Soil Cd, Pb, Cu, and Zn were digested with a mixture of concentrated
nitric acid, hydrofluoric acid, and hyperchloric acid in a polyvinyl fluoride crucible,
followed by graphite furnace atomic absorption spectrophotometry
(GFAAS-AA800, PerkinElmer Inc.) for Cd and Pb and
flame atomic absorption spectrophotometry (Hitachi Z-5000) for Cu
and Zn, respectively.