Overall separation:

The overall chemical isolation procedure from start-of-dissolution to final <sup>165</sup>Er isolation in 0.01 M HCl (~400 µL) was 4.9 ± 0.7 h (*n* = 13). The optimized procedure had a decay-corrected <sup>165</sup>Er recovery of 64 <sup>±</sup> 2% and a Ho/Er SF of (2.8 <sup>±</sup> 1.1)·10<sup>5</sup> and was validated for holmium target masses up to 180 mg, with the final <sup>165</sup>Er fraction containing 370 ± 180 ng of residual holmium and no detectable radionuclidic impurities (Supplementary Figure S6) after separation (*n* = 4).
