Phosphorus-32
Phosphorus-32 is a radioactive isotope of phosphorus, containing one more neutron than the common and stable isotope of phosphorus, phosphorus-31.
Phosphorus is found in many organic molecules, and so, phosphorus-32 has many applications in medicine, biochemistry, and molecular biology where it can be used to trace phosphorylated molecules and radioactively label DNA and RNA.
Decay
Phosphorus-32 has a short half-life of 14.269 days and decays into sulfur-32 by beta decay as shown in this nuclear equation:1.711 MeV of energy is released from this decay. The kinetic energy of the electron varies with an average of approximately 0.5 MeV and the remainder of the energy is carried by the nearly undetectable electron antineutrino. In comparison to other beta radiation-emitting nuclides, the electron is moderately energetic. It is on average stopped by about 1 m of air or 5 mm of acrylic glass.
The sulfur-32 nucleus produced is in the ground state, so there is no nuclear gamma ray emission, which is unusual for a decay of this energy.
Production
Phosphorus-32 has important uses in medicine, biochemistry and molecular biology. It only exists naturally on earth in very small amounts and its short half-life means useful quantities have to be produced synthetically. Phosphorus-32 can be generated synthetically by irradiation of sulfur-32 with moderately fast neutrons as shown in this nuclear equation:The sulfur-32 nucleus captures the neutron and emits a proton, reducing the atomic number by one while maintaining the mass number of 32.
This reaction has also been used to determine the yield of nuclear weapons.
Uses
Phosphorus is abundant in biological systems and, as a radioactive isotope, is almost chemically identical with stable isotopes of the same element. Phosphorus-32 can be used to label biological molecules. The beta radiation emitted by the phosphorus-32 is sufficiently penetrating to be detected outside the organism or tissue which is being analysed.Biochemistry and molecular biology
The metabolic pathways of organisms extensively use phosphorus in the generation of different biomolecules within the cell. Phosphorus-32 finds use for analysing metabolic pathways in pulse chase experiments, where a culture of cells is treated for a short time with a phosphorus-32-containing substrate. The sequence of chemical changes, which happen to the substrate, can then be traced by detecting which molecules contain the phosphorus-32 at multiple time points following the initial treatment.DNA and RNA contain a large quantity of phosphorus in the phosphodiester linkages between bases in the oligonucleotide chain. DNA and RNA can therefore be tracked by replacing the phosphorus with phosphorus-32. This technique is extensively used in Southern blot and Northern blot analysis of DNA and RNA samples respectively. In both cases, a phosphorus-32-containing DNA probe hybridises to its complementary sequence, where it appears in a gel. Its location can then be detected by photographic film.