## 3 Results and Discussions
### Analytical Performance  
The calibration curves for each element and the correlation coefficients are presented in Table 2, together with the LOD values and LOQ values, the extraction recoveries and the enhancement factors. As it can be observed, good linearity was obtained for all elements since the correlation coefficients were above 0.999. Moreover, low LOD values (0.01-0.21 ug g\({}^{-1}\)) and LOQ values (0.03-0.63 ug g\({}^{-1}\)) were obtained. Under optimum conditions for the d-SPE method, extraction recoveries (ER %) for Ba, Cd, Ni, Pb, Cu and Zn ranged from 58.7 to 95.1% and enhancement factors (EFs) ranged from 12 to 19.  
The intra-day and inter-day results expressed as relative standard deviation (RSD)% values for the toxic and nutrient metals and the RSD values for seven replicate ICP measurements of a standard solution are shown in Table 3. As it can be observed, the RSD values for the intra-day study were less than 1.9%, while the inter-day values were less than 4.7%. The RSD% values from the seven replicate ICP measurements of a standard solution containing the examined elements were less than 1.4% (instrumental RSD%). Finally, relative recoveries ranged between 90.1 and 117.8% for the examined analytes.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Element**} & \multirow{2}{*}{**Calibration Curves**} & \multirow{2}{*}{**R**} & \multirow{2}{*}{**Linear Range**} & \multirow{2}{*}{**LOD**} & \multirow{2}{*}{**LOQ**} & \multirow{2}{*}{**ER \%\%**} & \multirow{2}{*}{**EF \({}^{2}\)**} \\  & & & & & (\(\mu\)g g\({}^{-1}\)) & & (\(\mu\)g g\({}^{-1}\)) & & (\(\mu\)g g\({}^{-1}\)) \\ \hline Ba & y = (80.8 \(\pm\) 2.0)x \(-\) (7.6 \(\pm\) 1.2) & 0.9990 & 5–100 & 0.01 & 0.03 & 73.8 & 15 \\ Cd & y = (68.2 \(\pm\) 1.3)x \(-\) (321.8 \(\pm\) 31.9) & 0.9990 & 5–500 & 0.03 & 0.09 & 91.2 & 18 \\ Pb & y = (3.7 \(\pm\) 0.3)x \(-\) (180.2 \(\pm\) 10.1) & 0.9990 & 25–500 & 0.21 & 0.63 & 69.3 & 14 \\ Ni & y = (24.5 \(\pm\) 0.3)x \(-\) (66.9 \(\pm\) 5.8) & 0.9996 & 5–500 & 0.08 & 0.24 & 79.7 & 16 \\ Cu & y = (2551 \(\pm\) 77.9)x \(+\) (71826 \(\pm\) 363.9) & 0.9997 & 20–1000 & 0.10 & 0.30 & 58.7 & 12 \\ Zn & y = (60.6 \(\pm\) 1.5)x \(+\) (544 \(\pm\) 37.5) & 0.9997 & 20–5000 & 0.10 & 0.32 & 95.1 & 19 \\ \hline \hline \end{tabular}  
\({}^{1}\) Extraction recovery (ER). \({}^{2}\) Enhancement factor (EF).  
\end{table}
Table 2: Validation results for the proposed method.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**Intra-Day (\(\mu\) = 5)**} & \multicolumn{4}{c}{**Inter-Day (\(\mu\) = \(5\times 3\))**} & \multirow{2}{*}{**Instrumental RSD (\(\mu\) = 7)**} \\  & & \multirow{2}{*}{**RSD**} & \multirow{2}{*}{**RSD**} & \multirow{2}{*}{**\%\%**} & \multirow{2}{*}{**Round (\%)**} & \multirow{2}{*}{**RSD**} & \multirow{2}{*}{**\%\%**} & \multirow{2}{*}{**RSD**} \\ \cline{5-5}  
A comparison of the proposed method with other studies is shown in Table S4. As it can be observed, in most developed methods with graphite and graphene-oxide-derived materials, 30-200 mg of sorbent is usually required for the extraction procedure [57; 59; 60; 61]. Herein, 25 mg of NaGO was employed for the d-SPE procedure. Moreover, extraction and elution were performed within 5 min for each step, providing a fast and simple sample preparation method. In ref. [56], the adsorption time and desorption time were 23 min for each step, while in ref. [60], 10 min were required for the adsorption step. Compared with graphene, no chelating agents were required due to the active sites of the modified GO material [58; 62]. Moreover, with the d-SPE approach there is no requirement for sorbent packing in SPE cartridges or capillary columns [53; 54; 55; 57; 58; 60]. The RSD values and the LODs for the divalent metals are comparable to other sample preparation techniques [53; 54; 55; 56; 57; 58; 62]; however, to the best of our knowledge, that was the first study containing Pb, Cd, Ba, Zn, Cu and Ni for their extraction from meat samples.