The MS7 mass spectrometer is an instrument for the quantitative and qualitative analysis of trace impurities. It is capable of detecting elements present in quantities as small as 1 part in 10⁹ of the major constituent of a sample. After briefly discussing the mode of operation of the instrument, the authors of this article describe its use as an analytical tool and explain the factors which affect it sensitivity. A brief account is finally given of analyses carried out on materials used in semiconductor research.