The use of proton-transfer reactions to detect low levels of impurities inbulk oxygen using an atmospheric pressure ionization mass spectrometer

Atmospheric pressure ionization mass spectrometry (APIMS) is being routinel y used to quantify trace impurities in bulk gases used in the manufacture o f semiconductor devices. APIMS has been successfully applied for the quanti fication of ppt levels of O-2, H2O, CO2, and CH4 in Ar, N-2. and He. Howeve r, 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-t ransfer reaction between the ions of the bulk gas molecules and impurity mo lecules. 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 H2O in bu lk O-2. Clustering reactions have not been successfully used to detect CO2, N-2, and CH4 in bulk O-2 because the relevant cluster ions are very weak a nd 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 CO2, N-2, and CH4. We report the use of H-2, as a doping gas, in the source of an APIMS to facil itate 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 , below the lower explosion limit (LEL). Our results indicate a statistical Limit of detection of 0.5 ppb for CH4, 0.3 ppb for CO2, and 1.2 ppb for N- 2 (Int J Mass Spectrom 206 (2001) 7-12) (C) 2001 Elsevier Science B.V.