A technique for separation of a stream of large ions accelerated in a magnetic field. The resistance to deflection of the ion from its path is a measure of the momentum of the ion, and hence of its mass. In the earliest versions, the sample material was volatilized by heat and ionised by an electron beam. Later techniques of bringing the sample out of its liquid or solid phase, i.e. desorption, and ionisation of it, are fast-atom bombardment MS (FA-MS) by a beam of ions, and plasma desorption MS (PD-MS) by irradiation by daughter nuclides from 252Cf disintegration. In field-desorption MS (FD-MS) the sample is first ionised and then volatilized, a process that induces much less thermal stress on the sample than field-ionization MS, in which the sample is first volatilized and then ionised in an electrostatic field. In laser-assisted desorption MS (LD-MS) and matrix-assisted laser desorption/ionization MS (MALDI-MS) the sample exchanges energy with a solid or liquid matrix in order to minimise damage to the sample and isolate one sample molecule from others. In electrospray ionisation MS (ES-MS), atomization of solution particles of protein ions from which the solvent is evaporated in vacuo leaves multiply charged protein ions.