Precise biophysical characterization of macromolecular complexes is possible by mass spectrometry. Heck et al. report in their Communication on page 6247 ff. how two distinct icosahedral geometries of intact megaDalton capsids of the hepatitis B virus (a major human pathogen) can be measured and separated by mass spectrometry. Ion mobility measurements established that the viral structures are largely preserved in vacuo and two distinct conformers per capsid geometry could be distinguished.