Step 3: bDGA EXC

The final step of the <sup>165</sup>Er isolation procedure reduced trace metal impurities while concentrating the product into a small volume, low acidity solution suitable for radiolabeling using a commercially available bDGA resin. The loading (5 M HNO3, 5 mL) and rinsing (3 M HNO3, 15 mL) solution concentrations were chosen due to literature studies showing high erbium and low transition metal (Fe, Co, Ni, Cu, Zn) affinities on a similar DGA resin [48]. A small volume rinse (0.5 M HNO3, 2 mL) decreased column acidity and minimized subsequent <sup>165</sup>Er elution volume. Following this loading and rinsing routine, <sup>97</sup> <sup>±</sup> 2 % of the loaded <sup>165</sup>Er was recovered in 0.01 M HCl (1.2 <sup>±</sup> 0.2 mL) (*<sup>n</sup>* = 18). Smaller elution volumes were attainable by fractionation of the 0.01 M HCl elution solution, with the optimized (*n* = 5) elution profile shown in Table 3.

**Table 3.** Optimized <sup>165</sup>Er elution profile bDGA EXC column.

