Investigation of the Electrophoretic Mobility of the Actinides Th, U, Np, Pu, and Am in Different Oxidation States

The electrophoretic mobilities (μ_e) of the actinides Th and U–Am in different oxidation states (prepared in 1 M HCl and 1 M HClO₄) have been determined by capillary electrophoresis (CE)-inductively coupled plasma mass spectrometry (ICPMS) using 1 M acetic acid as the background electrolyte, which has proven to provide an excellent setup for trace analysis at environmentally relevant concentrations (1 × 10^–9 M). The values are independent of the respective acid solution. The μ_e of the Pu oxidation states +III to +VI have been measured. They agree with both the available literature data and the redox-stable analogues (Eu­(III), Th­(IV), Np­(V), U­(VI)) that have also been investigated. The trend in the μ_e for the actinides U–Pu was found to be An­(III) > An­(VI) > An­(V) > An­(IV). The μ_e values of Am­(III) (μ_e(Am­(III)) = 3.86 × 10^–4 cm²/(Vs)), U­(IV) (μ_e(U­(IV)) = 0.34 × 10^–4 cm²/(Vs)), and U­(VI) (μ_e(U­(VI)) = 1.51 × 10^–4 cm²/(Vs)) have been measured for the first time under these experimental conditions. Furthermore, the measured μ_e values show systematic trends that can be rationalized on the basis of the calculated species distribution of the actinides in 1 M acetic acid and the corresponding average effective charges (q_eff).