Equilibrating stable Cd isotopes with acidic soils

Cadmium (Cd) in the soil solution is in dynamic equilibrium with the reservoir of bioavailable Cd attached to the solid phase, i.e. the labile pool (CdE). Traditionally, CdE is estimated using the radioisotope 109Cd, which has severely restricted access to estimates of CdE. Using stable isotope dilution and isotope ratio measurement by inductive coupled plasma-quadrupole mass spectroscopy (ICP-QMS) would increase access to estimates of CdE; however, detail remains scant about the optimal conditions for equilibration and measurement. We report optimal conditions for spiking with 110Cd, batch equilibration and ICP-QMS measurement of the ratio of 110Cd to 111Cd using results for six acidic soils with total Cd concentrations of 0.19–6.4 mg Cd kg−1, suspended in three background electrolytes (10 mM CaCl2, 1 M NH4NO3, and 1 M NH4Cl). Our optimised procedure produces robust estimates of CdE. Application of this approach will greatly increase access to estimates of CdE and to the investigation of its role in Cd uptake by plants.