Definition of "isotopically" [isotopically]

• With regard to isotopes (adverb)

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• "Here, the center portion of each assembly is 155 mm across and holds the seed material consisting of metallic plutonium-zirconium (Pu-Zr) alloy (Pu is about 10% of alloy, and isotopically over 90% 239Pu) as 108 twisted tricorn-section rods 12.75 mm across with Zr-1%Nb cladding (Nb = Niobium)."
• "During ice ages isotopically light water depleted in oxygen 18 is locked up in the ice sheets."
• "This has the effect of depleting the residual water vapour in oxygen 18 and thus making subsequent precipitation isotopically lighter."