How Isotopes Are Measured

Atoms are instantly recognizable -- tiny clusters of lumpy protons and neutrons orbited by even smaller whizzing electron satellites. The most basic atom is hydrogen and it doesn't even have a neutron, just one proton and one electron. Except, of course, when that proton gets a little friend.

Two's Company

Hydrogen has two isotopes: deuterium and tritium. Unlike regular hydrogen, deuterium has one neutron and tritium has two. And hydrogen isn't alone, most elements have isotopes although the number of isotopes varies greatly depending on the element.

The additional neutrons don't greatly affect the chemical behavior of any particular atom, they usually just give it a little more mass. It's only when an isotope is unstable -- when it's radioactive and goes through the process of radioactive decay, spontaneously splitting and releasing energy to achieve a more stable state -- that it makes a great deal of difference how many neutrons an atom has.

As isotopes decay they break down into simpler, more stable isotopes, referred to as daughter elements or daughter nuclei. For example, potassium-40 decays into argon-40. Uranium-235, commonly used in nuclear bombs, eventually decays into lead-207 -- although considering uranium-235 has a half-life of 713 million years, it's not really a process worth waiting around for.

Three's a Crowd

Isotopes are measured using different methods of mass spectrometry, also known as mass spectroscopy. Basically, it works like this. A sample of an isotope is put into a machine (called either a mass spectrograph or a mass spectrometer) and ionized with a positive charge.

Then the positive ions are coaxed into a tightly focused beam and accelerated past a negative magnetic field, which deflects them. Depending on the atomic mass and positive charge of the isotopes, the level of deflection will vary. The data gathered from this process can detect which isotopes are present and in what quantities.

The Importance of Isotopes

Isotopes are increasingly important for study in a wide range of scientific fields, from chemistry to biology and medicine to astronomy. For example, geologists and biologists can make use of the fact that isotopes are scattered unevenly across the globe to delve into many facets of the world around us.

In addition to allowing scientists to track animal migration, isotopes can be used to gauge peoples' movements as well. Remember how hydrogen has a couple of isotopes? By measuring the level of deuterium and tritium in strands of human hair and comparing them to geographical data -- such as local sources of tap water -- researchers can tell where people have been, which has a number of useful applications.

"We have found significant variations in hydrogen and oxygen isotopes in hair and water that relate to where a person lives in the United States," says researcher Jim Ehleringer, a geology professor at the University of Utah.

His team's study, which was published in February 2008 in the Proceedings of the National Academy of Sciences, found higher concentrations of heavy hydrogen and oxygen in the hair of people living in the southeastern United States and Texas. People living in the northern Rocky Mountains had the lowest concentrations of hydrogen and oxygen isotopes.

This type of information can be used by forensic analysts to determine the origin of unidentified remains and to test alibis. It also has a range of potential uses for archaeologists to drug enforcement agents and everyone in between.

Article posted on April 27, 2009

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