The phrase "isotope composition" refers to the different forms (or variants) of atoms within a substance. It indicates the specific types of atoms and their relative amounts present in a sample. Full definition
Cope IL, Wilkinson JJ, Boyce AJ, Herrington RJ (2010) Oxygen Isotope Composition of Hematite: Pic de Fon Deposit, Republic of Guinea, West Africa. (nhm.ac.uk)
Mean values ± standard deviation of methane concentrations (as milligrams of CH4 L - 1) and carbon isotope composition in methane in shallow groundwater δ13C - CH4 sorted by aquifers and proximity to gas wells (active vs. nonactive) (pnas.org)
By studying the carbon isotope compositions of barley plants and their relationship with water - use efficiency, the researchers developed tools that plant breeders can use to improve selection efficiency for more water - efficient varieties. (frogheart.ca)