## 3 SPME Process
### Fiber Coatings
The extraction phase can be chemically bonded to the silica thus producing highly cross-linked phase networks [2]. The precursors for the synthesis of the colloids consist of a metal or metalloid element (alkoxysilanes tetramethylasilane or tetraethoxysilane (TMOS/TEOS) surrounded by various reactive ligands [109]. The sol-gel fibers therefore exhibit higher thermal stability, hydrolytic stability towards organic solvents as well as high and low pH solutions, and higher surface areas. Molecularly selective coatings can be prepared by using substances with specific functionalities such as crown ethers or \(\beta\)-cyclodextrintrin derivatives [2]. [PERSON] _et al._ (2008) published a review on the development of sol-gel methods including various applications (e.g., PAHs, aromatic amines, phenols and pesticides) in environmental, pharmaceutical and food analysis [2, 25]. A large number of new extraction phases with unique features have been introduced by application of the sol-gel technology. The most important advancements are summarized in Table 1. As there is no standardized sol-gel procedure, this new technology lacks inter-laboratory reproducibility [11]. Further disadvantages of the sol-gel technology are the large number of optimization factors and the fragility of the fused silica fiber used mostly as a support. Due to these limitations of the sol-gel procedures, simple chemical grafting has been developed. In this technique, the surface of the sorbents and supports are chemically modified in order to allow them to react with each other. Possible sorbents applicable to both metal wires and fused silica fibers are nanomaterials, antibodies, MIPs, ILs and PILs [24]. Electrochemical procedures are able to generate extraction films of variable thickness on an unbreakable metal support at a low cost and with a simple setup. Coatings show high thermal stability, have a long shelf life and some may provide higher capability to sorb polar compounds. Electrochemical procedures may be divided into electrodeposition, anodization and electrophoretic deposition (EPD) [110]. Electrodeposition can be used to create coatings with porous structure and refers to the deposition of a metallic or conductive polymer (CP) coating onto a base material by the electrochemical reduction of metal ions or electropolymerization of CPs from an electrolyte. This technique has been applied to the coating of metal supports with CPs, CPs nanocomposites, PIL composites and metal oxides [24]. Anodized metal wires were first developed by [PERSON] and co-workers in 2001 [111]. They anodized aluminum wires by direct current in sulfuric acid to obtain a porous layer of aluminum oxide on the aluminum surface [110]. Electrophoretic deposition (EPD) is a direct particle assembly method that is based on particle coagulation rather than an electrochemical reaction. Charged nanoparticles from a solution are deposited onto a substrate using an electric field [24]. Single-walled carbon nanotube (SWCNT) [112, 113, 114] and multi-walled carbon nanotube (MWCNT) [115] coatings have been prepared on the surfaces of platinum and stainless steel wires, respectively. In EPD, adsorption takes place in the spaces between the nanotubes and amorphous carbon in a three-dimensional network of nanotubes [24]. In electrospinning, a solution of a high molecular weight polymer with high viscosity is drawn into nanofibers by repulsive electrostatic forces. [PERSON] _et al._ (2010) were the first to prepare nanostructured polymeric based SPME coatings by electrospinning. They used a polymeric negative photoresist, SU-8 2100, which was converted to carbon by pyrolysis. The resultant SPME devices were used to extract both nonpolar and polar compounds [116]. [PERSON] _et al._ (2008) employed liquid phase deposition (LPD) to produce a nanomaterial-based in-tube SPME coating with increased extraction efficiency [117]. Due to the chemical bonding between the substrate and the coating, LPD offers highly stable nanomaterial thin film coatings.