Isotope

Atoms can be simply described as consisting of protons, electrons, and neutrons. Isotopes of the same element differ by the number of neutrons in the nucleus, resulting in different mass.

The δ-notation

Because relative differences in isotope ratios are more precisely detected than the absolute isotopic ratios, they are commonly reported in the δ-notation:

$\delta=(\frac{R_{x}-R_{std}}{R_{std}})\times 1000$

where $R$ is the ratio of the abundance of the heavy to the light isotope, $x$ is the measured sample, and $std$ is the standard. For the element oxygen, $R$ is given by ¹⁸O/¹⁶O.