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 M(2)H(+) 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 (T(eff)) 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 degrees 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 M(2)H(+) was observed even though no MH(+) was present in the spectrum. This is ascribed to T(eff) being mass dependent. A field sufficient to raise a M(2)H(+) to T(eff) for dissociation to MH(+) + M, raises MH(+) to a higher T(eff), leading to its immediate decomposition. (C) 2011 Elsevier B.V. All rights reserved.