The pertechnetate ion () is an oxyanion with the chemical formula . It is often used as a convenient water-soluble source of isotopes of the radioactive element technetium (Tc). In particular it is used to carry the 99mTc isotope (half-life 6 hours) which is commonly used in nuclear medicine in several nuclear scanning procedures.
A technetate(VII) salt is a compound containing this ion. Pertechnetate compounds are salts of technetic(VII) acid. Pertechnetate is analogous to permanganate but it has little oxidizing power. Pertechnetate has higher oxidation power than perrhenate.
Understanding pertechnetate is important in understanding technetium contamination in the environment and in nuclear waste management.
Chemistry
is the starting material for most of the chemistry of technetium. Pertechnetate salts are usually colorless. is produced by oxidizing technetium with nitric acid or with hydrogen peroxide. The pertechnetate anion is similar to the permanganate anion but is a weaker oxidizing agent. It is tetrahedral and diamagnetic. The standard electrode potential for / is only +0.738 V in acidic solution, as compared to +1.695 V for /. Because of its diminished oxidizing power, is stable in alkaline solution. is more similar to . Depending on the reducing agent, can be converted to derivatives containing Tc(VI), Tc(V), and Tc(IV). In the absence of strong complexing ligands, is reduced to a +4 oxidation state via the formation of hydrate.
Preparation of 99mTcO4−
is conveniently available in high radionuclidic purity from molybdenum-99, which decays with 87% probability to . The subsequent decay of leads to either or . can be produced in a nuclear reactor via irradiation of either molybdenum-98 or naturally occurring molybdenum with thermal neutrons, but this is not the method currently in use today. Currently, is recovered as a product of the nuclear fission reaction of , separated from other fission products via a multistep process and loaded onto a column of alumina that forms the core of a / radioisotope generator.
As the continuously decays to , the can be removed periodically (usually daily) by flushing a saline solution (0.15 M NaCl in water) through the alumina column: the more highly charged is retained on the column, where it continues to undergo radioactive decay, while the medically useful radioisotope is eluted in the saline. The eluate from the column must be sterile and pyrogen free, so that the Tc drug can be used directly, usually within 12 hours of elution. In a few cases, sublimation or solvent extraction may be used.
Examples
A complex that can penetrate the blood–brain barrier is generated by reduction of with tin(II) in the presence of the ligand hexamethylpropylene amine oxime (HMPAO) to form TcO-D,L-HMPAO. A complex that for imaging the lungs, Tc-MAA, is generated by reduction of with in the presence of human serum albumin. , which is both water and air stable, is generated by reduction of with carbon monoxide. This compound is a precursor to complexes that can be used in cancer diagnosis and therapy involving DNA-DNA pretargeting.
Compounds
Reactions
Radiolysis of in nitrate solutions proceeds through the reduction to which induces complex disproportionation processes:
Pertechnetate can be reduced by H2S to give Tc2S7. Pertechnetate is also reduced to Tc(IV/V) compounds in alkaline solutions in nuclear waste tanks without adding catalytic metals, reducing agents, or external radiation. Reactions of mono- and disaccharides with 99m yield Tc(IV) compounds that are water-soluble.
Uses
Pharmaceutical use
The half-life of is long enough that labelling synthesis of the radiopharmaceutical and scintigraphic measurements can be performed without significant loss of radioactivity. The energy emitted from is 140 keV, which allows for the study of deep body organs. Radiopharmaceuticals have no intended pharmacologic effect and are used in very low concentrations. Radiopharmaceuticals containing are currently being applied in the determining morphology of organs, testing of organ function, and scintigraphic and emission tomographic imaging. The gamma radiation emitted by the radionuclide allows organs to be imaged in vivo tomographically. Currently, over 80% of radiopharmaceuticals used clinically are labelled with . A majority of radiopharmaceuticals labelled with are synthesized by the reduction of the pertechnetate ion in the presence of ligands chosen to confer organ specificity of the drug. The resulting compound is then injected into the body and a "gamma camera" is focused on sections or planes in order to image the spatial distribution of the .