LASER-INDUCED SURFACE-IONIZATION IN A TIME-OF-FLIGHT MASS-SPECTROMETER

A laser-induced ionization scheme based on laser-metal interaction is described. It is found that when a low power, pulsed UV laser beam is directed to the repeller plate of a Wiley-McLaren time-of-flight mass spectrometer (TOFMS), which is held at much higher positive potential than the extraction grid, extensive ionization of gas phase species can take place. For monatomic species such as Ar and Xe, multiply charged ions up to Ar6+ and Xe9+ are obtained. For both aromatic and aliphatic organic molecules, classical electron-impact-type mass spectra are observed. Mass resolution is in the range of 300-400, comparable to that obtained using laser-induced resonant two-photon ionization (R2PI) in the same linear TOFMS. It is shown that this technique can be quite efficient in ionizing organic species. A detection limit of 5 ng and a linear dynamic range of four orders of magnitude for benzene are demonstrated. While not as sensitive as R2PI, it has a much wider applicability and more uniform ionization efficiency than R2PI. It also appears about 1000 times more sensitive than the incorporation of an electron gun in TOFMS. The technique can be interchanged with R2PI very readily by simply adjusting the position and focus of the laser beam. While the ionization mechanism involved in the technique is currently unknown, some preliminary investigation of the ionization process is reported. It is suspected that photoelectron generation from the plate surface may play some important role in producing the ions detected. © 1995 American Institute of Physics.