Abstract: A novel gas detection platform was developed to characterize gas concentration and partial pressure in high vacuum (UHV), required by extreme ultraviolet lithography, to evaluate the detection limit of an ultrahigh resolution mass spectrometer (QMS). The platform comprises the HV chamber, pumping/gauging/controlling units, air sampling valve combined with a metal capillary inlet, and a QMS to be tested. The platform detected the extremely low concentration and partial pressure of xenon in air. The results show that the existing 9 stable Xe-isotopes, except Xe (124) and Xe (126), could be clearly identified in the QMS spectrum and their concentration ratios could be derived. The lowest detectable concentration and partial pressure, analyzed in signal-to-noise ratio (SNR) method, were calculated to be 0. 319×10^-9 and 1. 46×10^-13 Pa, respectively. The evaluation results and QMS specifications were found to be in good agreement.