SINGLE-PHOTON IONIZATION MASS-SPECTROMETRY USING LASER-GENERATED VACUUM-ULTRAVIOLET PHOTONS

This paper provides an overview of the method of single photon ionisat ion mass spectrometry. A review of the theory of frequency upconversio n using third-order 4-wave sum mixing in isotropic media and experimen tal results of third harmonic generation (THG) using a frequency tripl ed 355 nm Nd:YAG pump source are presented. Vacuum Ultra-violet (VUV) photons of wavelength 118 nm are detected in an acetone ionisation cha mber. The emphasis of this paper is on the practical aspects of genera ting and detecting the VUV photons and using them for single photon io nisation (SPI) in the ion source of a mass spectrometer. Optimum gas p ressures for THG in Xe and Xe/Ar mixtures are established. For a pump beam of well defined mode structure the optimum gas pressures are in e xcellent agreement with theory. The major loss mechanism is attributed to re-absorption of VUV by the tripling gas. SPI mass spectra of hexa ne and the biomolecule valyl-valine are presented illustrating the pow er of the technique.