## 3 SPME Process
### Fiber Coatings  
The choice of the fiber coating mainly depends on the nature of the analytes. Before a sample is analyzed, the fiber should be cleaned in order to remove contaminants that might increase background noise in the chromatogram [65]. Frequently used commercially available fiber coatings for food and environmental analysis are polydimethylsiloxane (PDMS), polyacrylate (PA), Carboxen (CAR; a carbon molecular sieve), divinylbenzene (DVB), and Carbowax (CW; polyethylene glycol). These fibers are available in various coating combinations, blends or copolymers, film thicknesses, and fiber assemblies. The variety is constantly growing increasing the number of applications [11]. The most widespread examples of absorption or liquid fiber coatings are PDMS and PA. Mixed coatings in which the primary extraction phase is a porous solid are also available [100]. CAR/PDMS and DVB/CAR/PDMS fibers provide the best extraction efficiencies for a wide range of analytes with different polarities and molecular weights [65]. PDMS has the ability to withstand high temperatures up to 300 \({}^{\circ}\)C and shows great stability [65]. PDMS fibers are better suited to the analysis of nonpolar analytes, whereas PA fibers are more useful to extract polar analytes [65]. DVB is also a polar porous solid coating and therefore efficient in extracting polar compounds such as disulfides and trisulfides [101]. Bipolar compounds like alcohols, aldehydes, ketones, ethers and carboxylic acids are often most efficiently extracted by fibers consisting of a combination of e.g., non-polar material (PDMS) and a polar material (DVB) [65]. Another appropriate choice to extract analytes with different polarities are coating mixtures consisting of CARB/PDMS or DVB/CARB/PDMS [65]. However, a disadvantage of this coating material is the displacement effect of analytes with a lower affinity to the coating [102]. Apart from commercially-available sorbents, different new coating procedures have been applied to expand coating types in the commercially applicable and reproducible SPME devices. These new approaches also address biocompatibility, on-site compatibility, selectivity and sensitivity limitations [103, 104]. One can differentiate coating procedures based on the sorbent type into dipping and physical agglutinating methods, sol-gel technology, chemical grafting, electrochemical methods, electrospinning, liquid-phase deposition and the hydrothermal method [24]. Further new coatings are polypyrrole polymer coatings [105], molecularly imprinted polymers (MIPs) [106] and immunoaffinity systems [107]. The following section gives a brief overview of the variety of coating procedures. More detailed information, including advantages and disadvantages of various coatings, is available in other recent technology reviews [11, 24]. Table 1 lists the different coating procedures, sorbent materials and examples of applications. Physical coating processes were the first to be developed and constitute the most convenient procedures [11]. They are compatible with nearly all kinds of sorbent materials. Application of new coatings such as carbon nanomaterials (CNMs), ordered mesoporous materials, ionic liquids (ILs) and polymeric ionic liquids (PILs) is very simple. These coatings can be used for fused silica fibers as well as for metal wires. The Dipping process is conducted by placing a fiber in a concentrated organic solvent solution of the material to be deposited for a short time. Crosslinking of the deposited material can take place after evaporation of the solvent by drying. In case of a metal wire, support material can be attached using a sticky paste layer of the sorbent (epoxy or similar) [11, 24]. An alternative to physical coating procedures is the sol-gel technology developed by [PERSON] and co-workers (1997) [108]. Sol-gel process is based on the building of an inorganic network consisting of a colloidal suspension (sol) and gelation of this sol which forms a network in a continuous liquid phase (gel).