## 3 Analytical Procedures Using Microextraction Techniques for the Liquid Phase
This technique was initially used for isolating and enriching organic compounds, such as polycyclic aromatic hydrocarbons (PAH), organophosphate pesticides and chlorobenzenes [24]. It was also used in procedures for determining organotin compounds and other inorganic compounds. The DLLME technique is based on the use of a triple solvent system, just like in homogeneous liquid-liquid extraction (LLE) and cloud point extraction (CPE). Dispersive liquid-liquid microextraction consists of two stages (Figure 6):  
1. The introduction of an appropriate extraction and dispersing solvent mixture into an aqueous solution of an analyte-containing sample. The quantity of the extraction solvent used is usually approx. 1%-3% of the total mixture volume of various extraction solvents. At this stage, the extraction solvent is dispersed in the aqueous sample in the form of fine drops, in which analytes are enriched. At this stage, the solution becomes  
Figure 5: A diagram of a set for continuous flow microextraction [23]. cloudy [13]. The state of balance is achieved quickly due to the large surface area between the extraction solvent and the aqueous sample solvent so that the extraction hardly depends on time. * Centrifugation of the cloudy solution. After the completion of the microextraction process, centrifugation is a necessary step to separate the extracting agent phase. The enriched extracting agent phase is used to determine the required analytes using conventional analytical techniques [6,13]. The organic solvent is used on the basis of its density, which should be higher than the density of water, the extraction ability towards the analytes and compatibility with the chromatographic system used. Usually, these are: chlorobenzene, chloroform, carbon tetrachloride or tetrachloroethylene. The dispersing solvent must mix with water and the polar solvent. Acetone, methanol and acetonitrile are usually used as dispersing solvents [11,13]. At present, this technique is mostly used for analysing simple water samples and for preliminary tests. An additional purification stage would be required for samples characterised by a complex matrix composition [11,13]. Single-drop microextraction, apart from numerous significant advantages, has a disadvantage connected with the risk of destroying the drop. As a result, by modifying microextraction techniques to the liquid phase, changes were introduced, which involved immobilisation of the extracting liquid in a porous fibre drain, which was used in Hollow-fibre liquid-phase microextraction (HF-LPME) [12]. Figure 6: A diagram presenting consecutive stages of dispersive liquid–liquid microextraction. Based on [PERSON], F. _et al._[6]. This is extraction in the liquid-liquid system, where the extracting liquid is situated in the spaces of a porous fibre fixed at the tips of two needles (Figure 7, right-hand side) or at the end of one microsyringe needle (Figure 7, left hand side). Before the commencement of the extraction process, the capillary fibre is immersed in an organic solvent to \"keep\" the organic solvent in the pores of the capillary fibre. The capillary fibre is then immersed in the sample solution. To accelerate the extraction, the sample is intensively shaken or mixed. After the completed extraction, the extract is drawn into a syringe from the capillary tube. This technique gives the possibility of ensuring high repeatability and high selectivity of the extraction process due to the possibility of using a broad spectrum of appropriate extracting liquid and the type of porous fibre. Apart from protecting the extracting liquid, the fine pores of the fibre prevent weight materials from getting into it, which is desirable especially in the analysis of biological liquids. The extraction process using the HF-LPME technique can be performed both in two-phase and three-phase systems [25]. In two-phase systems, the acceptor solution is the same organic solution which was immobilised in the pores, while analytes are collected in the organic phase, which is compatible with the GC [26].