Gas phase fragmentation of protonated esters in air at ambient pressure through ion heating by electric field in differential mobility spectrometry

A planar differential mobility spectrometer has been used to study the ions formed at atmospheric pressure by a series of n-alkyl carboxylic acid esters (M). MH+ and M2H+ ions were present at low temperature. The combination of thermal energy and energy derived from collisional heating by acceleration in the asymmetric electric field caused ion decomposition at an effective temperature (Teff) higher than ambient. The products were the protonated carboxylic acids, F+. The electric field thresholds for the first observation of F+ decreased as the temperature of the supporting gas atmosphere was increased and the rate, 1.5 °C per Townsend, was the same for all the esters. A measurable mass dependence for thresholds existed where the higher the molar mass for the ester of a given acid, the higher the required field. Although MH+ is the well-established precursor of the protonated acid, an apparent direct formation of F+ from M2H+ was observed even though no MH+ was present in the spectrum. This is ascribed to Teff being mass dependent. A field sufficient to raise a M2H+ to Teff for dissociation to MH+ + M, raises MH+ to a higher Teff, leading to its immediate decomposition.

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► Ions were fragmented in a differential mobility spectrometry at ambient pressure.
► Decomposition was effected by temperature plus applied electric field.
► Electric field equivalence was 1.5 °C per Townsend.
► Electric field threshold for decomposition is mass dependence.