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Comprehensive and Detailed Explanation: Most Tc-99m kits contain stannous (tin II) chloride as the reducing agent. Pertechnetate (Tc-99m in the +7 oxidation state) does not readily bind to most ligands. Stannous ion reduces technetium to a lower oxidation state (typically +4 or +5), allowing chelation with the ligand (e.g., MDP, DTPA, sestamibi). Excess stannous ion must be carefully controlled; too little results in free pertechnetate, while excessive amounts can cause colloidal formation or instability. Ascorbic acid is sometimes used as an antioxidant or secondary reducing agent but is not the primary reductant in standard kits. Sodium pertechnetate is the starting material eluted from the generator. Aluminum oxide is the column material in the Mo-99/Tc-99m generator. Proper kit preparation technique, including correct volume of eluate, mixing, and incubation time, is critical for radiochemical purity and patient safety.