How Hot are Your Ions in Differential Mobility Spectrometry?

Ions can experience significant field -induced heating in a differential mobility cell. To investigate this phenomenon, the fragmentation of several para-substituted benzylpyridinium "thermometer" ions (R = OMe, Me, F, Cl, H, CN) was monitored in a commercial differential mobility spectrometer (DMS). The internal energy of each benzylpyridinium derivative was characterized by monitoring the degree of fragmentation to obtain an effective temperature, T-eff, which corresponds to a temperature consistent with treating the observed fragmentation ratio using a unimolecular dissociation rate weighted by a Boltzmann distribution at a temperature T. It was found that ions are sufficiently thermalized after initial activation from the ESI process to the temperature of the bath gas, T-bath. Once a critical field strength was surpassed, significant fragmentation of the benzylpyridinium ions was detected. At the maximum bath gas temperature (450 K) and separation voltage (SV; 4400 V) for our instrument, T-eff for the benzylpyridinium derivatives ranged from 664 +/- 9 K (p-OMe) to 759 +/- 17 K (p-H). The extent of activation at a given SV depends on the ion's mass, degrees of freedom, (N-DOF), and collision frequency as represented by the ion's collision cross section. Plots of T-eff vs the product of ion mass and N-DoF and the inverse of collision cross section produce strong linear relationships. This provides an attractive avenue to estimate ion temperatures at a given SV using only intrinsic properties. Moreover, experimentally determined T-eff correlate with theoretically predicted T-eff using with a self-consistent method based on two-temperature theory. The various instrumental and external parameters that influence T-eff are additionally discussed.