TEMPO-H and TEMPO (in some experiments also water) were evaporated into the evacuated stainless steel reservoir up to the indicated partial pressure and the carrier gas He (Nippon, 99.996%) was added up to 750 hPa. From there, about 5% of the gas mixture was expanded at room temperature in a 133 ms pulse through 7 magnetic valves into a pre-expansion chamber and guided through a 700 mm long, narrow slit nozzle into a large vacuum buffer. The size of the buffer ensures a large pressure gradient despite the high gas flow. This leads to adiabatic cooling and aggregation of the molecules within a few microseconds. Synchronisation of the mirror of a Bruker Vertex 70v FTIR interferometer to the gas pulses allowed to record cold, species-separated spectra in the OH/CH stretching range by a combination of a 150 W tungsten filament, CaF 2 optics, a suitable optical filter and a liquid-N 2 -cooled 2 mm×2 mm sandwich InSb/MCT detector. The vacuum buffer was permanently evacuated by a series of roots pumps. Their exhaust gas was recompressed by a screw pump to the original pressure, recovering the initial conditions in less than a minute. In this way, the gas could be circulated and reused about 50 times in 1000 pulses. During the circulation, the partial pressure of the water trace slowly accumulated from wall desorption, providing as a side effect an assignment aid for complexes including water. Furthermore, a small fraction of the TEMPO-H started to decompose into TEMPO during circulation, again providing an assignment aid from the time evolution of the spectra. By averaging the interferograms from 1000 pulses of the same gas filling, all spectra shown in Figure could be obtained from a total amount of about 0.25 g TEMPO and about 0.3 g TEMPO-H. See the ESI † for details.