Recent advances in green solvents-based liquid-phase microextraction techniques for chromatographic analysis of active components in traditional Chinese medicine

Traditional Chinese medicine (TCM) is a treasure of China and a crucial part of traditional medicine in the world, particularly in many oriental countries. TCM is the core and basis of traditional medicine in clinical practice for numerous diseases, and performs important function in nutraceuticals and dietary supplements. However, it is extremely difficult to extract each active ingredient from TCM to elucidate the mechanism of TCM clinical efficacy due to numerous compounds in TCM, especially trace compounds. Consequently, liquid-phase microextraction (LPME) techniques, one of the main extraction methods of active ingredients in TCM, have attracted a considerable attention from researchers. In recent years, many novel green solvents based-LPME methods have been reported, such as single-drop microextraction (SDME), hollow-fiber liquid-phase microextraction (HF-LPME), dispersive liquid-liquid microextraction (DLLME), and electro-membrane extraction (EME). Therefore, in this review, we present an up-to-date and comprehensive summary of various LPME techniques, novel green solvents, and their applications in the analysis of active ingredients within the complex TCM samples. We provide a detailed overview of the fundamental principles, modes, and the critical process parameters of the LPME techniques. In addition, we compare different types of green solvents (i.e., deep eutectic solvents, ionic liquids, magnetic ionic liquids, supramolecular solvents, switchable solvents, among others), and the advantages and disadvantages of these solvents are critically evaluated, highlighting their suitability for various applications. Finally, we elucidate the merits and demerits of different LPME methods, discuss their practical applications, and explore their future research directions. This review aims to provide a valuable resource for researchers and practitioners in the field of TCM, promoting research development and application of the advanced and environmentally friendly sample pretreatment techniques.