A neutral gas mass spectrometer for the investigation of lunar volatiles

We developed a compact time-of-flight mass spectrometer (TOF-MS) for the sensitive investigation of gas samples in planetary missions. The advantage of a TOF-MS for space research is its fairly simple and robust mechanical design and its moderate need for resources. Moreover, because a TOF-MS system records all masses at the same time it has high sensitivity. The performance as established by our prototype instrument is a mass resolution of about m/Δm=1100 and a mass range of 1000 amu (in principle the mass range is only limited by the accumulation memory). A TOF-MS is a pulsed system and mass spectra are recorded with high temporal cadence, typically 10,000 spectra per second. A high dynamic range is achieved by histogramming these mass spectra for a certain period, i.e., for accumulation periods of seconds a dynamic range about 107 is achieved. With our prototype instrument we established a detection limit of 10−16 mbar (corresponding to a number density of about 1 cm−3), which means that at a total pressure of 10−10 mbar we still have 6 decades of dynamic range. This instrument performance will allow for very sensitive measurements of the lunar exosphere, either from ground or from low-lunar orbit.