Analysis of anthraquinones in rhubarb (Rheum palmatum and Rheum officinale) by supercritical fluid chromatography.

The first report on the separation of five anthraquinones (chrysophanol, physcion, emodin, aloeemodin, and rhein) from rhubarb by supercritical fluid chromatography indicates that this technique is an interesting analytical alternative not just for non-polar substances. Within less than five minutes the compounds could be baseline resolved, using a mobile phase comprising supercritical carbon dioxide and methanol with 0.05% diethylamine. The optimum stationary phase showed to be an Acquity UPC(2) HSS C18 SB 1.8 µm column, operated at a flow rate of 2 ml/min and a temperature of 30 °C. Method validation confirmed that the developed procedure is selective, linear (R(2)≥0.999), accurate (recovery rates: 95.4% to 103.1%), and precise (intra-day≤6.9%, inter-day≤4.7%); the limit of detection was below 0.5 ng on-column. The analysis of plant extracts was feasible with acceptable repeatability (σrel≤3.8%), and it determined 0.3 to 0.7% of free aglyca in the native samples. After hydrolysis according to the European Pharmacopoeia, a rise in the total content up to 2.1% was observed, with rhein being the most dominant derivative in nearly all specimens.