Other isotopic research has focused on the geographic origins of things. Various stable isotope ratios in combination, sometimes including the isotopes of heavy elements like strontium and lead, have been used to determine whether the residents of ancient cities were immigrants or were born locally; to trace the origins of poached ivory and rhino horn to break up smuggling rings; and to determine the agricultural origins of cocaine, heroin, and the cotton fiber used to make counterfeit $100 bills. The latest wrinkle involves the potential use of hydrogen and oxygen isotopes in hair to determine where a person has been lately, by comparing the measurements with an international database for rainfall and local water supplies.
How Stable Isotopes Work
All of the earth and its atmosphere is made up of atoms of different elements, such as oxygen, carbon, and nitrogen. Each of these elements has several forms, based on their atomic weight (the number of neutrons in each atom). For example, 99 percent of all carbon exists in the form called Carbon 12 (written as 12C):
it has twelve neutrons in its nucleus. [Stable isotope researcher Eleanora Reber corrects me: "Carbon-12 has an atomic weight of 12, which is made up of 6 protons and 6 neutrons. The 6 electrons don't really count towards the weight, as they're so light. Carbon-13 still has 6 protons and 6 electrons, but has 7 neutrons, and Carbon-14 has 6 protons and 8 neutrons, which is basically too heavy to hold together in a stable way, so it is radioactive." Thanks to Kate for pointing out this discrepancy.]
But, one percent of the time, carbon exists in the form of Carbon 13 (13C), which has
thirteen neutrons six protons and six electrons but seven neutrons in its nucleus. Both forms react the exact same way (if you combine carbon with oxygen you get carbon dioxide, no matter what the number of neutrons). In addition, both forms are stable, that is to say, they don't change over time. (There are other forms that do change, and a good thing too: Carbon 14 decays and we can use its ratio to Carbon 13 for radiocarbon dating, but that's another issue entirely).
Carbon in the Food Chain
The ratio of Carbon 12 to Carbon 13 is constant in earthís atmosphere. There are always 100 12C atoms to 1 13C atom. During the process of photosynthesis, plants absorb the carbon atoms in earthís atmosphere, water, and soil, and store them in the cells of their leaves, fruits, nuts, and roots. But as a result of the photosynthesis process, the ratio of the forms of carbon gets changed as it is being stored. The alteration of the chemical ratio is different for plants in different parts of the world. For example, plants that live in regions with lots of sun and little water have relatively fewer 12C atoms in their cells (compared to 13C) than do plants that live in forests or wetlands. This ratio is hardwired into the plantís cells, and, hereís the best part, as it gets passed up the food chain (i.e., the roots, leaves, and fruit are eaten by animals and humans), the ratio remains virtually unchanged.
In other words, if you can determine the ratio of 12C to 13C in an animal, you can figure out what kind of climate the plants it ate during its life came from. The measuring takes mass spectrometer analysis; but thatís another story.
Not Just Carbon
Carbon is not by a long shot the only element used by stable isotope researchers. Currently researchers are looking at measuring the ratios of stable isotopes of oxygen, nitrogen, strontium, hydrogen, sulfur, lead, and many other elements that are processed by plants and animals and lead a simply incredible diversity of human and animal dietary information.
- The Stable Isotope Story
- History of Stable Isotope Analysis
- Modern Applications
- More Stable Isotope Resources