Ionic Liquid-Based Ultrasonic/Microwave-Assisted Extraction Combined with UPLC for the Determination of Anthraquinones in Rhubarb

Ionic liquid-based ultrasonic/microwave-assisted extraction (IL-UMAE) of five anthraquinones (physcion, chrysophanol, emodin, rhein, and aloe-emodin) from rhubarb was first studied. Several parameters of UMAE were optimized, and the results were compared with of the heat-reflux extraction (HRE), ultrasound, and microwave-assisted extraction (MAE and UAE). The optimal UMAE conditions were as follows: the solvent was 2.0 mol L−1 1-butyl-3-methylimidazolium bromide [bmim]Br solution, the ration of solid/liquid (g mL−1) was 1:15, time was 2 min, and microwave power was 500 W. Under these UMAE conditions, total content of five anthraquinones was 28.00 mg g−1. Compared with the conventional HRE, regular MAE and UAE techniques, the proposed approach exhibited higher efficiency (18.90–24.40% enhanced) and shorter extraction time (from 6 h to 2 min). The anthraquinones were then determined by ultra performance liquid chromatography (UPLC). Based on optimized conditions, contents of physcion, chrysophanol, emodin, rhein and aloe-emodin in rhubarb collected from different cultivated areas were 0.68–2.99, 5.03–15.40, 0.48–4.34, 0.025–3.93 and 0.26–2.56 mg g−1, respectively. This study suggests that IL-UMAE was an efficient, rapid, simple and green preparation technique.