## 3 Solvent Microextraction
During liquid-liquid microextraction (LLME), which is also called directly suspended droplet microextraction (DSDME), 10-100 \(\upmu\)L of an organic solvent is added to the centre of the stirring vortex of an aqueous sample. The direct interface between the solvent and water rapidly extracts and concentrates the analytes in the organic solvent, which is subsequently removed with a capillary tube or syringe and injected into a chromatographic system for analysis. Liquid-liquid-liquid microextraction (LLLME) is similar to LLME. This two-step process first extracts an ionisable solute into an organic layer before extraction and trapping it in a second aqueous layer with a pH capable of ionising the solute. Typically, this technique is used to extract acidic, basic or polar analytes from water into an acidic (for basic analytes) or basic (for acidic analytes) acceptor solution [45]. For example, [PERSON] _et al._[49] utilised LLLME to extract long-chain alkylphenols, such as 4-\(t\)-butylphenol, 4-\(t\)-octylphenol, 4-\(n\)-nonylphenol and bisphenol-A, from water. Dispersive liquid-liquid microextraction (DLLME) is a simple and rapid microextraction method that uses \(\upmu\)L volumes of a dense organic solvent with a few mL of dispersive solvent. A cloudy solution is formed when the appropriate mixture of extraction and dispersive solvents is injected into an aqueous sample containing the analytes of interest. After centrifuging the cloudy solution, the phase at the bottom of a conical tube is recovered and analysed by GC [43,45]. This microextraction technique is useful for non-polar analytes; it generally requires a halogenated solvent (tetrachloroethylene or carbon tetrachloride) and a water-soluble co-solvent (methanol (MeOH), acetone or acetonitrile (ACN)) that increases the solubility of the extraction solvent in water. Solvents with a lower density than water (octanol, toluene) have also been applied in DLLME [50, 51, 52]. Too, a mixture of two water-immiscible solvents (polar and non-polar) and auxiliary solvent was used for the extraction of ion-pair complexes from water samples in order to ensure that the resulting mixture has a density higher than that of water, [50]. In recent years, interest in DLLME has focused on using low-toxicity solvents with convenient and practical procedures. In this context, [PERSON] _et al._ developed a new mode of IL-DLLME based on hydrophilic and hydrophobic ionic liquids to determine two acidic phenolic compounds (2-naphth and 4-nitrophenol) in environmental water [53]. Membrane-protected solvent techniques include hollow-fibre-protected 2-phase microextraction [HF(2)ME], which is often called LPME or hollow-fibre LPME in the literature [45]; during this procedure, a porous polypropylene hollow fibre contains the extraction solvent within its pores and lumen. The organic solvent forms a thin layer within the wall of the hollow fibre, and the fibre is then inserted into a sample vial filled with the aqueous sample of interest. The analytes are extracted from the aqueous sample through the organic phase into the pores of the hollow-fibre before entering the acceptor solution inside the lumen [43]. Hollow-fibre-protected phase microextraction mode is HF(3)ME, often referred to as again LPME or 3-phase LPME. In this case, a water-insoluble, non-polar solvent saturates the wall of the fibre, whereas the lumen contains an acid or base; this system irreversibly extracts the analytes [45]. [PERSON] _et al._ proposed a three-phase hollow-fibre liquid-phase microextraction combined with a LC method using diode array detection (DAD) to determine six sulfonylurea herbicides, specifically triasulfuron, metsulfuron-methyl, chlorosulfuron, flazasulfuron, chlorimuron-ethyl, and primisulfuron-methyl, in environmental water samples [54]. Similar to other microextraction techniques, hollow fibres can be modified. A technique employing ionic liquids during HF-LPME is hollow fibre membrane liquid-liquid-liquid microextraction (HFM-LLLME) [55].