Introduction to Ion Mobility Methods

Basic experimental setup

To initiate an mobility experiment, ions are introduced into a region ("drift tube") across which an electric field is uniformly applied. The uniform field is typically achieved by connecting a series of evenly spaced rings (separated by a nonconducting material) with equal value resistors. The ions drift through the drift tube under the influence of the field, and collide with the buffer gas (typically He, N₂, or air, depending on the application). The time required for the ions to reach the detector depends upon the ion's collision cross section (averaged over all possible orientations of the ion), charge state, and the drift tube operating parameters (electric field strength, drift tube length, and the buffer gas pressure, temperature, and mass). More compact ions traverse the drift tube more quickly than elongated ions (for a given mass and charge state).

where t_D is the drift time of the ion, E is the electric field strength, L is the length of the drift tube, P is the buffer gas pressure, and T is the buffer gas temperature. The reduced mobility is normalized to a buffer gas pressure of 760 torr and temperature of 273.2 K to provide a basis for comparison of results.

The collision cross section can be calculated by:

where ze is the ion's charge, k_b is Boltzmann's constant, m_I and m_B are the masses of the ion and buffer gas, respectively, and N is the number density.

Comparison with trial conformers from molecular modeling

To obtain structural information from cross section measurements, cross sections can be calculated for trial conformers generated by molecular modeling methods and compared with experimental values. As mentioned above, the collision cross section gives an orientationally averaged result. The first approximation to the collision integral for an ion colliding with hard sphere buffer gas atoms is called the "projection approximation". This approach essentially finds the average "shadow" (as shown in the cartoon below) as a trial conformer is rotated through all possible orientations.

Selected reviews of ion mobility techniques

Revercomb, H. E.; Mason, E. A. Anal. Chem. 1975, 47, 970.

St. Louis, R. H.; Hill, H. H. Crit. Rev. Anal. Chem. 1990, 21, 321.

Jarrold, M. F. J. Phys. Chem. 1995, 99, 11.

Clemmer, D. E.; Jarrold, M. F. J. Mass Spectrom. 1997, 32, 577.

Liu, Y.; Valentine, S. J.; Counterman, A. E.; Hoaglund, C. S.; Clemmer, D. E. Anal. Chem. 1997, 69, 728A.

Hoaglund Hyzer, C. S.; Counterman, A. E.; Clemmer, D. E. Chem. Rev. 1999, 99, 3037.