TRACER CHEMISTRY 293 The half-livest of radioisotopes may be millionths of a second to millions of years. Carbon TM with a half-life of around 5000 years is extremely convenient for the tracer chemist who does not have to rush to his Geiger counter to measure the activity isolated in an experiment. On the other hand, Oxygen TM with a half-life of only 27.0 seconds makes it impossible to use it practically as a tracer. As a matter of fact there is no useful radioactive oxygen for tracer studies. nection with paper chromatography. A clear and permanent record of chromatographic studies is readily available by this method. This method enables the research chem- ist to locate, separate, and deter- mine quantitatively individual chem- icals in complex mixtures. Auto- matic counting and recording de- vices are now available which speed up research to a point never before realized and we all know how valu- able time is in costly research and developmental projects. MEASURING RADIOACTIVITY Basically, all radioactive sub- stances are measured by virtue of their ability to ionize matter. Var- ious nuclear radiation detectors exist and the type to be used de- pends upon the radiation charac- teristics of the isotope in question. Geiger counters and crystal count- ers measure individual nuclear particles, which enter their sensi- tive volume and are used to "count" radioactive samples. Ionization chambers measure total ionization within a fixed volrune and are most often used in personnel monitoring. The autoradiographic'method takes advantage of the sensitivity of film to all forms of radiation. Spe- cial films are available which en- able one to measure radioactivity with a high degree of accuracy and at the same time take a "picture" of the path of certain nuclear parti- cles. The autoradiographic method is especially of interest in con- t Half-life refers to the time in which the activity of an isotope is reduced by one-half. MEDICAL AND BIOLOGICAL USES In medicine, Iodine TM has been used to study the rate of uptake of iodine by the thyroid (3). While the technique is straightforward the interpretation of results is not al- ways simple. The patient is given a so-called "atomic cocktail" con- taining a measured quantity of tracer iodine. At various intervals the radioactivity of the iodine is measured by holding a Geiger counter at a fixed distance from the patient's neck. A graph is then plotted of activity versus time to determine the state of the thyroid. The above method has proved of great value only because the re- tention of iodine is so closely linked to the state of health of the thyroid and the fact that its radioactive twin exists. Let us see why it is possible to use Iodine TM as a tracer, by listing the characteristics that one should look for in an unstable isotope. 1. Half-Life--8 days, which means that it can be shipped from

294 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Oak Ridge without much decay in activity. The radioactive decay is nevertheless fast enough so that it is not likely to become a radiation hazard when it is necessary to give repeated small doses. 2. Radiation Characteristics-- Beta and gamma rays are emitted so that it is possible, and in this case most desirable, to measure the gamma rays which are able to pene- trate intervening tissue to reach the Geiger counter. The beta rays are not counted since they have limited penetrability in matter. 3. Chemical Characteristics-- I TM is chemically identical to ordi- nary iodine, so that the thyroid does not distinguish between the two. Most of us are aware of the use of I TM in the treatment of cancer of the thyroid, Phosphorus •2 in the treatment of certain blood dyscra- sias (4) and the application of col- loidal Gold •ø8 in ovarian tumors, prostate tumors, and metastases (5). Iodine tagged albumin is being used in the determination of plasma volume or total blood volume, and brain tumor localization iodinated globulins for research in antibody formation and diiodothyrosine for studies of thyroid function. The rate at which sodium ex- changes in and out of the eyeball and across blood vessels has been studied with the aid of Sodium% It is difficult to see how one could have discovered the following with- out the tracer method. In 75 seconds after Sodium •4 is injected into one arm it starts being excreted from a sweat gland in the other arm (6). Radioiron has been used to study the effectiveness of whole blood preservatives. The Sulfur •5 and Carbon TM labeled compounds are too numerous to mention. Labeled organic com- pounds of every description are employed every day to delve into the secrets of animal and plant metabolism. A list of these com- pounds and the sources of supply may be obtained from the Atomic Energy Commission (7) and from certain commercial firms. INr)vs•rR•^I• UsEs Let us see what the cosmetic chemist has to learn here. By irradiating piston rings so that part of the natural iron becomes radio- active iron the 1,ubrication value of various oils can be measured in a fraction of time required by older methods and with a much higher degree of accuracy. The transfer of one-millionth of an ounce of metal can be measured in this way. These studies will eventually lead to the design of better Diesel, auto and air engir•es, and lubricating oils (8). In metallurgy Phosphorus a• has been used to study the passage of phosphorus from slag into the mol- ten iron bath and sulfur has been followe.d from the melt into the slag employing Sulfur% Alpha-emitting radioactive sources such as polonium are being used in factories for the dissipation of electrostatic charges where this is a