Electrospray ionization mass spectrometry is a newer form of mass spectrometry which has the advantages of forming multiply charged species, relatively soft ionization, and coupling ability with High-Performance Liquid Chromatography to form an LC-MS hyphenated system.
The main premise behind electrospray is that the sample is introduced into the mass spectrometer in solution, the solution is sprayed out of a very fine needle pneumatically, so that the solution forms very fine droplets. The solvent in the droplets is evaporated away, and the dissolved ions get closer and closer together until coulombic repulsion gets so high that the droplets explode. All that's left are charged (or multiply charged) ions which are shot into the detector with the electric field. (Note that this work great for ionized/ionizable species). Another thing to consider is that non-volatile ions (like sodium salts) will clog the aperture...care must be taken to exclude these things which will "salt out"
However the mass spectrometer has a limit which traditionally had to do with the maximum mass a machine could analyse. The REAL limit was in mass/charge or m/z. As the charge gets higher, the total gets lower. This means larger species can be measured in these machines. Larger multiply charged species can be quantified. Proteins are large molecules.....did I forget to mention that electrospray is a gentle ionization technique? With the simple addition of some deconvolution software to look for all the peaks that could be differently charged envelopes of the same protein...you can accurately measure the molecular weight of said protein.