ICP-MS Determination of Chromium, Cadmium, Arsenic, Lead and Mercury in Vegetables with Microwave Digestion of Samples

Vegetable was collected from the growing area, washed with water, cut into segments and sampled by quartering; the sample was then crushed and 1.000 g of the crushed sample was treated with 7 mL of HNO₃ in a constant temperature digester at 120℃ for 30 min. The digested solution was then removed to a microwave digester to have a further digestion of the sample under 1 000 W for 25 min. After cooling, the digested solution was diluted with HNO₃ (2+98) solution and its volume was made up to 25.0 mL, which was used for ICP-MS determination of Cr, Cd, As, Pb and Hg. Isotopes having high relative abundance (i.e. ⁵²Cr, ¹¹¹Cd, ⁷⁵As, ²⁰⁸Pb and ²⁰²Hg) were selected as the species in the respective determinations to overcome effects of isobaric interferences to a definite extent. Besides, elements of C, H, O, N, Cl, S and Mo, which were existed in vegetables very often, might form polyatomic ions with Ar in the carrier gas, making interferences of polyatomic ions. DRC mode with gaseous NH₃ as reactant in the MS determinations of ¹¹¹Cd, ⁵²Cr and ⁷⁵As was adopted to eliminate such interferences. STD mode was applied to the MS determinations of ²⁰²Hg and ²⁰⁸Pb, since no polyatomic ion interference was encountered in these cases. Moreover, to overcome some new interferences arised in the DRC mode of determinations, setting up to RP_q values was found to be effective. It was shown by the testing results, linear relationships between values of intensities of analytical signals and the mass concentrations of the 5 related elements were found in the definite ranges, with their detection limits (3s/k) ranging from 0.2 to 3.2 μg·kg^-1. Recovery was tested by standard addition method, giving results of recovery in the range from 92.9% to 108%, and RSDs (n=5) in the range of 1.2%-5.8%.