## 3 Solvent Microextraction  
Solvent microextraction (SME) is a technique for sample preparation involving the extraction and concentration of liquid, gaseous and solid samples with solvent volumes in the \(\mu\)L or sub-\(\mu\)L range, thereby enabling high enrichment factors. The term liquid-phase microextraction (LPME) is also frequently used to describe this process [43]. This rapid inexpensive preparation technique uses minimal solvent volumes with negligible exposure to toxic organic solvents. LPME is normally performed using a small volume of a water-immiscible solvent and an aqueous phase containing the analytes of interest. From the introduction of the first paper on SME in 1996 [44] until now, different approaches have been developed in two broad categories: exposed solvent and membrane-protected solvent [45]. Exposed solvent techniques include single-drop microextraction (SDME), headspace single drop microextraction (HS-SDME), liquid-liquid microextraction (LLME, which is also called directly suspended droplet microextraction, DSDME), liquid-liquid-liquid microextraction (LLLME) and dispersive liquid-liquid microextraction (DLLME) [43,45] as shown as in Figure 2.  
Single Drop Microextraction (SDME) is a miniature liquid-liquid extraction: a drop of water immiscible organic extracting solvent (approximately 1-10 \(\mu\)L) is suspended from a syringe into the liquid or gaseous sample medium. After extraction, the liquid extractant is drawn back into the microsyringe and used directly to determine the analytes via GC. SDME is not exhaustive, and only a small fraction of the analyte is extracted and pre-concentrated for analysis [43]. Headspace (HS-SDME) enables the extraction and pre-concentration of volatile or semi-volatile compounds into a microdrop exposed to the headspace above the sample. The drop remains at the tip of the microsyringe throughout the extraction period before being retracted back into the microsyringe. In this mode, the analytes are distributed between three phases: the water sample, headspace and organic drop. HS-SDME can achieve a high degree of extract clean-up because non-volatile compounds and high-molecular-weight species are not extracted [43,45]. In all cases, GC is used to determine the target analytes. The major disadvantages of both techniques are the susceptibility of the drop toward dislodging during sampling, the size limitations of the drop and the volatility of the extraction solvent [45]. To resolve these drawbacks, air is deliberately introduced with the solvent drop, leading to a larger solvent surface area. The bubble also tends to support high-density solvents (e.g., CHCl\({}_{3}\)), which tend  
Figure 2: Scheme of some solvent microextraction techniques. to dislodge due to their weight [46]; otherwise, water-insoluble ionic liquids (ILs) are used. The latter are alternatives to organic solvents due to their high viscosity and surface tension, which helps the formation of a stable drop with a much larger volume [47]. However, the instability of the ionic liquid drop at the end of the needle remains the most significant limitation of SDME when coupling this technique to high-performance liquid chromatography (HPLC). Small solvent volumes (microliter level) are not sufficient for performing highly sensitive liquid chromatography determinations. Another alternative to organic solvents are the supramolecular assembly-based coacervates (e.g., surfactant micelles) that have been applied during analytical techniques to extract various organic compounds before their separation by LC. A vesicular-based coacervate was prepared by mixing decanoic acid in tetrabutyl ammonium hydroxide and distilled water and was used as the solvent in SDME. This technique, which is called single-drop coacervative microextraction (SDCME) [48], has been applied to extract chlorophenols from wastewater, superficial water from a reservoir and groundwater before liquid chromatography determination.