The device described is the combination of two mass spectrometers, with a surface sample placed between them. Its aim is to allow for detailed research on low-energy ion-surface interactions, involving and triggering surface chemistry. This task is fulfilled by a carefully chosen geometry: Projectile ions from an electron impact source are mass-per-charge selected using a quadrupole. Such continuous bombardment allows for good control of the surface condition. Species emerging from the collisions are focused onto a beam and analyzed using a purpose-built orthogonal pulsing time-of-flight mass spectrometer. Neutral species can be post-ionized using a second electron impact source. Neutral gases can be adsorbed to the surface from the gas phase in a controlled manner, using a feedback-controlled pressure regulator. In order to minimize the discrimination of secondary ions, the distance from the surface to the analyzing mass spectrometer system was kept as short as possible and the acceptance angle of the lens system as large as possible. This increased the sensitivity five orders of magnitude compared to its predecessor. The rigorous use of computer aided design software is responsible for the successful commissioning of the new device. This article describes first which parameters can be measured or controlled. Then, these are linked to the physical processes that occur in reactive ion-surface interactions. Next, the design goal and the design implementation are presented. In the end, a performance comparison, measurements of hydrogen surface chemistry with extensive use of isotope labeling, and measurements of post-ionized beryllium are presented.