Tracer Compounds

chemical compounds containing atoms of one or more elements whose isotopic composition differs from the main naturally occurring composition. The isotopes may be stable or radioactive, and their position in the molecules of the tracer compounds must usually be strictly defined.

The most frequently used isotopes (the type of radiation and half-life are given in parentheses) include D (stable), T ( β; 12.26 years), ¹³C (stable), ¹⁴C ( β; 5,730 years), ¹⁵N (stable), ¹⁸O (stable), ³²P ( β; 14.3 days), ³⁵S ( β; 87.2 days), ³⁶C1 ( β; 3.03 × 10⁵ years), ⁷⁹Br (stable), and ⁸¹Br ( β and γ; 35.4 hr).

Tracer compounds are produced mainly by chemical synthesis, isotope exchange, or biosynthesis. The chemical synthesis of such compounds is usually done by ordinary methods of preparative chemistry, with observation of the safety requirements for working with radioactive isotopes. The high cost and shortage of isotopic raw material dictate the use of micro and semimicro methods in the synthesis of tracer compounds. All the details of the synthesis, including the use of reaction byproducts, are carefully worked out on a model synthesis with an analogous raw material that does not contain an isotopic tracer. The quantity of substances used in the simulated and actual syntheses should be identical. The handling of tracer compounds containing radioactive tracers requires measures for protection from radiation.

Liquid or water-soluble waste products of the synthesis or washing of the reaction vessel are diluted with water to a specific activity of 0.1–1.0 microcurie and discharged into the common sewage system. Liquid waste products with a high specific activity, which require much greater volumes of water for sufficient dilution, as well as solid waste products, broken glassware, and used filter paper, are stored in safety containers and are removed for burial in accordance with the sanitary rules for work with radioactive materials.

The position of the isotope in the molecule of a tracer compound is usually indicated after the name of the compound. The superscript indicating the mass number of the isotope is to the left of the symbol for the element, and the number of atoms of a given isotope in the molecule is given by a subscript to the right of the symbol. For example, ¹⁴CH₃¹³COOH is acetic acid-1-¹³C-2-¹⁴C, and H₂NCH₂CH₂CH(¹⁵NH₂)COOH is α, γ-diaminobutyric acid- α-¹⁵N. The position of the isotope in the molecule is sometimes given by an asterisk rather than a mass-number superscript. In some cases, the position of the tracer may be given unequivocally only by a verbal explanation—for example, 0-C₆H₄(CH₂COOH)CH₂CH₂¹⁴COOH is o-phenyleneacetic- β-propionic acid (with ¹⁴C in the carboxyl group of the propionic acid radical).

Tracer compounds are widely used as isotope tracers.