TRACER CHEMISTRY 291 the same token, while the amount of scientific knowledge which is at our disposal concerning the phyco- colloids appears fairly considerable, the complexity of the subject is such that many features and many avenues remain to be explored and it may be confidently expected that fields of application for products so far developed and for products still to be developed will be con- stantly widened. Indeed, the in- terest created by these products and the advantages inherent in their use are such that the research laboratories of many manufacturers which are either actual or potential users of these products are today furnishing substantial aid in these scientific investigations. TRACER CHEMISTRY* By ALEX MESHBANE Tracerlab, Inc., New York, N.Y. 'i•HE PRESENT status of tracer chemistry is undoubtedly due to the tremendous efforts made during the last war to produce fissionable material in adequate quantity so that the war might be brought to a rapid end through the use of atom bombs. The first uranium "pile" was an attempt to produce a con- trollable nuclear reaction, where neutrons would be evolved in a chain reacting process. Today we see "piles" springing up all over the country and plans being formulated in foreign countries as well to take advantage of the ever growing de- mand for unstable isotopes and develop means for harnessing the tremendous energy available from controlled nuclear reactions. The discovery of radioactivity by Becquerel in 1895 was rapidly * Presented at the May 15, 1952, Meeting, New York City. taken advantage of by the medical profession who used the ionizing property of radium to effect the course of certain common diseases. The penetrating abilities of the rays from radium were soon em- ployed to study the internal incon- sistencies of various metals by a technique that we commonly refer to as radiography. These two broad applications were in use within a relatively short time after the discovery of radium. The employment of unstable, radio- active elements as tracers did not come into its own until around 1920 mainly because adequate, precise, radiation instruments had not yet been devised to measure these elusive and mysterious rays. In 1920 Hevesey and Zechmeister (1) using Thorium B, which is an iso- tope of lead, showed that there was no exchange between lead ions and

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.