Determination of Six Extractable Heavy Metals in Soil by ICP-MS

Aiming to the determination of 6 extractable heavy metals in soil, sample of soil was collected from field of farm-region, and treated by air-drying, crushing, grinding and sieving to prepare a portion of analytical sample having sizes less than 2 mm. 10.00 g of the prepared soil sample were extracted with 100 mL of 0.01 mol·L^-1 CaCl₂ solution (keeping the soil to solution ratio of 1 to 10) in a tightly-sealed polyethylene (PE) flask at (20±2)℃ for 120 min under constant shaking. An aliquot of 10 mL of clear solution was taken from the suspension with a filtering-syringe attached with 0.45 μm filtering membrane. Two drops of conc. HNO₃ were added to the solution, and mixed well. This solution is ready for use in ICP-MS analysis. A blank solution should be prepared simultaneously by the same procedure. The mode of collision cell with He gas at a flow-rate of 4 mL·min^-1 as reacting gas was used in MS, to eliminate most of the mass spectral interferences, and mixed standard solution of ¹⁰³Rh (5.0 μg·L^-1) and ¹⁸⁵Re (10 μg·L^-1) was added to overcome non-MS interferences, such as matrix effect and etc. Linear relationships were obtained between values of the signal intensities and the respective mass concentrations of the 6 heavy metal elements, values of detection limit (3s) were found in the range of 0.002-0.012 mg·kg^-1. Two standard samples (ASB-S2 and ESS-EC-1) were analyzed by the proposed method, giving values of extractable contents of the 6 heavy metals in consistency with the certified values, and values of RSDs (n=6) found were in the range of 1.2%-6.8%.