A deuterium-labeling study on the reproduction of hydronium ions in the PTR-MS detection of ethanol

The reactions of hydronium ions (H3O+) with ethanol (C2H5OH) and deuterium-labeled ethanols (CD3CH2OH and CH3CD2OH) were investigated by means of proton transfer reaction mass spectrometry (PTR-MS). Besides the protonation reaction of H3O+ with C2H5OH, three fragmentation processes yielding C2H5+, CH2OH+, and H3O+ ions were previously implied. In this paper, we report the branching ratios for those four channels at six different field strengths (E/N ratios) of the drift tube. The contribution of the channel that reproduces hydronium ions (H3O+) was determined by detecting H2DO+ ions at m/z 20 produced in reactions of H3O+ with deuterium-labeled ethanols (CD3CH2OH and CH3CD2OH). The reproduction of H3O+ ions was found to be significant between 108 and 162Td of the E/N ratio, however, it did not quantitatively account for the low detection sensitivity of ethanol by PTR-MS, suggesting the presence of unidentified reaction channel(s). The unidentified reaction channel(s) might be the H3O+-reproducing channel itself, because the contribution from this channel would be underestimated in this experiment due to loss process(es) such as an H/D exchange between H2DO+ and H2O. (C) 2009 Elsevier B.V. All rights reserved.