The most common use of electron guns is in cathode ray tubes, which were widely used in computer and television monitors until flat-screen displays rendered them obsolete. An electron gun can also be used to ionize particles by adding electrons to, or removing electrons from an atom. This technology is sometimes used in mass spectrometry in a process called electron ionization to ionize vaporized or gaseous particles. More powerful electron guns are used for welding, metal coating, 3D metal printers, metal powder production and vacuum furnaces.