Ultrasound and salt-assisted liquid–liquid extraction as an efficient method for natural product extraction
A simple, rapid and efficient ultrasound and salt-assisted liquid–liquid extraction (USALLE) method coupled with high-performance liquid chromatography (HPLC) has been introduced for extraction, clean-up and pre-concentration of oleuropein from olive leaves as a model analyte. In this technique, the plant sample was transferred into the extraction solvent (consisting of phosphate buffer and miscible organic solvents) and the mixture was exposed to ultrasonic waves. After ultrasonic-assisted extraction (UAE), phase separation was performed by addition of salt to the liquid phase. During salt-assisted liquid–liquid extraction (SALLE) the analyte was transferred into the supernatant organic phase. Various parameters that affect the extraction efficiency such as ultrasonic time and temperature, sample amount, type and volume of miscible organic solvent, type and concentration of salt and pH were evaluated and optimized. The calibration curve shows good linearity (r2 = 0.9934) and precision (RSD < 5.5%) in the range of 2.5–50 μg mL−1. The limit of detection (LOD) and limit of quantitation were 0.5 and 2.5 μg mL−1, respectively. The recoveries were in the range of 90.0–97.0% with RSD values ranging from 4.0 to 6.5%. Unlike the conventional extraction methods for plant extracts no evaporation and re-solubilization operations were needed in the proposed technique.