Tritium
More than 30 years ago, when deuterium (heavy hydrogen) was first discovered, heavy water (D₂O) was used for the determination of total body water. A small sample of heavy water was given either intravenously or orally, and time was allowed for it to mix uniformly with all the water in the body (about 4 to 6 hours). A sample was then obtained of the mixed water and analyzed for its heavy water content. This procedure was useful but it was hard to make an accurate analysis of low concentrations of heavy water.
More recently, however, tritium (³H) (radioactive hydrogen) has been produced in abundance. Its oxide, tritiated water (³H₂O), is chemically almost the same as ordinary water, but physically it may be distinguished by the beta rays given off by the tritium. This very soft (low-energy) beta ray requires the use of special counting equipment, either a windowless flow-gas counter or a liquid scintillator, but with the proper techniques accurate measurement is possible. The total body water can then be computed by the general isotope dilution formula used for measuring blood plasma volume.
The total body water is determined by the dilution method using tritiated water. This technician is purifying a urine sample so that the tritium content can be determined and the total body water calculated.