Different reactivities of H3O+(H2O)n with unsaturated and saturated aldehydes; ligand switching reactions govern the quantitative analytical sensitivity of SESI‐MS.

Rationale

The detection sensitivity of SESI-MS is seen to be much lower for saturated aldehydes than for unsaturated aldehydes. This needs to be understood in terms of gas phase ion-molecule reaction kinetics and energetics to make SESI-MS analytically more quantitative.

Methods

Parallel SESI-MS and SIFT-MS analyses were carried out of air containing variable and accurately determined concentrations of saturated (C5 pentanal; C7 heptanal; C8 octanal) and unsaturated (C5 2-pentenal; C7 2-heptenal; C8 2-octenal) aldehyde vapours. The influence of the source gas humidity and of the ion transfer capillary temperature, 250°C and 300°C, in a commercial SESI-MS instrument is explored. Separate experiments were carried out using SIFT to determine the rate coefficients, k
₇₃, for the ligand switching reactions of the H₃O⁺(H₂O)₃ ions with the 6 aldehydes.

Results

The relative slopes of the plots of SESI-MS ion signal against SIFT-MS concentration are interpreted as the relative SESI-MS sensitivities for these six compounds. It is seen that the sensitivities for the unsaturated aldehydes are variously some 20 to 60 times greater than for the corresponding C5, C7 and C8 saturated aldehydes. Additionally, the SIFT experiments revealed that the measured k
₇₃ are 3-4 times greater for the unsaturated than for the saturated aldehydes.

Conclusions

The trends in SESI-MS sensitivities are rationally explained by differences in the rates of the ligand switching reactions, which are justified by theoretically calculated equilibrium rate constants derived from thermochemical DFT calculations of Gibb’s free energy changes. Humidity present in SESI gas thus favours the reverse reactions of the saturated aldehyde analyte ions, effectively suppressing their signals in contrast to their unsaturated counterparts.