Composite metal oxide-coated tungsten cathodes were formulated as efficient and reliable thermo-electron sources for vacuum ultraviolet ionizers. Cathode suspensions were prepared by adding metal chlorides forms of four different metals; scandium, yttrium, indium, and neodymium (Sc, Y, In and Nd) to the triple calcium, strontium, and barium (Ca, Sr, and Ba) suspension. The improvement of electron emission and reliability of the cathodes has been achieved by adding Sc, Y, In, and Nd. The metal salts were activated to the oxide cathodes by a finely programmed calcination process under ultra-high vacuum (?10(?6) torr) and the inert atmosphere. Compared to the reference suspensions, the electron emissions of neodymium and yttrium type cathodes were significantly enhanced. The optimized formulation is the cathode containing as low as 1?mole% of yttrium exhibiting the greatest performance among them.