## 2 Solid-Phase Microextraction  
Solid-phase microextraction (SPME) is a fast, solvent-free sample preparation method that combines sampling, isolation, and enrichment in a single step [13]. This technique uses a small amount of an extraction phase, either immobilized or chemically bonded to a solid support, to \"capture\" analytes, whether volatile (via headspace) or non-volatile (via direct immersion). The process relies on the partitioning equilibrium of analytes between the sample matrix and the extraction phase. SPME is typically utilized prior to separation techniques (GC, LC, CE), but can also be directly coupled with mass spectrometry (MS), eliminating the need for chromatographic separation [14]. This technique offers an efficient, eco-friendly, cost-effective, automated, and continuously evolving approach to the extraction process. SPME extraction using fiber can be carried out in three main modes: (i) direct immersion (DI), (ii) headspace (HS) extraction above the sample, and (iii) a hollow-fiber membrane-protected approach (HF). Additionally, SPME integrates effortlessly with identification techniques such as GC-MS and LC-MS. Automated SPME injection systems combined with GC or GC-MS are the most widely used setups. When coupled with GC, SPME enables solvent-free sample preparation, particularly effective for determining volatile or semi-volatile organic compounds in complex sample matrices [15]. An interesting green SPME application has been reported by [PERSON] et al. for the determination of polycyclic aromatic hydrocarbons (PAHs) in various matrixes, including environmental water, urine, and brewed coffee [16]. The SPME fiber coating was prepared by CIM-80(Al) MOF material using an in situ growth method. A comprehensive review on the usage of MOFs as sorbents in SPME has been recently published [17]. A variety of techniques (i.e., X-ray powder diffraction (PXRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA)) have been used for the surface characterization of the SPME fiber. The manufactured SPME was tested both in direct immersion and headspace sampling modes. The separation of the PAHs was accomplished by using a FactorFour(tm) VF-5 ms capillary column, and their detection was performed in selected ion monitoring (SIM) mode. After optimization of the parameters of both sampling approaches, the optimum extraction time was 60 min, while a sample incubation temperature of 75 \({}^{\circ}\)C was selected for the HS-SPME extractions. A schematic representation of the proposed approach is shown in Figure 2. The authors also compared the performance of the MOF-based SPME fiber with the commercial PDMS one. Similar LOQs were obtained using both fibers, except for acenaphthylene and acenaphthene with the PDMS fiber, which were slightly lower (2.0 ng/L) than those obtained with the MOF phase (2.5 ng/L). Five HS-SPME applications have been reported for the determination of urinary volatile organic compounds (VOCs) [18], amphetamine analogs [19; 20], cyanide and thiocyanate [21], and hexanal and heptanal [22]. Ten years ago, [PERSON] et al. developed an interesting HS-SPME method for the determination of VOCs in autistic children [18]. The purpose of this study was to identify the VOCs as potential biomarkers for autism spectrum disorder. After screening four different commercially available SPME sorbents, the divinylbenzene/carboux/polydimethylsiloxane (DVB/CAR/PDMS) fiber was selected as optimum. The urine samples were thermostated at 40 \({}^{\circ}\)C and analyzed under both acid (pH 1-2) and alkaline medium (pH 12-14) in order to profile a range of urinary components with different physicochemical properties. Higher temperatures (i.e., 60 \({}^{\circ}\)C) led to fiber degradation. Metabolomic information patterns were obtained using GC-MS in scan mode at 30-300 \(m/z\). According to the authors, 2-methylpyrazine, 2,3-dimethylpyrazine, and iso-azole existed in higher levels in the urine of autistic children compared to the controls.