## 2 Solid-Phase Microextraction  
[PERSON] and [PERSON] [15] introduced solid-phase microextraction (SPME), generating interest in microextraction techniques for analytical chemistry. When using SPME, the analytes are isolated based on the equilibrium between the sample matrix and the extractive coating after selecting an appropriate extractive phase and reducing the volume to remove as many of the unwanted compounds as possible. This strategy leads to efficient clean-up and minimizes the matrix effect during mass spectrometry detection, which is a serious concern in liquid LC-MS systems. SPME configurations can be classified into static and dynamic techniques. Static procedures are typically carried out in stirred samples, including fibre SPME, thin-film microextraction (TFME), rotating disk sorptive extraction (RDSE), stir bar sorptive extraction (SBSE) and dispersive SPME. Fibre SPME, which is the most common format for this technique, utilises a sorbent coating on the outer surface of a fused silica fibre to extract the analyte(s) from the sample matrix; this process occurs through direct immersion (DI-SPME) or from the sample headspace in a closed container (HS-SPME). The dynamic techniques include capillary microextraction (CME) techniques, such as in-tube SPME(IT-SPME), in-needle and in-tip microextraction configurations. SPME focuses mainly on the development of new coatings and novel analytical strategies that improve the sensitivity [16]. A schematic diagram of some configurations is shown in Figure 1.  
Sol-gel technology was applied to prepare SPME fibres in 1997 [17]; since then, it has become one of the most popular approaches for preparing novel SPME coatings. This technology has already helped synthesise many novel sorbents for SPME with large surface areas, unique selectivity and high thermal and solvent stabilities; these characteristics contribute to the high sample pre-concentration factors. The versatility of these materials enables the creation of surface-bonded sorbent coatings on unbreakable fibre materials and on substrates with different geometrical formats. Sol-gel coatings are applied during the extraction of various analytes from different sample matrices in the fibre-SPME and in the SBSE configuration [18]. Therefore, a novel polar sol-gel precursor, cyanopropyltriethoxysilane (CNPrTEOS), was combined with PDMS for the SBSE of two non-steroidal anti-inflammatory drugs (NSAIDs) from aqueous samples [19]. Figure 1: Scheme of some solid phase microextraction techniques. Carbon nanotubes (CNTs) are interesting targets when studying new materials in SPME. CNTs are allotropic forms of graphitic carbon comprising a single rolled graphite lamella that forms a tube (single-wall carbon nanotubes, SWCNTs) or several single tubes arranged around a common axis (multi-wall carbon nanotubes, MWCNTs); the surface-to-volume ratios of these materials are significant [20,21]. A sol-gel amino-modified multi-walled carbon nanotube-PDMS (AMMWCNT-PDMS) was synthesised for use as a novel coating for the SBSE of phenols from environmental waters [22]. Metal-organic frameworks (MOF) are a new class of porous solid materials that are self-assembled by metal ions and organic ligands. Recently, [PERSON] _et al._[23] proposed a sol-gel coating for SBSE based on PDMS and a MOF to analyse oestrogens in environmental water samples. The selectivity required for SPME can be provided through molecular imprinting (MIPs), as demonstrated by [PERSON] _et al._[24]. MIPs are polymeric materials with a high binding capacity and good selectivity against a target molecule purposely introduced during the synthetic process. MIPs are typically synthesised through the co-polymerisation of functional monomers and templates. The functional monomers should possess specific functional groups, and the templates are always the target analytes or their analogous compounds. Cross-linkers are also required to form rigid polymer networks that stabilise the cavities for the target molecules, making the polymer mechanically and thermally robust.