## 3 Solid-Phase Microextraction SPME  
SPME microextraction is one of the static methods for isolating and concentrating analytes from solid, liquid, and gaseous samples. It involves the sorption of an analyte in a very thin layer of stationary phase deposited on the surface of a glass or quartz fiber. It was first applied in an analytical procedure in 1990 by [PERSON] [11]. It is now widely used in analytical laboratories in the preparation and testing of environmental [11], food, clinical, and pharmaceutical samples [12] primarily considering:  
1. Complete elimination of organic solvents from the analytical workflow;
2. Short time of the analyte extraction step;
3. Simplicity and speed of performing the analysis;
4. The possibility of sampling in situ and in vivo systems [13];
5. High sensitivity of substance determination at the ppt level;
6. An ability to automate the analytical procedure [14];
7. The possibility of desorption of analytes directly in the dispenser of the measuring device.  
Since 1993, commercial devices have been produced that allow microextraction to be carried out using the SPME technique. They contain a quartz fiber coated either with a polymer film or a polymer layer with a solid sorbent. The fiber is placed in a thin steel tube fixed to a micro-syringe-like holder. When the piston is pressed, the fiber slides out of the steel needle (Figure 4).  
The adsorption process takes place on the surface of a fiber that is placed in a vial with the test sample. After the adsorption has been completed, the fiber is retracted into the steel needle of the micro syringe. The adsorbed analytes are thermally desorbed in the dispenser chamber of a gas chromatograph, from which they, together with the carrier gas,  
Figure 4: Commercial SPME apparatus construction diagram.  
Figure 3: Solid-phase microextraction with sample flow: (**a**) in tube SPME, (**b**) solid-phase dynamic extraction—SPDE, (**c**) microextraction in packed syringe—MEPS, (**d**) polymer monolith microextraction—PMME, (**e**) fiber-packed needle microextraction—FNME, (**f**) microextraction “in needle”, (**g**) sorbent-packed in-tip SPME, (**h**) fiber-packed in-tip SPME.  
are transferred to the crucial chromatography column. Here, the analytes are separated, which is a key step in the process, and, in the chromatograph detector, they are detected.  
Depending on how the sorptive phase is placed about the sample, the SPME technique can be carried out as follows:  
1. In a direct immersion (DI) manner;
2. As adsorption from a headspace (HS) phase;
3. As adsorption with a protective membrane (Figure 5) [15].  
In direct DI-SPME microextraction (Figure 5a), the fiber is immersed in the analyzed solution. As a result, analytes are transferred directly from the sample matrix to the sorptive phase of the fiber. In HS-SPME microextraction, relatively volatile analytes that have passed from the liquid phase to the gas phase above the solution surface are adsorbed. During extraction from the headspace (Figure 5b), the fiber is not in direct contact with the solution and, as a result, is not exposed to damage that can be caused by non-volatile impurities present in the sample matrix. However, microextraction with a protective membrane is used for the microextraction of non-volatile compounds from solutions containing interferets with high molecular weights, such as humic acids or proteins (Figure 5c) [16].  
The type of organic compound requires selecting an appropriate fiber-coating material to be sorbed since the microextraction efficiency is determined by the affinity of the analytes to the stationary phase. A commonly used group of polymers as extraction phases are silicones, characterized by high thermal and chemical stability and ease of chemical modification. Therefore, these materials enable the selectivity of the sorptive phase to be changed. Extraction fibers coated with polydimethylsiloxane (PDMS), divinylbenzene (DVB), polyacrylate, carboxene (CAR), and polyethylene glycol (PEG/CW) [17] are commercially available.