2.3. Sample preparation and determination of NSC697923 total heavy metal concentrations

Standard reference materials (Geochemical Standard Soil GSS-1, GSS-3, GSS-5, GSS-6 and GSS-8) were used for quality assurance and quality control. The percentages of the 11 heavy metals recovered were: V, 99–115%; Cr, 104–119%; Co, 92–114%; Ni, 108–118%; Cu, 109–120%; Zn, 94–106%; As, 92–120%; Cd, 88–120%; Rb, 104–118%; Pb, 96–118%; and Tl, 95–122%. Duplicate samples were analyzed simultaneously for 15% of the samples and the standard deviation was controlled to within 5% in each digestion procedure.

2.4. Regression method

A regression model was used to characterize the relationship between the content of heavy metals and the distance from the roadside using Origin 9.0 software (OriginLab, Northampton, MA, USA). The curve fitted with the lowest χ2 and highest R2 values was taken as best model for predicting the relationship between the heavy metal content in the topsoil and the distance from the roadside.