292 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the lead atoms of lead tetraphenyl in amyl alcohol solutions. Around that time also tremendous strides were taken in studying the problem of separating out minute amounts of material by means of macroscopic precipitates with a high degree of accuracy through the medium of the natural radioactive substance, ra- dium. Someone has said, and I like this definition the best, that isotopes are twins that look and act alike but which are different in weight. Iso- topes have the unique ability that they can be followed in a specific batch of atoms through a compli- cated system irrespective of all the other atoms present and of all the chemical processes that may be going on. In terms of physics we are not just a batch of atoms but a batch of "stable isotopes," although precisely speaking this is not en- tirely true since there is a certain small percentage of unstable radio- active carbon (Carbon TM) in all of us. This fact is made use of in the more recently publicized re- ports which describe the dating of relics by radiocarbon analysis (2). PRODUCING RADIOACTIVE ISOTOPES Essentially, isotopes are produced by the interaction of nuclear par- ticles such as protons, deuterons, or neutrons with the nuclei of stable atoms. In order for this interac- tion to take place stable atoms must be bombarded by "accelerated" particles to penetrate the powerfully charged nucleus. Atom smashers such as the linear accelerator and the cyclotron produced limited quan- tities of isotopes before Oak Ridge. However, the efficiency of these cannot be compared to that of present-day piles. It would take five cyclotrons one year to produce one millicurie of Carbon TM at a cost of around one million dollars. One can buy Carbon TM produced at Oak Ridge, for around $30 per millicurie, and I might add now that the average tracer experiment uti- lizes less than one-thousandth this quantity of Carbon TM. Future pile design promises to reduce the above- mentioned cost even further. The following represent various modes of formation of some of the common isotopes: (a) N TM -1 t- n --• C •t -1 t- p Nitrogen (atomic mass 14) q- 1 neutron --• Carbon (atomic mass 14) q- 1 proton (b) C135 + n --• P3• + a Chlorine (atomic mass 35) q- 1 neutron --• Phosphorus (atomic mass 32) + 1 alpha particle (c) S a• + n -• pa• + p Sulfur (atomic mass 32) q- 1 neutron -• Phosphorus (atomic mass 32) q- 1 proton (b) and (c) illustrate two possible modes of formation of Phosphorus 32. Actually there are several more possibilities. There is hardly an element that does not have one or more radio- active isotopes. However, not all radioisotopes are useful for tracer studies since some lose their radio- activity so fast that they decay to stable atoms before they even leave the pile.

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