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Which rare-earth element leaves an ion-exchange column first during separation?

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A line graph showing electron probability distributions for gadolinium, with separate curves for 4f, 5s, 5p, 5d, and 6s orbitals. The x-axis shows distance from the nucleus in atomic units and the y-axis shows probability P2(r). — rare-earth element Electron probabilities, P²*(r)*, for the 4f, 5s, 5p, 5d, and 6s electrons of gadolinium.

During the ion-exchange process used to separate rare-earth elements, the elements are eluted based on the stability of their complexes. Among the rare-earth elements, lutetium is the first to leave the ion-exchange column, followed by ytterbium and the other lanthanides in sequence, with lanthanum being the last to emerge.1

While the retaining ions (such as copper or zinc) actually exit the column before any rare-earth elements, lutetium is the first of the rare-earth group to be collected. This separation is made possible by the use of complexing agents like ethylene diamine tetraacetate (EDTA), which exploit the varying stability of the rare-earth complexes.1