Improved synthesis methods are provided for preparing conjugates of proteins and chelating agents. In the synthesis methods, non-stable bonds are hydrolyzed after the conjugate is synthesized, for example by treating the conjugate with a quenching agent. The synthesis method provides conjugates that are substantially free of non-stable bonds between the protein and the chelating agent, such that the chelating agent is less likely to dissociate with the conjugate after the conjugate has been stored for extended periods. The conjugate may be useful for therapeutic or diagnostic methods.