## 3 Results
### Comparison  
To investigate the extraction performance of the prepared materials, 4 mg of Fe\({}_{3}\)O\({}_{4}\)-MWCNTs and 6 mg of M-M-ZIF-67, which was synthesized based on the same amount of the former material, were applied to extraction of the nine OCPs using the proposed method. The results are shown in Figure 6 based on three replicate analyses (\(n=3\)). The peak areas of all of the analytes for M-M-ZIF-67 \begin{table}
\begin{tabular}{l c c c c} \hline
**OCPs** & **Linear Range (\(\mu\)g L\({}^{-1}\))** & **Determination Coefficients** & **LOD \({}^{\text{a}}\) (\(\mu\)g L\({}^{-1}\))** & **RSD \({}^{\text{b}}\) (\%) (\(n=6\))** \\ \hline \(\alpha\)-HCH & 2–200 & 0.9953 & 0.12 & 8.5 \\ \(\beta\)-HCH & 1–200 & 0.9946 & 0.13 & 1.0 \\ \(\gamma\)-HCH & 1–200 & 0.9919 & 0.15 & 6.2 \\ \(\delta\)-HCH & 1–200 & 0.9947 & 0.07 & 4.0 \\ \(\alpha\)p\({}^{\prime}\)-DDE & 1–200 & 0.9951 & 0.17 & 1.5 \\ p\({}^{\prime}\)p\({}^{\prime}\)-DDE & 1–200 & 0.9989 & 0.45 & 1.3 \\ p\({}^{\prime}\)p\({}^{\prime}\)-DDD & 2–200 & 0.9934 & 0.41 & 3.1 \\ \(\alpha\)p\({}^{\prime}\)-DDT & 2–200 & 0.9932 & 0.74 & 3.9 \\ p\({}^{\prime}\)p\({}^{\prime}\)-DDT & 2–200 & 0.9916 & 1.03 & 3.1 \\ \hline \end{tabular}  
* LOD means limit of determination;\({}^{\text{b}}\) RSDs means relative standard deviations, and were determined by performing six replicate analyses of the spiked samples with 10 ug L\({}^{-1}\) of each of the organochlorine pesticides.  
\end{table} 
Table 2: Analytical parameters of zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes as an adsorbent for magnetic solid-phase extraction of nine organochlorine pesticides in an ultrapure water sample. Figure 5: Optimization of the desorption process for 50 ug L\({}^{-1}\) organochlorine pesticides using zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes: (**A**) desorption solvent; (**B**) desorption time; and (**C**) frequency of desorption. Extraction conditions: sample volume, 5 mL; amount of sorbent, 6 mg; and extraction time, 20 min. The error bars show the standard deviation of the mean (\(n=3\)). are 2.7-3.0 times higher than those for Fe\({}_{3}\)O\({}_{4}\)-MWCNTs, indicating that ZIF-67 plays a greater role in the extraction process. The performance of the current method was compared with some recently reported methods used for analysis of OCPs in water. The comparison data (Table 3) show that the proposed method based on the M-M-ZIF-67 adsorbent exhibits good sensitivity (as the LOD), less use of sample and adsorbent. However, the Fe\({}_{3}\)O\({}_{4}\)-MWCNTs based adsorbent can make the phase separation process easier and faster without additional centrifugation or filtration procedures, and also can avoid the time-consuming column passing operations encountered in SPE. Therefore, the developed M-M-ZIF-67 based magnetic solid-phase extraction method showed agreement in accordance with the principles of green analytical chemistry [41]. Figure 6: Effect of the sorbent on the adsorption capacities for the nine organochlorine pesticides. Extraction conditions: sample volume, 5 mL; amount of sorbent, 6 mg zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes and 4 mg magnetic multiwalled carbon nanotubes; and extraction time, 20 min. Desorption conditions: 2 mL of acetonitrile; ultrasonication time, 5 min, and ultrasonication performed one more time. The error bars show the standard deviation of the mean (\(n=3\)).