elements - How do I correctly typeset metastable radionuclide symbols?

Does anyone know if there is an officially sanctioned way to typeset symbols like technetium-99m (⁹⁹Tc^m or ^99mTc)? I have seen both, although in more recent publications, I think, the latter predominates.

For my part, it looks somewhat wrong to place the symbol for metastable in the position you'd normally expect the charge. How would you write the ion, ⁹⁹Tc^m+? That looks quite clumsy.

So does anyone know if there's some IUPAC (or similarly authoritative) guidance on this?

Answer

According to the international standard ISO 80000 Quantities and units – Part 9: Physical chemistry and molecular physics, Amendment 1, a metastable nuclide is indicated by adding the letter m (in roman type) to the mass number of the nuclide, as in the following examples:

$$\mathrm{^{133m}Xe}$$ $$\mathrm{^{99m}Tc^+}$$