Ion-mobility Spectrometry - Ion Mobility

Ion Mobility

In the traditional method of drift-time IMS, commonly referred to as just IMS, produced ions travel through a drift tube which has an applied electric field and a carrier buffer gas that opposes the ion motion. At the end of the tube is a detector. Based on an ion’s mass, charge, size and shape (the ion mobility), the migration time through the tube is characteristic of different ions, leading to the ability to distinguish different analyte species. The area of an ion that gas molecules strike is an ion’s collision cross-section, related to the ion size and shape. The greater this collision cross-section is, meaning the larger the ion size, the more area available for buffer gas to collide and impede the ion’s drift – the ion then requires a longer time to migrate through the drift tube.

The physical quantity ion mobility K is defined as the proportionality factor of an ion's drift velocity v_d in a gas and an electric field of strength E,

Ion mobilities are commonly reported as a reduced mobilities, correcting to standard gas density n₀, which can be expressed in standard temperature T₀ = 273 K and standard pressure p₀ = 1013 mbar:

The ion mobility K can be experimentally determined by measuring the drift time t_D of an ion traversing within a homogeneous electric field the potential difference U in the drift length L:

The ion mobility K can also be calculated by the Mason equation:

where Q is the ion charge, n is the drift gas number density, μ is the reduced mass of the ion and the drift gas molecules, k is Boltzmann constant, T is the drift gas temperature, and σ is the ion’s collision cross section with the drift gas. This relation holds approximately at a low electric field limit, where the ratio of E/n is small, at ≤ 2 x 10−17 J•C−1•cm2

A drift tube’s resolving power R can be calculated as

where L is the tube length, E is the electric field strength, Q is the ion charge, k is Boltzmann’s constant, and T is the drift gas temperature.

With a low electric field applied, the thermal energy of the ions is greater than the energy gained from the electric field between collisions. With these ions having similar energies as the buffer gas molecules, diffusion forces dominate ion motion.