USE OF A CLUSTERING REACTION TO DETECT LOW-LEVELS OF MOISTURE IN BULKOXYGEN USING AN ATMOSPHERIC-PRESSURE IONIZATION MASS-SPECTROMETER

Atmospheric pressure ionization mass spectrometry (APIMS) is being rou tinely used to quantify trace impurities in bulk gases used in the man ufacture of semiconductor devices, APIMS has been successfully applied for the quantification of ppt levels of O-2, H2O, CO2, and CH4 in Ar, N-2, and He. However, it has not been successfully used to quantify t race impurities in bulk O-2 due to the low ionization potential of O-2 . APIMS relies on charge-transfer reaction between the ions of the bul k gas molecules and impurity molecules. Since all the relevant impurit y molecules have ionization potentials higher than that of O-2, APIMS has not been used to analyze for impurities in O-2. We report here the detection of sub-ppb levels of H2O in O-2 by making use of the cluste ring reaction between O-2(+) and H2O. The declustering region in an AP IMS, which is normally used to break apart unwanted sand interfering c lusters, has to be carefully adjusted to keep intact the weakly bound cluster O-2(+). H2O. Our results indicate a statistical detection limi t of Less than 300 ppt for the detection of H2O in O-2.