Abstrakt
Glyoxal (m.w. 58 g/mol) is a highly reactive molecule important for atmospheric chemistry and associated with some basic biological processes. It is associated with catalytic reactions forming basic organic molecules in an inorganic environment [1] and is one of the products of oxidation of anthropogenic and biogenic volatile organic compounds (VOC) [2].
Thus, there is a need to analyse trace amounts of glyoxal vapour in ambient air and in sample headspace. A previous PTR-MS study [3] revealed a signifcant humidity dependence of the intensity of the m/z 59 product ion (protonated glyoxal) of the H3O+ reaction.
Moreover, ion product at m/z 31 (probably protonated formaldehyde CH2OH+) resulting from this reaction correlated with the sample humidity. In the present investigation, a selected ion fow tube, SIFT, was used to investigate ion-molecule reactions of glyoxal with H3O+, NO+ and O2 .+ reagent ions at variable humidities.
Humidity of the helium carrier gas was varied in a controlled manner. Formation of OC(H)C(H)O(NO)+ and OC(H)C(H)O.+ ions were observed in the NO+ and O2 .+ reactions respectively.
For the H3O+ reactions we observed formation of protonated formaldehyde at m/z 31 in addition to protonated glyoxal. However, the humidity dependence of protonated formaldehyde production has an opposite trend in comparison with PTR-MS results.
The ion chemistry under thermal conditions is theoretically described using B3LYP/6-311(p,d) calculations and using numerical modelling of the ion-molecule reaction kinetics. The results indicate the important intermediate step is the formation of the C2H2O2 .H+(H2O) ion, which seems to initiate a dissociation process leading into the formation of protonated formaldehyde