## 3 Sample Preparation Techniques and Development of Extraction Sorbents
### Solid-Based Extraction
#### 3.2.3 Solid-Phase Extraction (SPE)  
Solid-phase extraction (SPE) involves two different phases partitioning where compounds of interest are retained between a solid phase (sorbent) and a liquid phase (sample) [74]. SPE process comprises four steps, which are column conditioning, sample loading, washing, and elution. The procedures are pertinent despite the types of sorbents chosen, formats (cartridge, disks, 96-well plate, and pipette tip), and the operational mode, whether it is automated (online SPE) or conventional procedure (off-line SPE). The ability to enrich trace compounds simultaneously, along with matrix interferences' elimination; rapid, easy operation; low organic solvents' consumption; and the ability to provide a high preconcentration factor, make SPE superior to other sample preparation techniques [75]. In short, SPE is widely used and a reliable sample preparation technique. Therefore, these advantages have attracted significant interests among researchers, which led to substantial development of SPE. To date, numerous SPE methods have been developed using various materials including graphene [76], magnetic nanocomposites [77], and mesoporous silica doped with titanium [78]. A summary of past studies on the application of SPE as a sample preparation technique for OPPs' determinations is shown in Table 7. Nevertheless, some sorbents suffer from several shortcomings including high values of LOD, low extraction recovery, and incapability to simultaneously extract large number of OPPs. C\({}_{18}\)-functionalised Fe\({}_{3}\)O\({}_{4}\)@mSiO\({}_{2}\) microsphere was prepared by [PERSON] et al. [79] for enrichment of selected OPPs from water samples. The SPE method showed high LOD for the targeted analytes, ranging from 1.8-5.0 \(\upmu\)g L\({}^{-1}\), which exceeded the MRLs for drinking water (0.1 \(\upmu\)g L\({}^{-1}\)) as indicated by the European Union Directive. Hence, numerous SPE sorbent materials have been developed to enhance extraction efficiency and selectivity, primarily in terms of structures and functionalities. This is because the primary aspect of SPE depends on the sorbent phase as it controls the extraction performance and sensitivity and precision of analysis [42]. Therefore, consideration on the physicochemical properties of sorbent and target analytes should be taken into account during method development to promote good adsorbent-adsorbate interaction [80]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Year** & **Sorbent** & **Analyte** & **Matrix** & **LOD** & **Recovery (\%)** & **Analytical** & **Ref.** \\ \hline
2019 & Magnetically modified graphene & Chlorpyrifos & Water & 14 ng mL\({}^{-1}\) & 44 & HPLC-UV & [70] \\
2018 & Zirconium dioxide-reduced & Ethion & Water & 1.5 ng mL\({}^{-1}\) & 93-97 & NCD-IMS & [64] \\  & grapheme oxide (ZrO\({}_{2}\)-rGO) & Profenofos, leptophos, & & & & & \\
2016 & Polydimethylsiloxane (PDMS) & isotemphos, prohibitions and & Water & 0.072–0.091 ng mL\({}^{-1}\) & 82-94 & \({}^{33}\)P QNMR & [72] \\  & methamidophos & Diehlorsors, diazmon & & & & & \\
2016 & carbon nanotubes (OF-HAVON) & paraithion-m and fenitorthion & Water & 5–10 ng L\({}^{-1}\) & 67–84 & GC-MS & [65] \\
2013 & Polydimethylsiloxane/polyliophene & Quinalphos,brace, malathion, & & & & & \\
2009 & PDMS/PITH & paraithion, and fenitorthion & & & & & \\  & polydimethylsiloxane/polyvinyl & Thorate, fenitorthion, malathion, & & & & \\  & alcohol (PDMS/PVA) & paraithion, and quinalphos & Honey & 0.013–0.081 ng mL\({}^{-1}\) & 81–124 & GC-FPD & [66] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of past studies on the application of SBSE as a sample preparation technique for OPPs’ determinations. To date, molecularly imprinted polymers (MIP) have become a notable approach for selective SPE, in which it is based on molecular recognition mechanisms. The MIPs possess recognition sites that are complementary in shape (structure) and physicochemical properties with the target analyte (template molecule) [81].