The use of mass spectrometers on fusion research devices dates back more than two decades. Still, application is mostly restricted to such more trivial tasks as assessment of the residual gas composition, leak detection, and optimization of discharge cleaning procedures. Proper instrumentation provided, however, mass spectrometry can also serve as a powerful diagnostic for the plasma discharge itself. This application is associated with some not so usual rarified gas dynamics. In a sense the plasma can be visualized as a gigantic ion pump. The complexity of the physics involved presents some formidable problems for data interpretation. Nevertheless, many insights into the details of recycling, e.g., plasma pumping, wall adsorption, and outgassing are obtained. The capability of the method to study aspects of impurity transport and retention will also be demonstrated. The utilization of short pulses of a probing gas, a hydrogen isotope or a rare gas for instance, into the established plasma and the analysis of the subsequent response plays a central role in such studies. The consequent application of gas dynamics also affords the determination of mass resolved ion fluxes ‖‖Bt in the plasma edge. The presentation will preferentially focus on results obtained on ASDEX, owing to the specific advantages offered by a divertor experiment.