Liquid-phase-based extraction techniques are based on the diffusion of analytes from the liquid matrix (donor phase) to the acceptor liquid phase. Although they are commonly used for the pretreatment of liquid samples, such as liquid–liquid extraction (LLE) and LPME, their applications to online SP-LC systems are much less than those of solid-phase-based extraction techniques. A membrane is usually used as the online phase separator, which serves as a physical selective barrier between phases. Assisted with the membrane, the acceptor phase can easily flow through (or flow along) the liquid sample and transfer the extracts into LC system, facilitating the online coupling. The current membrane based
liquid-phase-based extraction techniques available for online coupling with LC include hollow fiber liquid phase microextraction
(HF-LPME), dialysis and MD.