The digested samples were assayed on a PerkinElmer ELAN 9000 ICP-MS equipment by a standard protocol. The main instrumental parameters were as follows: RF power 1000 W, Ar GSK744 flow rate 1 L min− 1, washing time 35 s, and 3 replicates per sample. Quantification was performed by internal standard calibration using PerkinElmer multi-element ICP-MS calibration standards. Blank samples were assayed and no significant concentrations of the studied elements were found. Concentrations of elements in the soil samples are expressed in dry soil weight terms.
2.6. Statistical analysis
Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) were performed by using SPSS 19.0 software and XLSTAT 2009.1.02 for Windows Excel. Hierarchical cluster analysis was performed by using Euclidean distance and the Ward agglomerative algorithm. To get a better insight into the latent structure of the data, the principal component extracted correlation matrix was subjected to a varimax orthogonal rotation.