Abstract: A method and apparatus are described for detecting the presence and amount of elemental oxygen in a sample cell. The intensity of extreme ultraviolet light passing through the sample cell at a wavelength band overlapping at least one of the Schumann-Runge absorption lines of oxygen is detected and compared with a predetermined non-absorbed condition of the ultraviolet light. The non-absorbed condition may be achieved by narrowing the wavelength band such that the band does not overlap the Schumann-Runge absorption line, or may be achieved by detecting the ultraviolet light passing through a reference cell. Also described is a novel ultraviolet source in which a cold zone captures neutral atoms of the emission gas to reduce the affect of resonance absorption of emitted ultraviolet light by such neutral atoms.