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 (see Radioactivity). In scientific usage, however, the term is applied also to less abundant nonradioactive, or stable, isotopes that are suitable for use in tracer techniques. See Isotope.

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 (see Particle Detectors). 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 analyzed. Tracers have important applications in many fields of scientific research and in medicine, agriculture, and industry.