## 3 Results and Discussion
### Real Food Sample Analysis  
The food quality may be affected by metal contamination or bioaccumulation through the food, eventually reaching the human body. Human health is affected by environmental pollution, and contaminated food is considered as a cause of human exposure to metal ions [41]. The developed solid-phase-based extraction was applied for preconcentration of Cr(III), Fe(III), Co(II) and Ni(II) in some food samples including _Piper nigrum_, _Vicia fuba_, _Curcumin_, _Pimpinella anisum_, _Cuminum cyninum_, cinnamon, _Coriandrum sativum_, _Prunus dulcis_, _Coriandrum sativum_, _Lens culinaris_ and _Cicer arietinum_. Table 4 shows the concentrations of Cr(III), Fe(III), Co(II) and Ni(II) in the tested real food samples (n = 3), indicating the trace levels within the safe region stated by international organizations related to food quality [42; 43]. The comparison of the limits of detection between the developed extraction processes using biomass-derived adsorbent and other methods from the literature [14; 18; 19; 42; 43] is presented in Table 5. For Cr(III) and Co(II) extraction, the present work exhibits LODs lower than dispersive solid-phase extraction (USE-AA-D-SPE) [14], while for Fe(III), the LODs were higher than those reported by [19; 42], and for Ni(II), they were higher than those reported by [43]. This confirms that the developed biomass-derived adsorbent prepared in this work has a similar performance to graphene- and carbon-nanotube-based materials for preconcentration purposes.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Real Food Samples**} & \multicolumn{2}{c}{**Cr(III)**} & \multicolumn{2}{c}{**Fe(III)**} & \multicolumn{2}{c}{**Co(II)**} & \multicolumn{2}{c}{**Ni (II)**} \\  & **(mg kg\({}^{-1}\))** & **(mg kg\({}^{-1}\))** & **(mg kg\({}^{-1}\))** & **(mg kg\({}^{-1}\))** \\ \hline _Pimpinella anisum_ & 0.08 \(\pm\) 0.001 & 0.68 \(\pm\) 0.015 & 0.11 \(\pm\) 0.010 & 0.09 \(\pm\) 0.005 \\ \hline _Cuminum cyninum_ & 0.009 \(\pm\) 0.002 & 0.82 \(\pm\) 0.023 & 0.01 \(\pm\) 0.009 & 0.05 \(\pm\) 0.003 \\ \hline _Cinnamon_ & 0.03 \(\pm\) 0.009 & 1.14 \(\pm\) 0.026 & 0.09 \(\pm\) 0.005 & 0.06 \(\pm\) 0.001 \\ \hline _Coriandrum sativum_ & 0.002 \(\pm\) 0.000 & 2.17 \(\pm\) 0.180 & 0.12 \(\pm\) 0.004 & 0.09 \(\pm\) 0.005 \\ \hline _Prunus dulcis_ & 0.079 \(\pm\) 0.005 & 2.26 \(\pm\) 0.085 & 0.13 \(\pm\) 0.007 & 0.07 \(\pm\) 0.001 \\ \hline _Coriandrum sativum_ & 0.012 \(\pm\) 0.006 & 3.45 \(\pm\) 0.240 & 0.09 \(\pm\) 0.004 & 0.05 \(\pm\) 0.002 \\ \hline _Lens culinaris_ & 0.032 \(\pm\) 0.004 & 1.02 \(\pm\) 0.09 & 0.07 \(\pm\) 0.005 & 0.09 \(\pm\) 0.001 \\ \hline _Cicer arietinum_ & 0.076 \(\pm\) 0.005 & 1.83 \(\pm\) 0.047 & 0.01 \(\pm\) 0.001 & 0.07 \(\pm\) 0.003 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Application of the proposed extraction for preconcentration of Cr(III), Fe(III), Co(II) and Ni(II) from real food samples (n = 3).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Coexisting ion**} & \multirow{2}{*}{**Added as**} & **Concentration** & \multicolumn{3}{c}{**Recovery \%**} \\  & & **(mg.L\({}^{-1}\))** & **Cr(III)** & **Fe(III)** & **Co(II)** & **Ni (II)** \\ \hline
**Cd\({}^{+2}\)** & Cd(NO\({}_{3}\))\({}_{2}\) & 10 & 97 & 98 & 100 & 98 \\ \hline
**Mg\({}^{+2}\)** & MgCl\({}_{2}\) & 200 & 95 & 96 & 96 & 91 \\ \hline
**Ca\({}^{+2}\)** & CaCl\({}_{2}\) & 150 & 94 & 99 & 95 & 93 \\ \hline
**Cu\({}^{+2}\)** & Cu(NO\({}_{3}\))\({}_{2}\) & 10 & 96 & 95 & 93 & 98 \\ \hline
**NO\({}_{3}\)\({}^{-}\)** & KNO\({}_{3}\) & 1200 & 98 & 96 & 92 & 96 \\ \hline F\({}^{-}\) & NaF & 500 & 97 & 95 & 96 & 93 \\ \hline
**CO\({}_{3}\)\({}^{-2}\)** & Na\({}_{2}\)CO\({}_{3}\) & 1000 & 97 & 98 & 100 & 98 \\ \hline
**SO\({}_{4}\)\({}^{-2}\)** & Na\({}_{2}\)SO\({}_{4}\) & 800 & 100 & 96 & 96 & 91 \\ \hline
**Na\({}^{+}\)** & NaNO\({}_{3}\) & 12000 & 94 & 99 & 95 & 93 \\ \hline \hline \end{tabular}
\end{table}
Table 3: _Cont._