## 3 Results and Discussion
### Investigation of the Analytical Performance  
The preconcentration factor (PF) was calculated from Equation (3):  
\[PF=\frac{Initial\ sample\ volume}{Final\ sample\ volume} \tag{3}\]  
The limits of detection (LOD) was calculated using Equation (4):  
\[LODs=\frac{(3\text{+STD})}{m} \tag{4}\]  
where STD is the standard deviation of seven blank readings and \(m\) is the experimental preconcentration factor, calculated considering the ratio of the slopes of the calibration curves with and without preconcentration procedure. The performance of biomass-derived adsorbent for extraction application showed efficient recovery percentage for Cr(III), Fe(III), Co(II) and Ni(II), exhibiting a preconce  
Figure 6: Optimization of the dispersive solid-phase extraction using the biomass-derived adsorbent: effect of pH (**A**), effect of sample volume (**B**) and effect of eluent acid (**C**). tration factor of 50 and LODs of 1.4, 2.4, 1.9 and 3.0 \(\mathrm{\SIUnitSymbolMicro g}\).L\({}^{-1}\) for Cr(III), Fe(III), Co(II) and Ni(II), respectively, while the LOQs were reported as 4.1, 7.3, 5.7 and 8.9 \(\mathrm{\SIUnitSymbolMicro g}\).L\({}^{-1}\) for Cr(III), Fe(III), Co(II) and Ni(II), respectively. In addition, the optimized dispersive solid-phase extraction process exhibited an RSD\(\%\) of 4.3, 4.1, 3.2 and 4.9 for Cr(III), Fe(III), Co(II) and Ni(II), respectively, calculated from seven replicates of a metal solution of 0.3 \(\mathrm{mg}\).L\({}^{-1}\). Furthermore, the addition/recovery of food sample extracts was evaluated by adding 0.1, 0.3 and 0.6 \(\mathrm{mg}\).L\({}^{-1}\) from the heavy metal mixture solution of Cr(III), Fe(III), Co(II) and Ni(II) (Table 1). For all tested concentrations of Cr(III), Fe(III), Co(II) and Ni(II), the recovery percentage was in the range of 87 to 100, with major cases between 90 and 100. The linearity of the addition/recovery tests from _Piper nigrum_ samples is shown in Figure 7, exhibiting correlation coefficient (R\({}^{2}\)) values of 99, 98, 1 and 99 for Cr(III), Fe(III), Co(II) and Ni(II), respectively. Moreover, the precision and reproducibility were investigated by studying the intraday and interday precision for Cr(III), Fe(III), Co(II) and Ni(II) from the solution of 0.4 \(\mathrm{mg}\).L\({}^{-1}\) (N = 8) (Table 2). The values of RSD\(\%\) during intraday and interday investigation were in the range of 2.1 to 8.5 for all tested cases, indicating high accuracy and reproducibility of the developed extraction process. The presence of foreign (interfering) ions may occupy the surface-active groups in the biomass-derived adsorbent, leading to a low extraction efficiency due to the competition between ions. Therefore, evaluation of the ability of the developed extraction procedure using the biomass-derived adsorbent was conducted by testing the presence of some common ions including Al\({}^{+3}\), K\({}^{+}\)/Cl\({}^{-}\), Pb\({}^{+2}\), PO\({}^{-3}\), Cd\({}^{+2}\), Mg\({}^{+2}\), Ca\({}^{+2}\), Cu\({}^{+2}\), NO\({}_{3}\)\({}^{-}\), F, CO\({}_{3}\)\({}^{-2}\), SO4\({}^{-2}\) and Na\({}^{+}\). The recovery percentage of Cr(III), Fe(III), Co(II) and Ni(II) is presented in Table 3 showing values between 90 and 100, indicating the high tolerance ability of the developed