Atmospheric pressure ionization mass spectrometry (APIMS) is being routinely used to quantify trace impurities in bulk gases used in the manufacture of semiconductor devices. APIMS has been successfully applied for the quantification of ppt levels of O 2 , H 2 O, CO 2 , and CH 4 in Ar, N 2 , and He. However, it has not been successfully used to quantify trace impurities in bulk O 2 because of the low ionization potential of O 2 . APIMS relies on charge-transfer reaction between the ions of the bulk gas molecules and impurity molecules. As all the relevant impurity molecules have ionization potentials higher than that of O 2 , APIMS has not been used to analyze for impurities in O 2 . A recent publication describes the use of a clustering reaction, as opposed to a charge-transfer reaction, to detect trace levels of H 2 O in bulk O 2 . Clustering reactions have not been successfully used to detect CO 2 , N 2 , and CH 4 in bulk O 2 because the relevant cluster ions are very weak and are declustered in the low-pressure declustering region normally used in an APIMS. O 2 has a relatively low proton affinity, and protonated oxygen is expected to undergo proton transfer reactions with CO 2 , N 2 , and CH 4 . We report the use of H 2 , as a doping gas, in the source of an APIMS to facilitate the protonation of trace impurities in O 2 . For safety considerations, a buffer gas like He or Ar has to be added to keep the concentration of H 2 below the lower explosion limit (LEL). Our results indicate a statistical limit of detection of 0.5 ppb for CH 4 , 0.3 ppb for CO 2 , and 1.2 ppb for N 2 . (Int J Mass Spectrom 206 (2001) 7-12) © 2001 Elsevier Science B.V.