Although neutron stars are mostly made of neutronium, the pressure at the surface is not very high which allows regular atomic matters to exist. Emission spectrum can reveal the chemical composition of distant stars. However, neutron stars are surrounded with extremely strong magnetic field which is enough to distort the atomic structures. Atomic nuclei should be more resistant against the magnetic field because they are much more compact and tightly bound, but I am not sure if atomic nuclei emit characteristic spectrum like the electron clouds.
If we can determine the composition of the neutron star crust, will there be any variations? Neutron stars formed due to iron core collapse should have an iron crust. Some neutron stars are formed due to the electron capture of the O-Ne-Mg core. Would these neutron stars have a different chemical composition in the crust? Many neutron stars are accreting hydrogen and helium gas from companion stars. Will the accreted matters show up on the surface?