A Photoionization Detector for Gases and Vapours

SENSITIVE ionization methods for the measurement of low gas and vapour concentration require the ionization of the test gas or vapour under conditions where an inert carrier gas is unionized. Such methods as the ionization of carbon compounds in a hydrogen flame1 and the reaction between gases and vapours and rare gas metastables2 are well established and successful for this purpose. An alternative method is to irradiate the gas mixture with ultra-violet radiation of such a frequency that photoionization of the test gas occurs without any accompanying ionization of an inert carrier gas. In practice an electrical discharge in one of the rare gases or in nitrogen or hydrogen generates photons of energy sufficient for the ionization of almost all polyatomic compounds, but with insufficient energy to ionize simple diatomic gases.