Isotopic Tracer

I

Introduction

Isotopic Tracer, name applied to an atom of an isotope used to observe the movement of certain materials in chemical, biological, or physical processes. The term tracer is applied commonly to any radioactive isotope employed in tracing the course of nonradioactive substances. In scientific usage, however, the term is also applied to less abundant nonradioactive, or stable, isotopes that are suitable for use in tracer techniques.

Tracers may be used to follow the movement of substances in large or small amounts as well as at molecular or atomic levels. The observations may be made by the measurement of radioactivity in the case of radioactive tracers, or of the relative abundance of isotopes in applications employing stable isotopes as tracers. Instruments used to detect radiation include the electroscope, the scintillation counter, and the Geiger-Müller counter. In investigations using stable isotopes as tracers, the instrument most commonly employed is the mass spectrometer, a device that can determine the relative amounts of various isotopes in a sample of the substance being analysed. Tracers have important applications in many fields of scientific research and in medicine, agriculture, and industry.

II

Bulk-Material Tracing

Although the movement of large masses of solids or fluids may be studied by many diverse methods, including visual observation of tracer dyes, the use of radioactive tracers in bulk-material applications offers such advantages as greater speed of operation, reliability, and convenience. For example, radioactive tracers may be used to mark the boundary between different grades of oil flowing in a pipeline by injecting at the juncture of two types of oil a radioactive material that emits penetrating gamma rays. Radiation detectors placed in the pipeline will detect the gamma rays as this interface passes a given point. The detectors can then be made to operate valves that channel the two grades of oil into different outlets.

Tracers are used in industry to detect microscopic amounts of wear. The lubricating quality of an oil, for example, can be evaluated after prolonged operation in an experimental engine by measuring the amount of wear on piston rings and cylinder walls and by the amount of steel deposits in the oil. Such experiments are time consuming and difficult to apply on a routine basis. In the tracer technique, the piston rings are made radioactive by exposing them to neutrons in a nuclear reactor (see Nuclear Energy). After the engine has been operating for a relatively short period, radioactive material worn from the piston ring can be detected in the oil and walls of the cylinder, and the amount of this material serves as an index for evaluating the quality of the oil.

Radioactive tracers may also be used to control the transfer of dyes in multicolour textile printing. Colour-printing machines consist of several rollers, each furnished with a dye bath of a different colour. One colour may be carried over by the fabric from one roller to the next, thus producing so-called colour soiling of the textiles. If such soiling is not discovered in time, hundreds of yards of valuable fabric may be spoiled. One method of avoiding spoilage is frequent replacement of the dye solutions. To eliminate the necessity for this costly procedure, the offending, or pirate, colour is labelled by adding radioactive phosphate to the dye bath. Contamination of subsequent dye baths with the pirate colour is monitored by radiation detectors, which are dipped automatically into the solutions at frequent intervals. When the pirate colour reaches a critical concentration, the dye bath is replaced by a fresh one.

III

Tracing of Molecules and Atoms

Most substances are compounds consisting of molecules linked together in chemical combination or are mixtures of compounds. Only one type of molecule in a compound may be of importance in a particular tracer application. Especially important in biochemical research is the ability to distinguish within a single compound similar molecules derived from two different sources. This information can be made available by the use of the technique called isotopic labelling.

Tracers are used in botanic and agricultural research to study the absorption of nutrients by plants and to trace metabolic pathways, especially those involved in photosynthesis.

In biological research, molecules labelled with radioactive isotopes have been especially useful in elucidating the metabolic pathways of biochemical synthesis and degradation. The routes of many nutrients and toxins through ecosystems have also been mapped by tracer techniques.

Tracer procedures are used in medical diagnosis and research to measure such functions of organs and tissues as their uptake of hormones, minerals, vitamins, blood or blood components, and drugs. Organ output of hormones or other proteins and wastes can also be measured with great speed and accuracy.

IV

Chemical Research

Procedures of tracing, labelling, and double labelling are of greatest significance in biochemical research, permitting the investigator to follow the paths of breakdown and formation of normal components of the body. The use of such procedures had made it possible to trace the origin of each atom in a complicated molecule such as the haem pigment of haemoglobin, which has the formula Fe(C₃₂H₃₀N₄) (COOH)₂. The carbon atoms have been shown to come from the precursor ethanoic acid CH₃COOH, which contains two groups, each with one carbon atom. In addition, by double labelling it is possible to determine which of the carbon atoms in the haem pigment come from the COOH group of the ethanoic acid molecule and which come from the CH₃ group. The nitrogen in the haem-pigment molecule is derived from the intermediate compound aminoethanoic acid.

In organic-chemical research, tracers have been used to investigate many chemical reactions involving the migration and rearrangement of atoms and groups of atoms. Labelling and double-labelling procedures have demonstrated the mechanism of some obscure and complicated reactions and have revealed that presumedly simple reactions were often more involved.

In inorganic chemistry, tracers have made possible the study of systems in which no net chemical reaction occurs but in which, for example, two oxidation states of the same element are present. Tracer techniques have shown that, without net chemical reaction, an interchange of atoms takes place between such forms as cobalt(II) and cobalt(III). Such interchanges, which are known as exchange reactions, are a logical extension of the chemical principle of dynamic equilibrium. See Nuclear Physics.