Luckily for us, atoms have provided an answer. Atoms comprise of protons, neutrons and electrons. It is the number of protons that give an atom its identity; they make carbon behave like carbon and hydrogen behave like hydrogen. However, while the number of protons in an atom must stay the same, the number of neutrons and electrons can change. In a neutral atom (i.e. one without a positive or negative charge), the number of electrons equals the number of protons so we can essentially ignore them. However, the number of neutrons can alter, which in turns changes the mass of the atom.
If we take carbon for example, it has 6 protons in its nucleus, which make it behave like carbon. However, you can have different versions of carbon, or isotopes, according to how many neutrons in the nucleus. Carbon can therefore exist as carbon-12 (6 protons plus 6 neutrons), carbon-13 (6 protons plus 7 neutrons) and carbon-14 (6 protons plus 8 neutrons). Carbon-14 (or 14C) is an unstable isotope of carbon, which means that over time (and only after an organism is dead) it loses a proton and radioactively decays into other atoms. This makes it so useful as a dating tool because we can measure the amount of 14C left in a dead organism – be that wood, bone or cereal grain – and work out how long ago that organism died.
The 12C and 13C isotopes of carbon are known as stable isotopes because they do not decay into other atoms. Although they’re no good at determining the age of a particular context where a bone or cereal grain is found, the difference in their masses because of their different number of neutrons means that they behave differently in living systems. A heavier isotope will be slower, meaning that it evaporates more slowly, is converted in biosynthetic pathways more slowly and therefore tends to be left behind when a process takes place. We measure the relative amounts of the light and heavy isotopes using a gas chromatograph-combustion-isotope ratio mass spectrometer. We then use the following equation to convert these values into a δ value, which we often refer to as a ‘signature’ of the relative amounts of light and heavy isotopes in a substance. The higher the δ value, the more of the heavier isotope is present.