Abstract: Molecular ionization detection and analysis including processes and apparatus for photoionizing trace constituents in a carrier gas or effluent from a chromatograph in a manner that substantially increases the effective sensitivity. The invention uses a pulsed gas valve to introduce a high density gas sample into a vacuum system where the trace constituents in the jet are photoionized by an intense vacuum ultraviolet [VUV] pulse from a flashlamp. The ions are extracted from the beam under free molecular flow conditions. This source may be used with the ion energy analyzer to eliminate any interference due to background gases in the vacuum system and to provide mass analysis of the eluting trace constituents.