A double‐focusing mass spectrograph for the analysis of low concentrations of impurities in solids has been developed. The instrument is of the Mattauch type and is designed for either photographic or electrical ion detection. The construction and performance of the instrument are discussed. Bulk concentrations below 0.1 part per million, and surface contaminants of less than 0.1 monolayer, can be detected in short exposures using a photographic plate.