No abstract

The dissociation of the proton bound dimer of dimethyl methylphosphonate to the protonated and neutral molecule has been studied in a planar differential mobility spectrometer. The internal energy of the ions is the sum of the thermal component and the electric field component and results in an effective temperature, T eff , that is significantly greater than that of the ambient atmosphere temperature, T. The measured rate constant for dissociation, which rises exponentially with field strength, together with activation energy and pre-exponential factor previously determined under thermal conditions, allow the calculation of T eff as a function of electric field strength. T eff is a linear function of electric field intensity at constant T over the range of T from 60°C to 140°C. The efficiency of the available collision energy in causing dissociation decreases with increasing T, from 52% at 60°C to 40% at 140°C.

The kinetics for the decomposition of the symmetrical proton-bound dimers of a series of 2-ketones (M) from acetone to 2-nonanone have been determined at ambient pressure by linear ion mobility spectrometry (IMS) and by differential mobility spectrometry (DMS). Decomposition, M2H(+) →MH(+) + M, in the IMS instrument, observed under thermal conditions over the temperature range 147 to 172 °C, yielded almost identical Arrhenius parameters Ea = 122 kJ mol(-1) and ln A = 38.8 for the dimers of 2-pentanone, 2-heptanone, and 2-nonanone. Ion decomposition in the DMS instrument was due to a combination of thermal and electric field energies at an effective ion internal temperature whose value was estimated by reference to the IMS kinetic parameters. Decomposition was observed with radio frequency (RF) fields with maximum intensities in the range 10 kV cm(-1) to 30 kV cm(-1) and gas temperatures from 30 to 110 °C, which yielded effective temperatures that were higher than the gas temperature by 260° at 30 °C and 100° at 110 °C. There was a mass dependence of the field for the onset of decomposition: the higher the ion mass, the higher the required field at a given gas temperature, which is ascribed to the associated increasing heat capacity with the increasing carbon number, but similar, internal vibrations and rotations.