Stable Isotopes and M&Ms
Students are often confused by the concepts of stable isotope signatures and conventional notation. This activity is designed to introduce students to the del notation (isotopic signature), fractionation, and end member mixing of stable isotopes using familiar objects disguised as isotopes. The activity does not explain the physical manifestation of why isotopic fractionation occurs, but does explain the concepts that one easily observes when interpreting stable isotope data sets.
Undergraduate majors, upper division
Skills and concepts that students must have mastered
Students should be proficient in chemistry.
How the activity is situated in the course
Stable isotopes are an important aspect of interpreting ocean processes (biological, physical, chemical) and these processes affect the abundance of heavier isotopes. Since these changes are small but measureable, we use a specific (del) notation to compare samples directly. Changes in (del) are extremely useful in paleoceangraphy studies, carbon cycling, and trophic studies.
Content/concepts goals for this activity
- Students will be able to determine del values based on given references.
- Students will be able to determine isotopic signatures from end member mixing.
Higher order thinking skills goals for this activity
Once students have shown understanding of the fundamental mechanisms that affect stable isotope signatures, they should be able to apply their understanding to new situations.
Other skills goals for this activity
Description and Teaching Materials
In this activity, students will use two types of M&Ms to simulate the behavior of different isotopes. The plain M&Ms will represent the lighter isotope and the peanut M&Ms the heavier isotope. By counting the ratios of lighter and heavier isotopes, students should gain an understanding of del notation, end member mixing and fractionation. A problem at the end of the activity will be used to assess students' understanding and application of the concepts presented here.
Teaching Notes and Tips
Should may become confused at first, when counting the 'isotopes'.
The key is provided under supporting materials and will help guide students in performing the correct calculations.
Students successfully completing the activity should be able to determine end member mixing of 13C or 18O reservoirs through another problem set.
They should be prepared to interpret changes in (d) values based on specific isotope applications (i.e. 15N, 13C, 18O) .
References and Resources