Tandem Mobility Measurements of Ionic Liquid Cluster Volatilities

We illustrate the potential of a pair of steady-state ion mobility filters run in series, similar to a triple quadrupole mass spectrometer, not only to operate under atmospheric pressure but also to determine reaction kinetics at controlled pressure and temperature. Singly charged cluster cations of the ionic liquid 1-ethyl-3-methyl-imidazolium tris(pentafluoroethyl)trifluorophosphate (EMI-FAP) produced by a bipolar electrospray source are isolated steadily at atmospheric pressure by a first ion mobility spectrometer (IMS). A second IMS then determines the composition of evaporative decay products: (EMI-FAP)_<i>n</i>-1EMI⁺→(EMI-FAP)_<i>n</i>-2EMI⁺+EMI-FAP (2 ≤ <i>n</i> ≤ 30) after passing through a heated parabolic flow reactor tube at controlled temperatures <i>T</i> (23-40 °C). Measured product/parent abundance ratios are converted into reaction constants <i>k</i>_<i>n</i> via a previously described theory. At <i>n</i> < 11, evaporation rates <i>k</i>_<i>n</i> exhibit idiosyncrasies characteristic of various magic numbers, some of which are also reflected in a cluster compactness parameter inferred from the measured mobility cross section and mass. An approximately continuous variation of volatility with <i>n</i> is found for 11 ≤ <i>n</i> ≤ 14, and for <i>n</i> ≥ 19, with a region of low volatility in between. This especially stable domain of four clusters (15 ≤ <i>n</i> ≤ 18) is reached on the right (<i>n</i> = 18) with a discontinuous (4-fold) volatility decrease, suggesting a transition to a liquid-like structure. On the left it merges continuously (though with a steep slope) with the trend at smaller sizes (11 ≤ <i>n</i> ≤ 14).