## 4 Conclusions  
In areas close to mining operations, the preliminary use of pXRF may be able to easily identify the location of the mining wastes characterized by the highest metal or metalloid concentrations, exploring even the occurrence of elements of potential economic interest. Conversely, users may encounter unreliable results when pXRF data are not validated, especially when dealing with low concentrations. Care must be taken regarding the concentration effects  
Figure 3: Comparison between pXRF and confirmatory data (ICP-MS and DMA) in “fast lab-prepared” samples (only dried and sieved \(<2\) mm) of two datasets from a Pb-Zn and Cu-Sb mining sites, marked with different colours. since the absence or occurrence of very low concentrations can be interpreted by pXRF as higher background levels and also regarding actual LODs, which may be higher than those indicated by the factory producer due to different operating conditions. In this study, the overall analytical quality was found to be mainly influenced by the composition of samples due to interferences, matrix effects, and low concentrations more than the heterogeneity of the sample. Although milling is a highly time-consuming procedure, the sample preparation should be carefully defined according to the desired data quality. However, the loss in comparability due to \"fast lab-prepared\" samples was negligible in comparison to compositional differences. A summary of the results is presented:  
* Lead (Pb), zinc (Zn), and manganese (Mn): the determination of these base metals by pXRF was often found to be successful (from quantitative to definitive quality levels), and no notable analytical issues related to interferences or concentration effects were observed;
* Arsenic (As): the element met the \"quantitative\" quality level in both datasets. However, better comparability is found in sites where Pb is not dominant, and As concentrations are elevated enough due to As-K and Pb-L X-ray overlaps, which reduce the accuracy of As determination, so that the traditional laboratory analysis may be preferential. A concentration effect was observed leading to a decrease in comparability for As < 100 mg/kg;
* Chromium (Cr): the metal showed moderate comparability between the two analytical approaches in the Cu-Sb matrix, characterised by relatively higher Cr concentrations and occurrence of false positives by using pXRF in the carbonate-hosted Pb-Zn samples that should be removed by the user via manual spectral interpretation. Overall, Cr never exceeded the \"qualitative\" level when determined via pXRF;
* Copper (Cu): comparability of pXRF with ICP-MS results was strongly dependent on the element concentration and acceptable for Cu > 100 mg/kg, whereas below such values, Cu concentrations detected via pXRF were false;
* Iron (Fe): similar to Cu, its detectability by pXRF was strongly dependent on the concentration. Comparability between the two analytical approaches was found to be optimal for values >10,000 mg/kg, which are common in many (but not all) natural samples;
* Mercury (Hg) and antimony (Sb): these trace elements were found quantitatively determined by pXRF only when occurring with concentrations > 10-20 mg/kg. Conversely, when the real sample concentrations were below such values, as in the case of Hg, pXRF produced false-positive results, which should be manually removed by the user via spectral interpretation. Unfortunately, this result reveals a serious limitation in using pXRF, as the operator may be precluded from detecting low concentrations of Hg in soils that exceed those threshold levels specified by national regulations that define the use of uncontaminated soil. This is the case with the Italian legislation, for instance, where the highest threshold level, defined by major land use, i.e., for commercial and industrial purposes, is fixed at 5 mg kg\({}^{-1}\), whereas for public, private, and residential use, it is 1 mg kg\({}^{-1}\) (Italian Legislative Decree 152/06 [9]).