Research Isotopes
Most people are familiar with radioisotopes that are used in the diagnosis and treatment of cancer. However, both stable and radioactive isotopes—and compounds labeled with these isotopes— are used in many research applications that provide insight into such far-ranging topics as global warming, the effects of acid rain, and human metabolic activity. Some of these applications are discussed here.
Some Uses of Stable Isotopes for Research
Radioisotopes are of course unstable and, depending on the isotope, may decay through a number of modes: spontaneous fission, alpha decay, beta decay, electron capture, and isomeric transition, to name the dominant modes of decay. Regardless of the mode, nearly all decays are accompanied by the emission of x-rays, gamma rays, or sometimes neutrons. It is the detection of this emitted radiation that allows them to be effective tracers. Stable isotopes, on the other hand, by definition do not decay and therefore do not emit any radiation. Their presence must be detected by means of a mass spectrometer. A stable isotope ratio mass spectrometer (IRMS) is one common type of spectrometer that quickly quantifies how much of a given isotope is present in a sample. The benefit of using stable isotopes as tracers is that they pose no radiation hazard and can often be used in noninvasive diagnoses.
One widely used technique to detect the bacterium Helicobacter pylori in the human stomach by means of IRMS is to have the patient ingest urea labeled with carbon-13, one of the two stable isotopes of carbon (the other is carbon-12). This bacterium, if present, metabolizes the urea thereby producing carbon-13 labeled carbon dioxide (13-CO2), which is subsequently exhaled. Carbon dioxide is a natural product of respiration, and the natural ratio of carbon-13 to carbon-12 is 0.0111. By a mass spectrometric analysis of the breath, should this ratio be significantly greater than 0.0111, then this is a good indication of the presence of Helicobacter pylori. Otherwise, the carbon-13 is digested and eliminated through the urine. This noninvasive procedure is readily preferred over more traditional techniques such as taking a sample of the patient's stomach lining.
All electro-magnetic mass spectrometers employ the same basic physics. The atoms of the element are first given an electric charge (made into ions), then linearly accelerated in an electric field. These ions are then introduced into a magnetic field perpendicular to their direction of motion. That is, the magnetic field applies a force to the ions, but because it is at a right angle to the direction of motion, the trajectory of the ion is bent into a circle. The radius of curvature depends on the charge to mass ratio of the ion. Thus, if two different isotopes of the same element have the same charge, the heavier isotope will have a smaller radius of curvature. The isotopes are thereby separated, and cups can be strategically located to collect the separated isotopes.
Another example that employs using two isotopes of calcium offers insight into calcium retention by the human body. The technique uses calcium-42 and -44. A calcium-42-labeled compound is given intravenously to the patient and eliminated rather readily through the urine. Simultaneously, a calcium-44-labeled compound is ingested by the patient. This has to be metabolized and passed through the intestinal tract, only to be absorbed into the blood stream, then eliminated through the urine. The ratio of calcium-44 to -42 as determined by mass spectrometry will tell the researcher how long calcium is retained by the body before it is eliminated. Obviously, this technique can be applied to many biologically important elements significant not only to humans but to other organisms as well.
Some Uses of Radioisotopes for Research
As you will recall, a radioisotope is unstable; its presence can be ascertained and quantified by detecting the emitted radiation. An example of a radiotracer important to oceanographic and climate research is silicon-32, a beta emitter.
The element silicon is taken up by microscopic organisms known as diatoms found in great abundance in the world's oceans. Some diatoms have siliceous exoskeletons, and when they die, they sink to the ocean bottom. (Diatoms are also important to paleontologists, as they are an aid in locating oil deposits.) All diatoms are photosynthetic and convert atmospheric C02, O2, and N2 into usable organic molecules much in the same way terrestrial plants do. In this regard, these siliceous diatoms are very important to the ocean's carbon cycle. Carbon is fixed by the diatom at the surface and then transported to the deep ocean bottom when it dies. A similar process takes place with regard to the nitrogen cycle.
Unfortunately much of the C02 in the atmosphere is generated by human activity. This molecule is a great reservoir of solar energy that leads to the so-called "greenhouse effect." Many scientists believe an enhanced level of C02 in the atmosphere is one contributor–perhaps the dominant contributor–to global warming. If the ocean is unable to remove C02 from the atmosphere and sink the carbon to its bottom, then there should be even more concern about global warming and its negative effects.
How quickly the diatoms sink affects the rate at which carbon is stored by the deep ocean and the rate at which C02 is removed from the atmosphere. Diatoms sink faster when they are heavier, that is, when their silicon to carbon (or nitrogen) ratio is higher than other diatoms that do not have ratios as high. One factor that may affect these ratios—and thereby the ocean's ability to store carbon and nitrogen—is the concentration of iron in the seawater to which the diatoms are exposed.
Researchers have found that how much iron present in diatoms can significantly alter silicon uptake. (Seawater iron concentration varies around the world, and how much iron incorporated by diatoms also varies.) One such study found that when diatoms were grown under iron-deficient conditions, the silicon/carbon ratio increased when compared to noniron-deprived diatoms. Similar analyses were done with respect to zinc deprivation.
Returning to the importance of various isotopes to research, the silicon to carbon (or nitrogen) ratio as a function of iron concentration is found by "feeding" silicon-32 to the diatoms and varying the amount of iron present when they are grown. Counting the beta decays with a liquid scintillator is somewhat complicated because silicon-32 beta decays to phosphorus-32, which is also a beta emitter. These two energy spectra unfortunately overlap making it difficult to identify which electron came from which emitter when looking at the energy spectrum generated by the liquid scintillator. Fortunately, nature has made the endpoint energy of phosphorus-32 significantly higher than silicon 32, so that if one first calibrates the detector with a pure phosphorus-32 source, then counts silicon-32/phosphorus-32 decays from diatoms, and looks in that energy region above the endpoint energy of silicon-32, one can measure how much phophorus-32 is present. From this, one can then determine how much silicon was taken up by the diatoms at one iron concentration compared to another. This method works only if the parent and daughter radioisotopes are in equilibrium, that is, sufficient phosphorus-32 has grown in to decay at the same rate as the silicon.
So, the seemingly unrelated isotope silicon-32 has much to say about the ocean's carbon cycle and global warming.
Another radioisotope used to study the effects of acid rain and possible sources of Alzheimer's disease in humans is aluminum-26. Aluminum is one of the most abundant elements found in the Earth's crust. Acid rain, formed from sulfur dioxide and nitrous oxide emissions, leaches aluminum from the soil where it is normally bound and allows it to be transported to lakes and streams. In sufficiently high concentrations, it is toxic to many aquatic plants and animals (especially fish) and trees. Its pathway to waterways can be modeled through the uses of aluminum-26 as a tracer.
Similarly, aluminum-26 is used as a tracer to study aluminum uptake in mice and humans. High concentrations of aluminum in the brain have been found associated with Alzheimer's disease, though it has not been determined that they are correlated or not. One pathway for uptake is through drinking water. By feeding aluminum-26-spiked water to mice, its uptake, concentration, and retention in the mouse brain can be studied. From this, its uptake and retention in humans can be modeled. A benefit from this study is that in addition to shedding light on any correlation between aluminum and Alzheimer's disease, it may help establish an upper limit to acceptable aluminum concentration in drinking water.
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