## 3 Analytical Procedures Using Microextraction Techniques for the Liquid Phase
DI-SDME requires the use of a mixing organic solvent and analytes, which are characterised by higher solubility in the organic solvent than in the sample solution [11,13]. In the HS-SDME technique, gaseous analytes from the liquid phase, which are in the gaseous phase, dissolve in the solvent drop at the end of the microsyringe needle placed over the surface of the sample. After the extraction, lasting for a defined period of time, the microdrop is withdrawn back into the syringe needle and then it is injected to the detector or chromatograph for quantitative determination of analytes [11]. The methodological solutions described above should be treated as a static variant of this technique. Dynamic variants are also known and, in this case, a small liquid column which is in the syringe throughout the extraction is the drop [6]; the sample is then introduced into the syringe, where analytes are dissolved at the solvent phase. Figure 3: Diagram of a set for direct immersion single-drop microextraction (DI-SDME) and headspace single-drop microextraction (HSSDME). Based on [PERSON] _et al._[14]. Determining the different forms of mercury and tin in the water samples is troublesome due to the fact that the concentration of organometallic compounds in water is relatively low. Therefore, there is more and more information on the possibility of using the liquid-liquid-liquid microextraction technique (LLLE) to enrich analytes and purify extracts for the analysis of organomercury and compounds in water samples and other environmental samples. The LLLIME technique uses three solutions--the donor solution, the organic solvent phase, which constitutes a specific organic membrane separating the two aqueous phases, and the acceptor solution [15, 16, 17, 18]. In general, the principle of operation in this system can be presented in the following way: analytes are extracted from the starting solution to the organic solvent phase and then it is extracted again to drops of the acceptor aqueous solution suspended in an organic solvent, usually at the end of a syringe needle. A diagram of a set for performing tests using this technique is presented in Figure 4. (In this system, the analytes are extracted from the donor solution into the organic solvent phase and back-extracted simultaneously into the acceptor phase while stirring) [15, 17, 18, 19]. The liquid-liquid-liquid microextraction is becoming more and more popular as a technique for analyte enrichment and purification of samples containing organometallic compounds prior to analysis, in particular using capillary electrophoresis [15]. Literature contains information about numerous modifications which have been introduced into the LLLME system over the past several years. For example, the use of hollow-fibre liquid-liquid-liquid microextraction for the determination of organomercury compounds [16, 20, 21, 22]. Figure 4: A diagram of the set for liquid–liquid–liquid microextraction. Based on [PERSON] _et al._[14]. For instance various techniques of simple microextraction to the liquid phase were improved to eliminate some defects of the traditional installations. In 2000, the principles of the continuous-flow microextraction technique (CFME) were first described. In this technique, a drop of extractant solvent is introduced using a microsyringe into the extraction chamber so as to place it on the outlet tip of the PEEK tube. This tube is used as a kind of \"holder\" for a solvent drop and for filling the extraction chamber by pumping the sample through it in a continuous manner at a constant flow rate. As a result, the solvent drop placed at the end of the tube has continuous contact with the sample solution. At the end of the extraction process, the extract is collected using a microsyringe [23]. Figure 5 presents a diagram of the construction of the set for tests using the CFME technique. The dispersive liquid-liquid microextraction (DLLME) is a relatively \"young\" extraction technique, developed in 2006 [3].