This paper provides an overview of the method of single photon ionisation mass spectrometry. A review of the theory of frequency upconversion using third-order 4- wave sum mixing in isotropic media and experimental results of third harmonic generation (THG) using a frequency tripled 355 nm Nd:YAG pump source are presented. Vacuum Ultra-violet (VUV) photons of wavelength 118 nm are detected in an acetone ionisation chamber. The emphasis of this paper is on the practical aspects of generating and detecting the VUV photons and using them for single photon ionisation (SPI) in the ion source of a mass spectrometer. Optimum gas pressures 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 excellent agreement with theory. The major loss mechanism is attributed to re-absorption of VUV by the tripling gas. SPI mass spectra of hexane and the biomolecule valyl-valine are presented illustrating the power of the technique.