A platinum foil 8 μm thick was etched electrolytically in 3% potassium cyanide solution to form an edge which was much more efficient than a razor blade edge for producting positive ions of oxygen and nitrogen by field ionization. The etched foils were used in a mass spectrometer ion source to ionize these gases. Etched gold foil was also used to ionize the gases and was more effective than a razor blade. Attempts to improve the efficiency of razor blades for field ionization by etching and other procedures were not successful.