Stable isotopes

Where Found

Stable isotopes comprise the bulk of the material universe. Some elements are found in only a single form, while others have several isotopes. For study and application, it is necessary to separate the various isotopes from one another. A number of methods have been developed to accomplish separation.

Primary Uses

Analysis of stable isotopes and isotopic composition is used extensively in a wide variety of fields. These include soil and water analysis, plant tissue analysis, determination of metabolic pathways in plants and animals (including humans), archaeology, forensics, the geosciences, and medicine.

Technical Definition

An isotope is one of two or more species of atom that have the same atomic number (number of protons) but different mass numbers (number of protons plus neutrons). Stable isotopes are those that are not radioactive. Because the chemical properties of an are almost exclusively determined by atomic number, different isotopes of the same element will exhibit nearly identical behavior in chemical reactions. Subtle differences in the physical properties of isotopes are attributable to their differing masses.

Description, Distribution, and Forms

There are approximately 260 stable isotopes. While most of the 81 stable elements that occur in nature consist of a mixture of two or more isotopes, twenty occur in only a single form. Among these are sodium, aluminum, phosphorus, and gold. At the other extreme, the element tin exhibits ten isotopic forms. Two elements with atomic numbers less than 84, technetium and promethium, have no stable isotopes. The atomic weight of an element is the weighted average of its isotope masses as found in their natural distribution. For example, boron has two stable isotopes: boron 10 (an isotope with mass number 10), which accounts for 20 percent of naturally occurring boron, and boron 11, which accounts for 80 percent. The atomic weight of boron is therefore (0.2) (10) + (0.8) (11) = 10.8. In those elements that have naturally occurring isotopes, the relative abundance of the various isotopes is found to be remarkably constant, independent of the source of the material. There are cases in which abundances are found to vary, and these are of practical interest.

History

In the early part of the twentieth century, the discovery of radioactivity, radioactive elements, and the many distinctly different products of radioactive decays showed that there were far more atomic species than could be fit into the periodic table. Although possessing different physical properties, many of these species were chemically indistinguishable.

In 1912, Joseph John Thomson, discoverer of the electron, found that when a beam of ionized neon gas was passed through a properly configured electromagnetic field and allowed to fall on a photographic plate, two spots of unequal size were exposed. The size and location of the spots were those that would be expected if the original neon consisted of two components—about 90 percent neon 20 and 10 percent neon 22. Later Francis William Aston improved the experimental apparatus so that each isotope was focused to a point rather than smeared out. The device he developed, known as a mass spectrograph, allows much greater precision in the determination of isotope mass and abundance.

Obtaining Isotopes

All methods for separating stable isotopes are based on mass difference or on some isotopic property that derives from it. The difficulty of isotope separation depends inversely upon the relative mass difference between the isotopes. For example, the two most abundant isotopes of hydrogen are ordinary hydrogen (hydrogen 1) and deuterium (hydrogen 2). These isotopes have a relative mass difference of (2-1)/1 = 1, or 100 percent. The mass difference between chlorine 35 and chlorine 37, by contrast, is only (37-35)/35 = 0.057, or 5.7 percent.

There are two types of separation methods. The only single-step method is electromagnetic separation, which operates on the principle that the curvature of the path of a charged particle in a magnetic field is dependent on the particle mass. This is the same principle on which the mass spectrograph is based. Though it is a single-step technique, the amount of material that can be separated in this way is extremely small. All other processes result in a separation of the original material into two fractions, one slightly enriched in the heavier isotope. To obtain significant enrichment, the process must be repeated a number of times by cascading identical stages. Such multistage methods include gaseous centrifugation, aerodynamic separation nozzles, fractional distillation, thermal diffusion, gaseous diffusion, electrolysis, and laser photochemical separation. For example, in centrifugation a vapor of the material to be separated flows downward in the outer part of a rotating cylinder and upward in the center. Because of the mass difference, the heavier isotope will be concentrated in the outer region and can be removed to be enriched again in the next stage.

Uses of Stable Isotopes

Most stable isotope applications are based on two facts. First, isotopes of a given element behave nearly identically in chemical reactions. Second, the relative abundances of isotopes for a given element are nearly constant. The three principal types of applications are those in which deviations from the standard abundances are used to infer something about the environment and/or history of the sample, those in which the isotopic ratio of a substance is altered so that the substance may be traced through a system or process, and those in which small differences in the physical properties of isotopes are used to understand process dynamics.

As an example of the first type of application, consider that the precise isotopic composition of water varies with place and time as it makes its way through the Earth’s complex hydrologic cycle. Knowledge of this variation allows for the study of storm behavior, identification of changes in global climatic patterns, and investigation of past climatic conditions through the study of water locked in glaciers, tree rings, and pack ice. The cycling of nitrogen in crop plants provides an example of stable isotope tracer methods. Fertilizer tagged by enriching (or depleting) with nitrogen-15 is applied to a crop planting. Subsequent analysis makes it possible to trace the quantities of fertilizer taken up by the plants, remaining in the soil, lost to the by denitrification, and leached into runoff water.

Bibliography

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