## 3 Results
### Validation Assessment: pXRF vs. ICP-MS and DMA-80  
The comparability is determined by the degree to which the pXRF concentration of an element in a sample matches the concentration value obtained with the confirmatory analysis via reference methods. The comparability is assessed by the statistical equivalence of pXRF and ICP-MS (and DMA-80 for Hg) log\({}_{10}\)-transformed data and acceptable repeatability (RSD%) as described by the U.S. EPA [11], which indicates that pXRF results can reach definitive-, quantitative-screening-, or qualitative-screening-level criteria. The first step in the evaluation of the quality of the pXRF analysis was performed under optimal sample preparation conditions (\"lab-prepared\" samples), where samples were finely ground and prepared in the same manner as samples for traditional laboratory analysis. The comparability between pXRF and confirmatory analysis on site-specific samples (soils, stream sediments, and mine wastes) are shown in the log-log plot in Figure 1 and in the statistical summary shown in Table 3.  
The quality of pXRF analytical determination is variable among elements and matrices, in agreement to previous studies [13; 27; 32]. In general, a good comparability was found in both datasets for major elements as base metals (Table 3), according to previous investigations reporting significant correlations between digested samples through reference methods and pXRF for base metals [16; 33]. In general, deviations from reference data may be due to uncertainties deriving from both pXRF and/or ICP-MS/DMA-80 analytical procedures, as demonstrated by the variability of the recovery for the different techniques (Table S1). Lead (Pb) was found to be the most accurate pXRF analyte and was the only element that reached the definitive data quality in both datasets among the nine elements investigated. Such high quality was the result of excellent comparability with ICP-MS analysis in a very wide concentration range, from 5 mg/kg to around 40,000 mg/kg (4 wt.%). Definitive quality levels were also obtained in the case of Zn in the Pb-Zn matrix and in the case of Sb in the Cu-Sb matrix. In addition, As, Fe, and Mn reached quantitative screening levels in both matrices, whereas Cu and Hg met the quantitative screening level only in the Cu-Sb-rich matrix due to the highest content of such metals in the host rocks and in the ore minerals [25; 34]. In the case of As, results from pXRF were found to be slightly less correlated to ICP-MS results for the Pb-Zn site most likely due to the fact that As determination is affected by \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Element**} & \multirow{2}{*}{**CRM Value (mg/kg)**} & \multicolumn{2}{c}{**Mean**} & \multicolumn{2}{c}{**Mean**} \\  & & \multicolumn{2}{c}{**Accuracy—Recovery (\%)**} & \multicolumn{2}{c}{**Repeability—RSD (\%)**} \\ \hline  & **PACS-3** & **MESS-4** & **PACS-3** & **MESS-4** & **PACS-3** & **MESS-4** \\ \hline As & 30.3 & 21.7 & **164\%** & **121\%** & 3.9\% & 6.1\% \\ Cr & 90.6 & 94.3 & 120\% & 106\% & 7.4\% & 4.1\% \\ Cu & 356 & 32.9 & 104\% & **160\%** & 3.5\% & 2.4\% \\ Fe & 41060 & 37900 & 107\% & 102\% & 0.6\% & 1.4\% \\ Hg & 2.98 & 0.09 & n.d. & n.d. & n.d. & n.d. \\ Mn & 432 & 298 & 107\% & 103\% & 3.7\% & 3.4\% \\ Pb & 188 & 21.5 & 93\% & 93\% & 3.4\% & **13.0\%** \\ Sb & 14.7 & 1.07 & n.d. & n.d. & n.d. & n.d. \\ Zn & 376 & 147 & 109\% & 102\% & 2.0\% & 3.3\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: QA/QC of pXRF data from CRMs. Data presented in bold are out of the acceptable quality range. high Pb concentrations. The same issue was observed for the CRM analysis. Hence, for the Pb-Zn site, a better As determination was obtained by ICP-MS, e.g., [25], although statistics indicated that As could still be identified via pXRF at the quantitative screening level. Chromium (Cr) and Hg (Table 3) reached low to very low concentrations in the Pb-Zn carbonate matrix. Their peaks were incorrectly interpreted by the factory calibration producing meaningless data (Figure 1).