Abstract: A mass spectrometer for the analysis of substances in a gas comprises means for generating a plasma in an enclosure through which a gas flows, conduit means for conveying the gas from the enclosure to a sampling member, and a mass analyzer for mass analyzing ions characteristic of the substances which pass through an aperture in the sampling member. The conduit means is constructed so that a line-of-sight path does not exist between the interior of the enclosure and the aperture in the sampling member. Substances for analysis may be introduced into the gas before or after it passes through the plasma enclosure. When the plasma comprises a pulsed DC glow discharge the spectrometer is useful for the analysis of traces of electrophillic substances in the gas.