Elution of Eu with 2—hydroxy isobutyric acid, HIBA, from experimental 10 μm, highly (>40%) crosslinked porous resins (XP-11, XP-3016) gives tailed elution peaks. Elution volume of the peaks decreases and column efficiency increases when the flow rate of the mobile phase increases. The increased tailing and height equivalent to a theoretical plate observed for Lu/HIBA (and Eu/HIBA) compared to Ln-isotopic exchange for XP-3016 resin is due to the slow release of the LnL4−complex ion from the micropores. On the other hand, a mono-disperse 10 μm, 5% cross-linked cation-exchanger (XP-7) has high column efficiency and a 3 cm column separated the Yb-Lu pair, but it is unsuitable in an HPLC system where the ionic strength of mobile phase changes.