Stable Isotopes

What are stable isotopes?

Stable isotopes are different forms of the same chemical element. These different forms weigh a slightly different amount, so that some forms weigh more than others. The heavy forms of the element are slightly slower to form new reactions, and so the different forms tend to separate and concentrate in different areas. For example, the heavy isotope of hydrogen in water doesn’t evaporate quite as fast as the light form of hydrogen, and so more heavy isotopes of hydrogen are left in the ocean. Another example is nitrogen – nitrogen is found in protein, which makes up muscles, hair, organs, etc… Proteins are regularly broken down by the body, and the light isotope of nitrogen is “easier” to break down than the heavy isotope of nitrogen. As a result, the heavy isotope of nitrogen tends to get left behind in tissues. As you go up a food chain, animals are eating more and more of the heavy isotope of nitrogen, and so you can actually use nitrogen to see what kinds of things an omnivore, like a bear, may be eating – plants, or animals?

How are stable isotopes measured?

Stable isotopes can be measured using a machine called an IRMS (Isotope Ratio Mass Spectrometer). First, you put a small amount of sample in tiny tin cups and roll them up into a ball. This is just a container for the sample. Then, you drop each sample into the IRMS, where it is immediately combusted. The gas produced is then ionized (charged) and shot at an angle so the ion beams separate based on weight (kinda like using a prism to separate the beams of light). A counter measures the signal from each ion beam to determine the isotope composition of the sample.

Why is it important to measure stable isotopes?

Stable isotopes are an example of a biological marker – something which you can use to find out information about the ecology or physiology of an animal. Think of an actual marker or highlighter – a writing utensil you use to highlight information. In a textbook or a cookbook for example, there is more information than you need. You use a marker to highlight information that is most relevant to you at a particular time, so you can find it again in the future without having to read the entire book again. In the same way, biological markers are used to find information out about how systems work. Often, using markers are the only way of finding the answers to certain questions. For example, if a human patient is having digestive problems, markers are sometimes employed in finding the cause for the illness. Patients are given capsules to swallow which contain markers that show up on xrays – as the markers move through the digestive tract, xrays are taken to track the progress and identify where and what is causing the problems.

Stable isotopes are particularly useful because they often form natural gradients – differences between things of interest. For example, different types of plants may have different stable isotope signatures (a number which represents the types of stable isotopes of a particular element found in a sample). If a gradient exists between the things you are interested in, you can use the stable isotopes to track patterns. A small example of the types of questions stable isotopes can be used to answer include:

  • What type of food is an animal eating?
  • How much of a certain type of food is an animal eating?
  • How has an animal’s diet changed over time?
  • Where did an animal come from?
  • How long ago was an animal in a certain area?
  • How have migration patterns changed over time?
  • How do mothers invest in growing a newborn animal?
  • How do animals extract nutrients from food?
  • How does an animal’s metabolism work?

Clearly, stable isotopes are a powerful tool but they are complex to use and much research needs to be done in identifying natural gradients as well as how stable isotope signatures are changed by animals.