Isotopes are variations of chemical elements containing different numbers of neutrons. Because isotopes are recognizable, they provide an efficient way to track biological processes during experimentation. There are many potential uses for isotopes in experimentation, but several applications are more prevalent.

Each chemical element has a unique number of protons, a fact that gave rise to the periodic table. Similarly, an isotope of any given element has its own unique number of neutrons; the designation of an isotope is determined by the sum of protons and neutrons in the nucleus (referred to as the mass number). An element can have any number of isotopes. For instance, carbon-12 and carbon-13 both have six protons, but the latter contains one additional neutron. Because the number of neutrons in an atom's nucleus has a negligible effect on chemical properties, isotopes provide an efficient means of studying various biological processes without significantly affecting their natural course.

Biogenic substances (those produced by naturally occurring life processes) can have significant variations of carbon, nitrogen and oxygen isotopes, which makes them an easier target for analysis. Food safety applications make it possible to track the country of origin of certain food products, like beef, using carbon and nitrogen isotopes. Agencies and manufacturers are also able to determine the method of feeding for livestock -- organic or conventional -- by analyzing carbon, nitrogen and sulfur isotopes. By studying carbon and oxygen isotope data, it's possible to determine where in the Mediterranean various olive oils come from, and how "natural" fruit juice products are.

Unusual isotopes can be used as markers in chemical reactions. This can be helpful, particularly in the field of cell biology, where research labs like Johns Hopkins University's Pandey Lab are finding new ways to study cancer and other life-threatening conditions. For instance, Stable Isotope Labeling with Amino Acids (SILAC) in cell culture is a process by which sister-cell populations are differentiated in vitro using varying forms of amino acids. The amino acids are incorporated into the proteins being studied and, because they behave identically to one another despite their differing nuclear composition, newly synthesized proteins can be more closely studied alongside their controlled (naturally occurring) counterparts.

Radioactive isotopes are often used to measure the age of materials containing carbon. One popular radioactive dating method is called carbon dating -- the dating of organic materials. Because the life of a radioisotope isn't affected by any influence outside of the nucleus, its predictable rate of decay acts like a clock. Studying radioisotopes in the surroundings of animal fossils, for instance, provides a way of estimating the age of those fossils.