Green and miniaturized ultrasonic-assisted extraction using natural deep eutectic solvents to extract phenolic compounds from tea samples

Natural deep eutectic solvents (NADES) have emerged as promising green solvents, particularly useful in sample treatment procedures. For the first time, different types of NADES were prepared and evaluated as potential phenolic extraction solvents from tea samples (Camellia sinensis). The NADES, composed of lactic acid:glycerol:water in a molar ratio of 1:1:3, provided the best results for the simultaneous extraction of the target phenolic compounds. This NADES was also characterized by using nuclear magnetic resonance (NMR), and the presence of nuclear Overhauser enhancement effects confirmed its supramolecular structure. The experimental conditions affecting the miniaturized ultrasonic-assisted extraction (UAE) were optimized to maximize extraction efficiency by performing a design of experiments. The proposed sample treatment was combined with ultra-high performance liquid-chromatography coupled to tandem mass spectrometry detection (UHPLC-MS/MS) to determine 21 phenolic compounds including catechin derivatives and other minor compounds. The UAE-UHPLC-MS/MS method was successfully characterized and applied to the analysis of various commercial green, black, white and red tea samples. In all cases, 18 phenolic compounds were determined with concentrations ranging from 0.3 to 29,125 mg kg⁻¹. The proposed sample treatment is a high-throughput, easy, fast and robust alternative to conventional procedures, which reduces organic solvent consumption and costs, aligned with green sample preparation principles.