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.
Activation Analysis
Another booklet in this series, Neutron Activation Analysis, discusses a new process by which microscopic quantities of many different materials may be analyzed accurately. Neutron irradiation of these samples changes some of their atoms to radioactive isotopes. A multichannel analyzer instrument gives a record of the concentration of any of about 50 of the known elements.
One use of this technique involved the analysis of a hair from Napoleon’s head. More than 100 years after his death it was shown that the French Emperor had been given arsenic in large quantities and that this possibly caused his death.
The ways in which activation analysis can be applied to medical diagnosis are at present largely limited to toxicology, the study of poisons, but the future may bring new possibilities.
Knowledge is still being sought, for example, about the physiological role played by minute quantities of some of the elements found in the body. The ability to determine accurately a few parts per million of “trace elements” in the various tissues and body fluids is expected to provide much useful information as to the functions of these materials.