The phrase "isotopic composition" refers to the different types or versions of atoms that make up a substance. It tells us the ratio or proportion of the different isotopes of an element within that substance. Full definition
On the basis of anomalous isotopic compositions of oxygen and magnesium, 14 oxide grains from two primitive meteorites (Bishunpur and Semarkona) have been identified as circumstellar condensates. (science.sciencemag.org)
It is possible to reconstruct past precipitation changes by measuring the stable hydrogen isotopic composition in terrestrial plant waxes because rainfall is the primary source of hydrogen stored in plant material. (senckenberg.de)
LeGrande, A.N., 2006: The Climate Imprint on the Oxygen Isotopic Composition of Water: Observations, Proxies, and Coupled Isotopic Model Simulations. (pubs.giss.nasa.gov)