Is it ensure, insure, or assure?
late 14c., "to make wealthy," from Old French enrichir "enrich, enlarge," from en- "make, put in" (see en- (1)) + riche "rich" (see rich).
Figurative sense is from 1590s. Scientific sense of "to increase the abundance of a particular isotope in some material" is first attested 1945. Related: Enriched; enriching.