Fractionation of Milligram Quantities of Gadolinium and Terbium Ions by Annular Free‐Flow Isotachophoresis

The fractionation of milligram quantities of gadolinium (Gd) and terbium (Tb) ions is demonstrated using annular free-flow isotachophoresis (ITP) with ammonium as the leading ion and hydronium as the terminating ion, and α-hydroxyisobutyric acid (HIBA) and acetate as lanthanide complexing ligands. Buffer conditions were designed using numerical simulations in COMSOL Multiphysics, allowing for rapid screening of candidate buffer systems before laboratory experiments. The leading electrolyte contained 15 mM ammonium acetate, 5 mM acetic acid, and 3.5 mM HIBA at pH 4.9, while the terminating electrolyte consisted of 15 mM glutamic acid, 5 mM acetic acid, and 3.5 mM HIBA at pH 3.1. After 5 h of electrophoresis at 2 kV and at a mass loading of 7.077 mg Gd and 7.074 mg Tb, 84.08% of the Tb mass was recovered in the first 7 fractions removed from the separation column, and 86.51% of the Gd mass was recovered from the remaining 11 fractions. Overall recoveries of Gd and Tb were 95.07% and 99.13%, respectively.