Temperature of Ions in IMS and Its Effect on Ion Geometries

Beyond its influence on IMS separation parameters for a fixed geometry, the gas temperature affects ion geometries. As ion-molecule collisions must be sufficientiy frequent for a steady drift, ions are thermalized their internal, rotational, and translational modes are equilibrated at a single temperature. At low E/N where the ion drift is much slower than the Brownian motion of gas molecules, relative ion-molecule velocities conform to the Maxwell-Boltzmann distribution and ion temperature equals T of the gas. 

In that regime, IMS resolving power is not influenced by interconversion dynamics and Equation 1.23 applies. However, measured fi corresponds to no particular geometry and extracting structural informalion from the data requires averaging over a representative geometry ensemble using Equation 1.25. 

FICURE 1.18 Conventional IMS spectra of dTG and dGT isomeric oligonucleotide ions measured in He at 7= 300 and 85 K. (From Gidden, J., BushneU, JJi., Bowers, M.T., J. Am. Chem. Soc., 123, 5610, 2001.) The arrows indicate unstable minor conformers that convert into the major conformers at higher T. 

Full equilibration of ions at a known temperature in IMS allows measuring temperature-dependent rate constants for structural transitions, from which accurate activation energies and preexponential factors could be determined in an assumption-free manner using Arrhenius plots. In contrast, structural characterization techniques implemented in vacuum, such as various laser spectroscopies (threshold photoionization,photodissociation,or photoelectron spectroscopy ), MS/MS by collisional or other dissociation, or chemical reactivity studies lack a direct ion thermometer. In those methods, ion temperature is estimated as the source temperature (possibly with semiempirical adjustments) or gauged using various indirect thermometers, and vibrationally or electronically hot ions are the ever- 

Despite strenuous efforts to eliminate that possibihty, it has 

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