Mobility, Electric Field, and Pressure

The ion mobility spectrum has many forms that share one common feature The ion current intensity is measured as a function of an ion s mobility in a gas. As with other types of spectrometry, the ion mobility spectrum is obtained by correlating a change in a spectrometer s parameter with a physical property of the ions. In light spectrometry, the number of photons is recorded as a function of photon energy in mass spectrometry, the number of ions is recorded as a function of mass, and in ion mobility spectrometry (MS), the number of ions is recorded as a function of an ion s collision cross section, which is related to its mobility. The type of IMS depends on the instrumental parameter that is scanned to produce the intensity versus mobility spectrum. To understand the many types of mobility spectra, we must first consider the relation among mobility, electric field, and pressure. 

The mobility of an ion swarm is defined as the ratio of the swarm s average velocity to the electric field through which it migrates  

In addition to the electric field, tanperature and pressure affect the velocity of an ion swarm. At constant pressure, an ion swarm s velocity will increase with increasing tanperature, and at constant tanperature, an ion swarm s velocity will decrease with inCTeasing pressure. To correct for the variation in an ion swarm s velocity as a function of tanperature T and pressure P, mobilities are commonly reported as reduced mobilities Kq at standard number density (N = 2.687 x 10 cm- ) using standard tanperatures (Tq = 273.15 K) and pressures (Pq= 760 torr) according to the following relation  

FIGURE 8.1 Dependence of A on EIN. (From Thomson et al.. Mobility, diffusion, and clustering of potassium(-r) in gases, J. Chem. Phys. 1973, 58, 2402-2411. With permission.) 

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