FORMATION OF IONS A mass spectrometer works with electrically charged particles and, before a mass spectrum can be obtained, the substance under examination must be ionized if it is not already ionic. Most research into mass spectrometry has been carried out on positive ions and these are discussed in detail. Negative ions are discussed as the occasion demands. A molecule (M) can be ionized by removal or addition of an electron to give species (M +· and M -· respectively) having a mass which, for practical purposes, is identical to that of the original molecule, the mass of an electron being so small. Optionally, the molecule may be ionized by addition or subtraction of other charged species (X + ) to give ions ([M + X] + or [M - X] - ); in these cases, the resulting ions contain all or most of the original molecule but have masses that are different from that of the original so they are called quasi-molecular ions. Finally, the original substance may be a salt (M + X - ), in which instance, it is ionized already and the oppositely charged species need only be separated prior to mass spectrometry. Both currently and historically, the most widespread means of ionization has been the removal of an electron from a molecule to give M +· through use of another electron ( electron ionization , EI). Therefore, this discussion on the formation of a mass spectrum begins with EI, the other methods being introduced later.