A "heavier isotope" refers to a version of an atom that has more particles in its nucleus than the usual form of that atom. This extra mass makes the isotope heavier than the typical version. Full definition
It also estimates the total loss by measuring the fraction of heavier isotopes of certain atoms versus their lighter counterparts. (newscientist.com)
Initial SAM results show an increase of 5 percent in heavier isotopes of carbon in the atmospheric carbon dioxide compared to estimates of the isotopic ratios present when Mars formed. (spaceref.com)
One common technique for tracking a drug's fate is to label its molecular framework with heavier isotopes of hydrogen (either deuterium or tritium). (science.sciencemag.org)