What is an (isotopic) fingerprint or signature?
The fingerprint or signature is the combination of δ values of one or more elements and/or elemental ratios and/or other measured parameters that characterise the compound or produce.
All organisms take up elements from the environment, although different elements are taken up at different rates and in different forms. Their elemental and isotopic compositions are in most cases not the same as in the environment (water, soil, food), but nevertheless, they are typical for a region of origin. Plants or animals of the same species from different environments also live in different conditions regarding the temperature, humidity, insolation, soil composition and food sources. Therefore the isotopic compositions of elements in their bodies may slightly differ, however, the difference must not occur in all elements. If we analyse only one element for its isotopic composition, then we simply speak about its isotopic composition – for instance, isotopic composition of nitrogen in mussels cultured at different locations. If we analyse two or more isotopes (beside nitrogen also carbon and maybe even sulphur), then the combination of their δ values (δ13C, δ15N and δ34S) represents the isotopic fingerprint of mussels. In such case, at least one isotope most probably will show some differences and can provide enough information to estimate the origin of mussels. Nevertheless, to estimate the source area of produce, isotopes are not always enough; additionally to the isotopes, the chemical (elemental) composition, e.g. the ratio between different elements, can be used to determine their origin, or even further, the isotopic and elemental ratios in specific compounds of the organisms. Sometimes we even have to analyse the positions of individual isotopes in a specific molecule (for instance the position of deuterium – heavy hydrogen – in the ethanol molecule in wine).