# Is enthalpy affected by isotopes?

Enthalpy is a sum of $$U$$ and $$PV$$, and I thought that for different isotopes, internal energy values may differ. Is it true? There are three stable isotopes of $$\textrm{Mg}$$, mass number 24, 25, and 26. If enthalpy is different, which one is a standard for enthalpy $$0$$? I read somewhere on the web that enthalpy of a covalent bond is affected by isotopes, but I'm confused when there are no covalent bonds.