Molybdenum-99/technetium-99m generators

Technetium-99m (half-life about 6 hours, too short to transport far) is generated on demand from its parent molybdenum-99 (half-life about 66 hours) using column chromatography. Fission-produced 99Mo, as molybdate (MoO4^2-), is adsorbed onto an acid-washed alumina (Al2O3) column, typically 2-10 g of alumina, inside a lead-shielded housing. As 99Mo decays it forms pertechnetate (TcO4^-), which, carrying only a single negative charge, binds the alumina far more weakly than molybdate. Passing sterile 0.9% saline through the column selectively elutes sodium pertechnetate while molybdate stays bound, typically recovering more than 80% of the theoretical available 99mTc per elution ('milking'). Because 99mTc regenerates continuously from ongoing 99Mo decay, transient equilibrium is reached roughly 23-24 hours after the previous elution, which is when generators are normally milked for maximum yield.

Eluate quality is governed by US NRC 10 CFR 35.204 (99Mo breakthrough limit of 0.15 microcurie of 99Mo per millicurie of 99mTc at administration) and by aluminium-ion breakthrough limits (commonly under 10 microgram/mL), both checked at every elution because aluminium interferes with downstream cold-kit labelling, especially sulfur colloid. A generator is used for around one to two weeks before its 99Mo activity decays away, after which it is stored for roughly ten half-lives (about 28 days) before disposal as low-level waste. This generator underpins essentially all Tc-99m-based bone, cardiac, renal, hepatobiliary and sentinel-node imaging performed worldwide.