3.1. Development of UA-LDS-DLLE procedure
It is well-known that ultrasound energy could cause cavitation bubbles resulting in higher temperatures and pressures in the cavitation zone, which generated physical disruption of sample matrix including mechanical erosion of solids and colloid particles rupture. The solvent would easily penetrate into the interior of substances, which facilitate mass transfer of analytes from matrix into the solvent, usually resulting to a good yield [38,39]. Heating would cause a dramatic volume expansion with subsequent structure rupture of substances, releasing the analytes into the extraction solvent. The vortex agitation was used to swirl the fluids and create a vortex, which could enable the breakup of organic phase into fine droplets and the formation of a cloudy state, resulted to the rapid extraction of analyte in aqueous sample. The extraction equilibrium could be attained within 2 min agitation time applied when the rotation speed was greater than 1500 rpm [40]. To test the performance of the synergistic effect, UA-LDS- DLLE and acetonitrile–water dispersive liquid–liquid extraction (DLLE) were comparatively studied in this study. The mean recoveries of UA-LDS-DLLE and DLLE at 100 mg kg−1 for each analyte were in the range of 86.1–104.5% and 65.1–80.5% with RSDS (n1⁄43) less than 5.6% and 7.1%. So a synergistic effect is more powerful than a single energy. In addition, the extraction selectiv- ity could be adjusted by choosing a preferential solvent, control- ling the concentration or pH of acid solution and the suitable ratio of sample quantity to organic solvent volume.