Isotopes are variants of chemical elements: while all isotopes of a given element contain the same number of protons, they vary in the number of neutrons they contain. Variation in neutron number produces chemically identical atoms with different masses, which can be exploited to allow tracing of specific individual atoms through a system.
Stable isotopes are generally defined as non-radioactive isotopic elements that do not decay over time. Radioactive isotopes may also be classified as stable isotopes when their half-lives are too long to be measured. These elements can often be found to occur in nature and include isotopes of carbon, nitrogen, hydrogen, oxygen, noble gases and metals. For example, three naturally occurring isotopes of hydrogen include protium (H) having one neutron, deuterium (2H) having two neutrons, and tritium having three neutrons.
Isotopically labeled compounds
Isotopically labelled compounds are compounds that incorporate isotopic elements within their molecular structure and are thus "labelled" by the isotope. These compounds are used to study chemical and biochemical reactions, metabolic pathways or cellular transport. Specifically, isotope-labelled compounds are routinely used for a variety of applications including magnetic resonance imaging (MRI), spectroscopy, nuclear magnetic resonance (NMR), and geochemical analyses. An example of using an isotopic label includes replacing the most common isotope of hydrogen, protium, with deuterium to observe hydrogen exchange reactions in water.
Isotope separation methods
Distillation and Diffusion Processes
Distillation or diffusion are processes which are for enrichment used when there are relatively large mass differences between different isotopes of an element.
Centrifuge processes for enrichment includes gas centrifugation and improvements upon the gas centrifuge techniques. This process involves rotating cylinders in order to move the heavier gas molecules containing a given isotope to the outer radius of the cylinder while collecting the lighter gas molecules containing the given isotope in the center of the cylinder.
Electromagnetic isotope separation processes involves first vaporizing the isotope containing molecules followed by ionizing the vapor with positively charged ions. A mass spectrometer, known as the Calutron, is then used to redirect a stream of cations onto a target for collection.
Other methods: laser enrichment, photochemical enrichment and plasma separation
Laser enrichment processes provide for lower energy inputs and thus more economical enrichment. One method currently under investigation is known as the Separation of Isotopes by Laser Excitiation (SILEX). Another laser method that is used to enrich uranium containing the 235U atom is known as molecular laser isotope separation (MLIS) which involves using infrared laser at UF6 molecules and a second laser to free a fluorine atom resulting in precipitation of the remaining UF5 compound out of the gas.
Plasma separation involves the principle of ion cyclotron resonance and uses superconducting magnets to energize a given isotope in plasma consisting of an ionic mixture.
Biochemical markers and probes are used to research the uptake of compounds by the body. For example, nutritional studies are commonly performed using isotopic labeled compounds.
Brain and kidney function
Studies of brain and kidney function are performed by tracing isotopes throughout these organs for both diagnostics and treatment applications.
Precursors for therapeutic radioisotopes or radiation therapy are used for a variety of therapies. For example, neuroendocrine tumors are treated by radiotherapy using hormone bound lutetium-177 and yttrium-90.
Tracing drug metabolism requires the use of isotopes bound to drug in order to understand the processing of the given pharmaceutical by the body.
A wide range of biochemical processes can be studied using stable isotopes. For example, a technique known as stable isotope labeling by amino acids in cell culture (SILAC) is used in proteomics research to help identify disease biomarkers.
The use of isotope labeling allows chemists to study the mechanisms of chemical reactions, as individual atoms can be followed through a system.
Isotopes are valuable for studying release and spread of pollutants in the environment.
The tracing of isotope movement, either within a local system such as an estuary or on a global scale, can allow study of circulation patterns.
Various compounds labeled with nitrogen-15 are used in the study of processes such as plant metabolism and fertilizer uptake.