With this knowledge in mind, further [ 201 Tl]Tl(I) radiolabelling reactions were conducted in water with the addition of small amounts of 1 M Na2CO3 (final pH = 11). As no new peaks could be detected by radioHPLC, we hypothesized that the acidic mobile phase resulted in decomplexation of 5 from [ 201 Tl]Tl(I). Monitoring of a reaction mixture of 5 and [ 201 Tl]Tl(I) under neutral solvent conditions, without inclusion of an ionic solvent modifier, resulted in the observation of a new peak, putatively assigned as [ 201 Tl(5)] + , with a retention time that is clearly distinct from that of unchelated 201 Tl. Unfortunately, under these conditions, we could not obtain reproducible radio-HPLC traces. We attribute this to adsorption of hydrophilic [ 201 Tl(5)] + to silica as in some repeated experiments, radioactive species were retained on the C18 column and did not elute. Such behaviour is sometimes observed in HPLC experiments using mobile phases without inclusion of a buffering or ion pairing reagent or acid. Through further extensive experimentation it was found that addition of 0.01% formic acid to the HPLC mobile So far, all evidence pointed to K222-based chelators complexing Tl(I) under both nonradioactive (stoichiometric) and radioactive (non-stoichiometric tracer) conditions. A crucial benchmark for any radiochelator is not only the ability to bind a given radiometal but stably retain it under physiologically relevant conditions. A common assay to evaluate the stability of radiolabelled bioconjugates is to incubate them with human blood serum over time. We incubated [ 201 Tl(K222-PSMA)] + in human blood serum at 37 °C, and monitored its stability.