What is isotope composition?
In principle, the simplest way to indicate the isotopic composition of an element is to give the abundance of each isotope in atom %. For instance, about 1 % of all carbon atoms on Earth is 13C or heavy carbon; about 0.36 % of all nitrogen atoms is heavy nitrogen (15N). The differences in abundance of heavy and light isotopes of an element between different reservoirs (such as oxygen in water and plants; or nitrogen in predators and prey) or environmental compartments are usually very small. It would be thus inconvenient to express them in atom % or isotope ratios (such as 18O/16O, 13C/12C). Therefore a relative δ (delta) value is used to express the stable isotope composition of elements, which describes the relative difference between the heavy-to-light isotope ratio in the material of interest and the reference material, expressed in ‰ (per mil). If the δ value of nitrogen (δ15N) of the sample is 0, this doesn’t mean that there is no 15N in it, but that the abundance of the 15N in the sample is exactly the same as in the reference material. If the δ value is positive, then the sample is enriched in 15N compared to the standard, and if it is negative, the sample is depleted in 15N compared to the standard. Reference materials are substances which have the isotope ratio of the element as close as possible to the average isotope ratio of the same element ibn Earth. For example, the reference material for oxygen is ocean water, and reference for nitrogen is air. By definition, the δ value of the reference material is 0 ‰.