techniques for their determination are essential. Previously described methods mainly utilized Liquid Chromatography for this purpose, as the photometric assay described in the current edition of the European Pharmacopoeia lacks selectivity. In this study, the development and application of an alternative approach, Ultra-High Performance Supercritical Fluid Chromatography, is presented, facilitating the qualitative and quantitative determination of seven anthraquinone derivatives in under six minutes. This was possible by using a Torus™ DIOL column and a modifier comprising methanol with 10 % acetonitrile and 2.5 mM oxalic acid; applying a flow rate of 1.6 mL/min and an elevated temperature of 55 °C enabled optimal resolution. Regarding selectivity, linearity (R² ≥ 0.9995), precision and accuracy (recovery rates between 96.6 and 103.3 %), all results were in line with the respective ICH requirements. Practical applicability of the assay was confirmed by analyzing different samples, including commercially available plant material, phytopharmaceuticals containing Frangula bark and wild collected samples. Except of the latter, glucofrangulin A always was the dominating compound; however, the overall anthraquinone content showed to be very variable ranging from 2.18 to 51.32 mg.