## 3 Sample Preparation Techniques and Development of Extraction Sorbents
### Solid-Based Extraction
#### 3.2.1 Solid-Phase Microextraction (SPME)
As a result, this led to the emergence of modified nanomaterials. However, despite the high extraction efficiency of the modified nanomaterials, several shortcomings were identified, which are associated with tedious preparation procedures and expensive precursors [52]. Therefore, [PERSON] et al. [52] prepared a modified halloysite nanotube (MHNTs) as SPME coating for OPPs' enrichment in water, cucumber, and apple samples prior to (gas chromatography-corona discharge ion mobility spectrometry (GC-CD-IMS) analysis. MHNTs were selected due to structure similarity with CNTs and being non-toxic and environmentally friendly in comparison to CNTs. The developed technique obtained low LOD (0.01-0.03 ng mL\({}^{-1}\)), indicating its sensitivity toward OPPs. Besides that, good recovery (84-97%) and good precision (3-9%) were achieved for all real samples. The good analytical performance of SPME-MHTs could possibly be due to high surface area of MHTs (102 m\({}^{2}\) g\({}^{-1}\)) and molecular interaction between the targeted analytes and MHTs via hydrophobic and polar interaction (NH\({}_{2}\) and OH groups). Therefore, the properties enhanced the extraction capability of the sorbent toward polar and semi-polar analytes. In recent times, metal-organic frameworks (MOFs) have also been used as SPME sorbent. MOFs has been getting much attention due to their pronounced physicochemical properties such as high surface areas, being porous, and having tunable pore size, functionality, and thermal stability, which make it a promising SPME coating material [52]. Many studies have reported the application of MOFs as SPME sorbent coating. However, [PERSON] et al. [53] and [PERSON] et al. [54] reported that some MOFs' structures collapsed when exposed to high temperatures. This restricts their application in SPME. Therefore, [PERSON] et al. [55] prepared a nitrogen-doped metal organic framework (MOF)-based porous carbon (C-(C\({}_{3}\)N\({}_{4}\)@MOF)) as a new SPME coating for determination of 14 selected OPPs in five fruits' samples coupled with GC-MS. The sorbent material was prepared via carbonation of a graphitic carbon nitride (g-C\({}_{3}\)N\({}_{4}\))-templated MOF (NH\({}_{2}\)-MIL-125). The incorporation of g-C\({}_{3}\)N\({}_{4}\) in MOF led to the generation of a porous carbon structure, forbid agglomeration, and collapse of pores during pyrolysis [56; 57]. The developed method exhibited low LOD (0.23 to 7.5 ng g\({}^{-1}\)) and good recovery, ranging from 82.6-118%. Results indicated that the C-(C\({}_{3}\)N\({}_{4}\)@MOF) method showed good sensitivity toward polar OPPs. This probably was due to the existence of nitrogen, which increased the polarity of the carbon material [58]. The extraction efficiency of the method was also promoted by the presence of \(\pi\)-stacking and hydrophobic interactions between the sorbent and OPPs as a result of the presence of graphitic sp2-hybridised carbons, C=O, and O-C=O in the C-(C\({}_{3}\)N\({}_{4}\)@MOF). In addition, the prepared coating material could be used over \(100\times\) without substantial loss of extraction efficiency. This indicates that the sorbent coating possessed good durability and reusability. To date, numerous studies have been reported to overcome the disadvantages of SPME, especially in terms of coating materials. Many modifications on coating materials have been introduced as an effort to enhance the extraction efficiency of SPME coating sorbent. However, in general, the recoveries of targeted analytes were still low despite the low LODs. This could probably be because sorbent coatings were lacking in thermal stability, by which they were susceptible to degradation when exposed to high temperature during the desorption process. Hence, further improvement needs to be carried out, particularly in terms of coating material that possesses high thermal stability, which is capable to endure SPME conditions since it is the key factor in achieving successful extraction of target analytes.