## 3 Results  
### Instrumental Neutron Activation Analysis (INAA) Validation  
All of the analytical procedures are performed to obtain as much information as possible. For this aim, the use of a nuclear analytical technique such as INAA is strongly recommended because it allows for high sensitivity and quantification of elements to be achieved at very low (ultra-trace) concentrations. On the other hand, using nuclear analytical techniques, the problem could regard the reference material used for the analysis. This is considered a critical point: SRMs should show a composition quite similar to the investigated matrices. According to the experience of this laboratory, largely involved in round-robin comparisons with other international laboratories for both verifying the instrumental drift and quality control, the authors solved this problem by analyzing secondary reference materials [35; 36; 37] along with primary certified standards. In particular, over the mixture of primary standard solutions (obtained starting from 1 mg mL\({}^{-1}\) of each one) above reported, USGS GRX-3 and NIST 1633b (coal fly ash) were used for the evaluation of the methodology. Furthermore, in the analysis of such matrices, high attention should be focused on the precision and accuracy, along with information about the area characterization on the anthropogenic sources present in the territory. Table 2 shows the comparison between our data and the relative certified values for the investigated SRMs. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Element**} & \multirow{2}{*}{**Product Nuclide**} & \multirow{2}{*}{**Half Life**} & \(\gamma\)**-ray Used** & **LOD** & **Radionuclide** \\  & & & **(keV)** & **(\(\mu\)g g\({}^{-1}\))** & **Interfering (keV)** \\ \hline Ga & \({}^{72}\)Ga & 14.3 h & 630.1 & 0.01 & \({}^{54}\)Mn (834.8 keV) \\ Hf & \({}^{181}\)Hf & 42.4 d & 482.2 & 0.25 & \\ Hg & \({}^{203}\)Hg & 46.9 d & 279.0 & 5.2 b & \({}^{75}\)Se (279.6 keV) \\ Ir & \({}^{192}\)Ir & 74.3 d & 316.5 & 0.001 & \\ K & \({}^{42}\)K & 12.52 h & 1524.7 & 260 & \\ La & \({}^{140}\)La & 40.27 h & 1596.2 & 3.5 b & \\ Mn & \({}^{56}\)Mn & 2.6 h & 1810.7 & 0.1 & \\ Mo & \({}^{99}\)Mo & 2.75 d & 141.0 & 1 & \\ Na & \({}^{24}\)Na & 15.0 h & 1368.4 & 2.0 b & \\ Nd & \({}^{147}\)Nd & 11.1 d & 531.0 & 1 & \\ Ni & \({}^{58}\)Co & 70.78 d & 810.7 & 80 & \\ Rb & \({}^{86}\)Rb & 18.66 d & 1076.7 & 0.4 & \\ Sb & \({}^{124}\)Sb & 60.3 d & 1690.7 & 6 b & \\ Sc & \({}^{46}\)Sc & 83.85 d & 889.2 & 0.9 b & \\ Se & \({}^{75}\)Se & 120.4 d & 264.6 & 9 b & \({}^{182}\)Ta (264.1 keV) \\ Sm & \({}^{153}\)Sm & 1.948 d & 103.1 & 0.41 b & \\ Sn & \({}^{113}\)Sn & 115.1 d & 391.1 & 40 & \\ Sr & \({}^{85}\)Sr & 64.0 d & 514.0 & 50 & e\({}^{+}\)+e\({}^{-}\) (511.0 keV) \\ Ta & \({}^{182}\)Ta & 115.1 d & 1221.2 & 0.2 & \\ Tb & \({}^{160}\)Tb & 72.1 d & 879.4 & 0.3 b & \\ Th & \({}^{233}\)Pa & 27.4 d & 311.8 & 0.1 & \\ U & \({}^{239}\)Np & 2.35 d & 277.6 & 0.03 & \({}^{200}\)Hg (279.0 keV) \\  & & & & & \({}^{76}\)Se (279.6 keV) \\ W & \({}^{187}\)W & 24.0 h & 685.7 & 0.01 & \\ Yb & \({}^{175}\)Yb & 4.21 d & 396.1 & 0.01 & \\ Zn & \({}^{65}\)Zn & 243.8 d & 1115.5 & 12 b & \({}^{46}\)Sc (1120.1 keV) \\ Zr & \({}^{95}\)Zr & 65.5 d & 724.2 & 80 & \\ \hline \hline \end{tabular}  
* m: minutes; h: hours; d: days; y: years; b: values expressed as ng g\({}^{-1}\); e\({}^{+}\)+e\({}^{-}\): annihilation.  
\end{table} 
Table 1: _Cont._  
\begin{table}