Phosphorus-32, abbreviated as 32P when written and “P-32” when spoken, is a common radioactive isotope of phosphorus that is used in scientific research and medical treatment. It is created artificially by subjecting phosphorus-31 to neutron capture, forcing it to absorb a neutron. 32P emits high energy beta particles with a maximum energy of 1.71 MeV that can travel 20 feet. While emitting these particles 32P atoms decay to stable, nonradioactive sulfur atoms.
32P is the third most common radioactive isotope used in research behind hydrogen-3 (tritium) and carbon-14. It has two major benefits that make it particularly useful. First, it has a half-life of 14.3 days. This half-life is long enough to perform experiments that take place over days to weeks while retaining radioactivity, but short enough to decay to safe levels in ten half-lives (143 days). In comparison, 3H and 14C have a half-life of roughly 12 and 5700 years, respectively. Second, 32P is a beta emitter so it can easily be monitored by a Geiger counter to check for spills and prevent contamination. 3H and 14C must be monitored by wiping the area and checking for radioactivity in a scintillation counter, a labor and time consuming step.
Even though the beta particles have a high amount of energy, they cannot pass through the layer of dead cells on the skin's surface. This means that there is a minimal exposure risk to the radiation unless 32P is directly applied to the skin or ingested. Most of the risk is eliminated by wearing a lab coat and gloves when working with the isotope. Scientists also work behind a clear plastic shield several centimeters thick to block particles. Liquid and solid waste that contains 32P is kept separate from normal trash and monitored for 10 half lives until the radioactivity is gone.
32P has a long history and its first use can be traced back to the 1920s. It was experimented with in the 1930s and 1940s to diagnose and treat several cancers, including leukemia and brain cancer. It was also used in the famous Hershey-Chase experiment in 1952 that proved that DNA, not protein, is the carrier of genetic information. Today 32P is occasionally used to treat some rare cancers but it is not commonly administered due to its ability to cause leukemia. It is also a treatment for polycythaemia vera, a condition where excessive amounts of red blood cells are produced.
32P is an important isotope used in scientific research. Animal or human cells or live animals are treated with 32P, which is incorporated into macromolecules that contain phosphorus. Researchers can then analyze these macromolecules by detecting the radioactivity. Such macromolecules include:
- Proteins, which can be phosphorylated at serine, threonine, and tyrosine amino acid residues. Many proteins are phosphorylated under certain conditions, such as a point in the cell cycle, and the phosphorylation can affect the protein’s activity, interactions, or location within the cell. 32P is often used to monitor the amount and conditions of phosphorylation, as well as to determine exactly where the phosphorylation sites are on the protein.
- DNA and RNA, which contain phosphorus in the phosphodiester bonds in their backbones. Fragments of DNA and RNA are often labeled in vitro with 32P and then analyzed by Southern or Northern blot to check for certain sequences of DNA or RNA or analyzed by EMSA to look at DNA or RNA interactions with proteins.
- Lipids, some of which contain phosphorus and are called phospholipids. Phospholipids are a major component of the cell membrane and can be labeled with 32P.