How do I correctly typeset metastable radionuclide symbols?
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:

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