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.
[Illustration: _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._]