## 3 Results and Discussion
### Repeatability, Accuracy and Precision
Repeatabilities for the intra- and inter-day values vary significantly amongst working groups and methods used. [PERSON] and [PERSON] reported repeatability values between 10-25% for V, Cr, Mn, Fe, Co, Cd, Zn, As and Se [58]. [PERSON] et al. defined a smaller range of 3-9% for their method validation [59]. Nevertheless, we showed inter-day and intra-day repeatability \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Analyte**} & **Theoretical** & **Measured** & **Accuracy (\%)** & **Intra-Day Cv** & **Inter-Day Cv** \\  & **Concentration** & **Concentration** & **(\% L\({}^{-1}\)) \(\pm\) SD** & **\(\pm\) SD** & **(\%)** & **(\%)** \\ \hline As & 14.50 & 12.16 \(\pm\) 0.79 & 119.29 \(\pm\) 7.52 & 9.58 & 6.48 \\  & 21.75 & 20.15 \(\pm\) 0.64 & 107.93 \(\pm\) 3.38 & 4.06 & 3.20 \\  & 41.15 & 37.54 \(\pm\) 1.36 & 109.61 \(\pm\) 4.53 & 3.54 & 4.23 \\  & 43.50 & 41.60 \(\pm\) 1.76 & 104.58 \(\pm\) 3.91 & 0.93 & 3.63 \\  & 82.30 & 77.88 \(\pm\) 0.90 & 105.68 \(\pm\) 1.23 & 0.96 & 1.15 \\ \hline Se & 11.60 & 6.83 \(\pm\) 0.32 & 128.73 \(\pm\) 7.89 & 1.59 & 4.71 \\  & 17.40 & 15.12 \(\pm\) 0.37 & 115.04 \(\pm\) 2.81 & 5.02 & 2.44 \\  & 34.80 & 33.07 \(\pm\) 0.68 & 105.24 \(\pm\) 2.12 & 3.00 & 2.04 \\  & 44.65 & 42.11 \(\pm\) 3.19 & 106.04 \(\pm\) 8.51 & 3.84 & 7.59 \\  & 89.30 & 85.32 \(\pm\) 3.90 & 104.67 \(\pm\) 4.64 & 1.30 & 4.57 \\ \hline Mo & 7.97 & 7.74 \(\pm\) 0.36 & 102.87 \(\pm\) 4.94 & 10.16 & 4.71 \\  & 11.95 & 11.53 \(\pm\) 0.66 & 103.64 \(\pm\) 5.74 & 3.95 & 5.68 \\  & 23.90 & 23.17 \(\pm\) 1.66 & 103.16 \(\pm\) 7.22 & 9.70 & 7.17 \\  & 49.65 & 48.68 \(\pm\) 1.11 & 102.00 \(\pm\) 2.32 & 1.06 & 2.28 \\  & 99.30 & 99.51 \(\pm\) 1.99 & 99.78 \(\pm\) 2.01 & 3.33 & 2.00 \\ \hline Cd & 0.82 & 0.73 \(\pm\) 0.11 & 115.22 \(\pm\) 10.52 & 9.85 & 10.12 \\  & 1.24 & 1.07 \(\pm\) 0.14 & 114.30 \(\pm\) 7.54 & 1.38 & 12.86 \\  & 2.47 & 2.19 \(\pm\) 0.04 & 112.65 \(\pm\) 1.89 & 9.08 & 1.69 \\  & 7.15 & 6.43 \(\pm\) 0.44 & 111.11 \(\pm\) 7.34 & 6.93 & 6.77 \\  & 14.30 & 12.68 \(\pm\) 0.35 & 108.37 \(\pm\) 3.12 & 5.79 & 2.76 \\ \hline Sn & 1.71 & 1.51 \(\pm\) 0.08 & 114.02 \(\pm\) 6.47 & 7.65 & 5.41 \\  & 2.57 & 2.17 \(\pm\) 0.11 & 118.50 \(\pm\) 6.10 & 3.54 & 5.24 \\  & 5.00 & 4.39 \(\pm\) 0.31 & 109.16 \(\pm\) 8.05 & 1.76 & 7.70 \\  & 5.14 & 4.71 \(\pm\) 0.36 & 113.82 \(\pm\) 8.27 & 8.36 & 7.13 \\  & 10.00 & 9.23 \(\pm\) 0.71 & 108.37 \(\pm\) 8.86 & 3.66 & 7.69 \\ \hline Ba & 3.60 & 3.44 \(\pm\) 0.24 & 104.64 \(\pm\) 7.04 & 5.74 & 7.08 \\  & 5.40 & 5.48 \(\pm\) 0.13 & 98.58 \(\pm\) 2.30 & 1.66 & 2.32 \\  & 10.80 & 10.31 \(\pm\) 0.27 & 104.73 \(\pm\) 2.81 & 7.13 & 2.64 \\  & 25.65 & 24.72 \(\pm\) 0.86 & 103.74 \(\pm\) 3.62 & 0.89 & 3.47 \\  & 51.30 & 49.81 \(\pm\) 2.65 & 102.99 \(\pm\) 5.38 & 2.79 & 5.33 \\ \hline TI & 2.27 & 2.18 \(\pm\) 0.05 & 103.89 \(\pm\) 2.17 & 0.95 & 2.13 \\  & 3.40 & 3.15 \(\pm\) 0.04 & 107.93 \(\pm\) 1.40 & 5.11 & 1.30 \\  & 6.80 & 6.16 \(\pm\) 0.26 & 110.31 \(\pm\) 4.64 & 0.97 & 4.26 \\  & 8.85 & 7.57 \(\pm\) 0.17 & 113.84 \(\pm\) 2.67 & 2.90 & 2.27 \\  & 17.70 & 15.61 \(\pm\) 0.34 & 113.41 \(\pm\) 2.47 & 2.73 & 2.20 \\ \hline Pb & 9.17 & 7.80 \(\pm\) 0.33 & 117.56 \(\pm\) 5.16 & 5.59 & 4.34 \\  & 13.75 & 11.51 \(\pm\) 1.05 & 119.46 \(\pm\) 2.16 & 2.00 & 9.23 \\  & 27.20 & 22.17 \(\pm\) 1.16 & 105.55 \(\pm\) 3.84 & 1.62 & 3.72 \\  & 27.50 & 26.05 \(\pm\) 0.97 & 105.56 \(\pm\) 6.23 & 5.19 & 5.21 \\  & 54.40 & 50.38 \(\pm\) 6.06 & 107.99 \(\pm\) 5.97 & 1.37 & 12.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: _Cont._(Cv) for all elements below 13% with our method. Regarding the results of the internal quality control, the measured values are in good agreement with the declared values of the control material from SeronormTM (Sero AS, Billingstad, Norway), as indicated in Table 4.