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

Glyoxal (C 2 H 2 O 2 ) 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 H 3 O + ions with C 2 H 2 O 2 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 C 2 H 3 O 2 + . The second step, in the presence of water vapour, is the association forming C 2 H 3 O 2 + (H 2 O) and interestingly also protonated formaldehyde CH 2 OH + . Hydrated protonated formaldehyde CH 2 OH + (H 2 O) 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 10 14 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. Protonated glyoxal reacts with water molecules to form protonated formaldehyde, interfering with SIFT-MS analyses of glyoxal in humid samples.