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.
Enthalpy is slightly affected by isotope composition, but mostly below enthalpy measurement errors.

Generally, the standard enthalpy is considered for natural mixture of stable isotopes, unless considered for particular isotope explicitly.