What is an isotope?

In nature, every element has a particular arrangement of protons and neutrons, called an isotope, which is favored. For example, the most abundant form of the element carbon has 6 protons and 6 neutrons and thus a total mass of 12. ¹²C makes up 99% of all the carbon. There is another form of carbon, ¹³C, which is comprised of 6 protons and 7 neutrons. ¹³C makes up only 1% of all the carbon. Similarly, the common isotope for oxygen is ¹⁶O (8 protons and 8 neutrons) and there are two rare isotopes (¹⁷O and ¹⁸O). For hydrogen, the abundant isotope has a mass of 1 (H) and the rare isotope has a mass of 2, which is called ²H or, more commonly, deuterium (D).

How do we use isotopes?

In nature, the reaction rate of the rare isotope is typically slower than the reaction rate of the abundant isotope. As a result, the ratio of the rare to abundant isotope (for example, ¹³C/¹²C, ¹⁸O/¹⁶O, D/H, etc.) can be very different for different compounds. For example, the ¹³C/¹²C of carbon in trees (as cellulose) or in coal is very different from the ¹³C/¹²C of carbon in the carbon dioxide (CO₂) we exhale. Thus we can tell the difference between CO₂ in human breath from CO₂ produced by burning coal by measuring the ¹³C/¹²C of the CO₂. In this way, isotopic measurements of compounds (like CO₂) can be used to identify, like using fingerprints, specific sources and sinks of these compounds.

How do we measure Isotopes?

In our laboratory we can measure the ratio of the rare to abundant isotopes for carbon, oxygen, nitrogen and hydrogen very precisely using an instrument called a mass spectrometer. We can distinguish between isotope ratios that differ by 0.005%. These instruments are relatively expensive ($200,000 to $400,000) since they are made to exacting specifications by only three manufacturers in the world. The mass spectrometers we use were manufactured by a company called Finnigan located in Germany. The people who work in our laboratory, both technicians and students (rarely the professor), have been trained to operate these instruments.