According to the NRC, which radionuclide impurity concentration must be determined after the first elution of a 99Mo/99mTc generator?

The correct choice is that the concentration of aluminum (Al^3+) must be determined after the first elution of a 99Mo/99mTc generator. This is essential because aluminum ions can be present as a result of the leaching from the generator system and can adversely affect the quality of the radiopharmaceutical preparations.

Monitoring aluminum concentration is critical since it can impact the efficacy and safety of the radiopharmaceuticals, particularly regarding the pH and stability of 99mTc radiopharmaceuticals. Elevated levels of aluminum can cause adverse reactions in patients, potentially leading to complications such as allergic reactions or interference with the binding of radiotracers to target tissues.

While determining the concentration of 99Mo, free pertechnetate, and 99mTc-NaCl are also important for assessing the generator's performance and ensuring quality control, the specific focus on aluminum ion concentration is mandated by regulatory standards to ensure patient safety and compliance.