Chemical ionization of glyoxal and formaldehyde with H3O+ ions using SIFT-MS under variable system humidity

Abstract

Glyoxal (C₂H₂O₂) is a highly reactive molecule present at trace levels in specific gaseous environments. For analyses by chemical ionization mass spectrometry, it is important to understand the gas-phase chemistry initiated by reactions of H₃O⁺ ions with C₂H₂O₂ molecules in the presence of water vapour. This chemistry was studied at variable humidity using a selected ion flow tube, SIFT. The initial step is a proton transfer reaction forming protonated glyoxal C₂H₃O₂⁺. The second step, in the presence of water vapour, is the association forming C₂H₃O₂⁺(H₂O) and interestingly also protonated formaldehyde CH₂OH⁺. Hydrated protonated formaldehyde CH₂OH⁺(H₂O) was also observed. Relative signals of these four ionic products were studied at the end of the flow tube where the reactions took place during 0.3 ms in helium carrier gas (1.5 mbar, 300 K) as the water vapour number density varied up to 10¹⁴ cm⁻³. The data were interpreted using numerical kinetics modelling of the reaction sequences and the mechanisms and kinetics of the reaction steps were characterised. The results thus facilitate SIFT-MS analyses of glyoxal in humid air whilst drawing attention to ion overlaps with formaldehyde products.