Determination of Labile Cadmium in Agricultural Soils by Isotope Dilution Plasma Mass Spectrometry and a Coprecipitation Separation Technique

Isotope dilution techniques were examined in order to determine the E-value (isotopically exchangeable pool or labile pool) of cadmium (Cd) in uncontaminated soil. In order to avoid spectral interference, coprecipitation separation was applied before taking inductively coupled plasma mass spectrometry (ICP-MS) measurements. An enriched Cd-113 solution was added to a soil suspension (15 g of soil and 150 mL of water) and then a 2 mL aliquot of it was sampled and filtered immediately at different exchange times. By addition of a sodium hydroxide solution to the filtered solution, Cd coprecipitated with major constituents in the filtered solution, such as iron and magnesium. After the supernatant was discarded, the precipitate was dissolved by adding 1.5 mL of 0.2 mol/L nitric acid. After this treatment, the Cd-113/Cd-114 ratio in the soil solution could be treasured with high precision, within RSD 5%. The correction of mass bias was not required. The proportion of E-values to total Cd concentration in the soil samples ranged between 50 and 60%. These results indicate that the E-value of Cd in uncontaminated soil can be measured by using both coprecipitation separation and isotope dilution ICP-MS.