We describe an apparatus for ion-molecule-reaction mass spectrometry. It is suitable for on-line gas analysis covering the mass range from 1 to 200 amu with a sensitivity of about 1 ppm. The gas to be analyzed is ionized by reaction with a primary ion beam at low collision energy. A mass-selected ion source allows the fast switching among three primary ions beams (Xe+, Kr+, and CF3I+), covering the recombination energy range from 10.23 to 14.67 eV. Calibration measurements and an example of application to the study of heterogeneous catalysis are presented.